ACTA BIOLOGICA SLOVENICA LJUBLJANA 2017 Vol. 60, Št. 1: 3-28 Am Occurrence, toxins and possibilities of control of bloom-forming cyanobacteria of European freshwaters: a review Pojavljanje, toksičnost in kontrola cvetenja cianobakterij v evropskih celinskih vodah: pregled Klara Jarniab, Tjaša Griessler Bulcab, Aleksandra Krivograd Klemenčiča* aFaculty of Civil and Geodetic Engineering, University of Ljubljana, Hajdrihova 28, SI-1000 Ljubljana, Slovenia bFaculty of Health Sciences, University of Ljubljana, Zdravstvena pot 5, SI-1000 Ljubljana, Slovenia Correspondence: aleksandra.krivograd-klemencic@fgg.uni-lj.si Abstract: Blooming of cyanobacteria is a common problem of eutrophic water bodies in Europe and worldwide and can cause severe problems with toxicity, taste and odour of the water. Toxins produced by cyanobacteria (cyanotoxins) are structurally diverse and their effects range from liver damage, including liver cancer, to neurotoxicity and thus they may present a serious threat for drinking water safety. Cyanobacterial blooms present major challenges for the management of rivers, lakes and reservoirs and are predicted to cause even worse problems in the future due to the climate change associated with global warming, increased availability of light to phytoplankton and rising levels of atmospheric CO2. This paper presents the literature review of occurrence, toxins (along with their effects on human health) and possibilities of control of bloom-forming cyanobacteria. Keywords: algal blooms, cyanobacteria, cyanobacterial control, cyanotoxins, Europe, freshwaters Izvleček: Cvetenje cianobakterij je pogost problem v evtrofnih vodnih telesih v Evropi in po svetu. Povzroča lahko resne težave zaradi toksičnosti, spremenjenega okusa in vonja vode. Toksini, ki jih izločajo cianobakterije (cianotoksini), so po zgradbi različni, njihovi učinki pa zajemajo vse od poškodb jeter, vključno z rakom na jetrih, do nevrotoksičnosti in lahko predstavljajo resno nevarnost pri zagotavljanju varne pitne vode. Cvetenje cianobakterij predstavlja velik izziv za upravljalce rek, jezer in zbiralnikov, predvideva pa se, da bo v prihodnosti ta problematika še naraščala zaradi klimatskih sprememb in z njimi povezanih učinkov globalnega segrevanja, povečane dostopnosti svetlobe za fitoplankton in naraščajočih koncentracij atmosferskega CO2. Članek predstavlja pregled literature o pojavljanju, toksinih (vključno z njihovimi učinki na zdravje ljudi) in kontroli cianobakterijskih vrst, ki cvetijo v evropskih celinskih vodah. Ključne besede: cvetenje alg, cianobakterije, kontrola cianobakterij, cianotoksini, Evropa, celinske vode 102 Acta Biologica Slovenica, 60 (1), 2017 Introduction When environmental conditions such as temperature, light and nutrient status are conducive, surface waters (both freshwater and marine) may host increased growth of algae and/or cyanobac-teria. If and when such proliferation is dominated by a single (or a few) species, the phenomenon is referred to as an algal or cyanobacterial bloom (CB) (Chorus and Bartram 1999). CBs are a common problem of stagnant water bodies in Europe (Eiler and Bertilson 2004, Jacquet et al. 2005) and worldwide (Paerl and Huisman 2009, Kosten et al. 2012, Michalak et al. 2013). They present major challenges for the management of rivers, lakes and reservoirs (Carey et al. 2012) and are predicted to cause even worse problems in the future due to the climate change associated with global warming, increased availability of light to phytoplankton and rising levels of atmospheric CO2 (Johnk et al. 2008, Kosten et al. 2012, O'Neil et al. 2012, Paerl and Huisman 2009, Paerl and Paul 2012, Zhang et al. 2012). Some lakes, rivers and estuaries have seasonal blooms that start in summer and last into autumn, some have persistent blooms that encompass all seasons, and some have blooms that occur as extreme peaks and crashes lasting just a few days or weeks (Havens 2008). In temperate regions, CBs generally occur during the late summer and early autumn and may last two to four months (Cook et al. 2004). This is also the time when demand for recreational water is the highest (Chorus et al. 2000). In regions with Mediterranean (mild, wet winter and warm, dry summer) or subtropical climates, the bloom season may start earlier and persist longer (Cook et al. 2004). The CBs increase the turbidity of eutrophied lakes and in turn supress growth of aquatic mac-rophytes affecting invertebrates and fish species in addition to oxygen depletion and odour problems (Paerl and Huisman 2009). Lastly, some cyanobacterial species produce toxic peptides and alkaloids, which are a major threat to the use of freshwater ecosystems, and reservoirs for drinking water, irrigation, fishing and recreation (Carmi-chael 2001). If cyanobacteria are present or even dominant for most of the year, the problems associated with high cyanobacterial biomass and the potential health threats from their toxins increase. Proliferation of toxic cyanobacteria often causes a reduction in biodiversity, leading to disruption of the trophic chain and to ecosystem imbalance (Sedmak and Elersek 2005). Potential toxic risks, to both animal and humans, may cause problems to local fisheries and to touristic and recreational activities (Chorus and Bartram 1999, Dokulil and Teubner 2000, Briand et al. 2003). Environmental conditions promoting bloom-forming cyanobacterial growth The mechanism of CB occurrences is very complex as they are not caused by a single environmental driver but rather by multiple factors occurring simultaneously (Dokulil and Teubner 2000, Heisler et al. 2008). Environmental conditions promoting growth of most common potentially toxic cyanobacteria in European stagnant waters are shown in Tab. 1. Onset of development and proliferation of CBs are closely associated with eutrophication and climatic conditions. Cyanobacteria can occupy almost all kinds of aquatic habitats as they are able to use different forms of carbon (C), nitrogen (N), phosphorus (P), and sulphur (S), they grow well in shade, are resistant to grazing and release allelochemicals to out-compete other organisms (Sharma et al. 2010). Cyanobacteria possess certain unique adaptations that make them a successful competitor. These include their ability to grow in warm waters, to utilize low total N (TN) to total P (TP) ratio, to access low dissolved CO2 concentration (in form of bicarbonate), and their ability of N fixation (Sharma et al. 2010). Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 5 Table 1: Recorded occurrencies of common potentially toxic cyanobacteria in freshwaters in Europe and environmental conditions promoting their growth. Tabela 1: Pojavljanje najpogostejših potencialno toksičnih cianobakterij v evropskih celinskih vodah in okoljski dejavniki, ki spodbujajo njihovo rast. Trophic state Optimal Optimal of water body temperature light intensity Additional info Country Aphanizomenon flos-aquae Ralfs ex Bornet and Flahault mesotrophic stagnant waters, reservoirs 20 °C - 28 °C 100 - 110 mmol phot. Fresh and salty waters, common in plankton, sometimes creates blooms Coexists with Britain M. aeruginosa Denmark France Germany Netherlands Poland Portugal Romania Slovenia Spain Sweden Turkey (Europe) Skuja (1948), Alvarez-Cobelas and Gallardo (1988), Aboal (1996), Kosi (1999), Dokulil and Teubner (2000), Tsujimura et al. (2001), Whitton (2002), Karlsson-Elfgren and Brunberg (2004), Aboal and Puig (2005), Dean and Sigee (2006), Ersanli and Gönülol (2006), O'Brien et al. (2006), Willame et al. (2006), Carrasco et al. (2007), Leao et al. (2009), Pérez et al. (2009), Kokocinski et al. (2010), Täuscher (2011), Caraus (2012) Aphanizomenon mesotrophic- 20 °C - ■ 28 °C 100 - 110 Freshwater, Belgium Caraus (2002), gracile eutrophic mmol phot. planktic, France Whitton (2002), Lemmermann stagnant m-2s-1 common Germany Willame et al. (2006), waters (ponds, in stagnant Luxembourg Carrasco et al. (2007), reservoirs) waters (ponds, Poland Kokocinski et al. (2010), reservoirs) Romania Täuscher (2011), Spain Caraus (2012), Mehnert et al. (2012) Chrysosporum eutrophic and 26 °C - - 30 °C Mostly in Greece Alvarez-Cobelas ovalisporum mesotrophic- Mediterranean Italy and Gallardo (1988), (Forti) eutrophic Europe, Middle Poland Bazzichelli and E.Zapomelová, reservoirs East, North Spain Abdelahad (1994), O.Skácelová, America and Turkey (Europe) Gkelis et al. (2005), P.Pumann, R.Kopp Australia Ersanli and Gönülol and E.Janecek (2006), syn. Carrasco et al. (2007), Aphanizomenon Kokocinski and Soininen ovalisporum (2012), Forti, Anabaena Sukenik et al. (2013) ovalisporum Forti Cuspidothrix mesotrophic- Freshwater, Britain Caraus (2002), issatschenkoi eutrophic planktic in France Whitton (2002), (Usachev) stagnant lakes and Germany Willame et al. (2006), Rajaniemi, waters (ponds, ponds in Hungary Carrasco et al. (2007), Komárek, reservoirs) Europe and Poland Leao et al. (2009), Willame, Hrouzek, Asia Portugal Kokocinski et al. (2010), Ka Romania Täuscher (2011), syn. Spain Caraus (2012), Aphanizomenon Horváth et al. (2013) issatschenkoi (Usacev) Proshkina-Lavrenko Acta Biologica Slovenica, 60 (1), 2017 102 Taxa Trophic state Optimal Optimal Occurrence Additional Country Source of water body temperature light info intensity Cylindrospermopsis mesotrophic- 29 °C • - 31 °C 80 - 120 Tropical and Germany Dokulil and Teubner raciborskii eutrophic mmol phot. subtropical, Poland (2000), (Woloszynska) stagnant m-2s-1 but appears to Portugal Saker et al. (2004), Seenayya and waters (ponds, be invading Stuken et al. (2006), Subba Raju reservoirs) temperate regions (as far north as Vienna) Wiedner et al. (2007), Carneiro et al. (2009), Leao et al. (2009), Kokocinski et al. (2010), Täuscher (2011), Mehnert et al. (2012), Kokocinski and Soininen (2012) Dolichospermum hypertrophic 20 °C • - 28 °C Freshwater, Britain Skuja (1948), circinale fishponds, planktic, often Czech Republic Alvarez-Cobelas and (Rabenhorst mesotrophic- forming heavy France Gallardo (1988), ex Bornet eutrophic water blooms; Germany Caraus (2002), and Flahault) stagnant cosmopolitan Romania Whitton (2002), P.Wacklin, waters (ponds, distribution Slovenia Karlsson-Elfgren and L.Hoffmann and reservoirs) with exception Spain Brunberg (2004), J.Komârek of subpolar Sweden Ersanli and Gönülol syn. Anabaena regions; Turkey (Europe) (2006), circinalis massive Täuscher (2011), Rabenhorst populations Zapomelova et al. (2011), ex Bornet and known mainly Caraus (2012), Flahault from Central Europe, South Database of Slovenian Environment Agency America and Australia Dolichospermum hypertrophic 20 °C - 28 °C Freshwater, Czech Republic Willame et al. (2006), crassum fishponds, planktic in Germany Carrasco et al. (2007), (Lemmermann) mesotrophic- ponds and Luxembourg Täuscher (2011), P.Wacklin, eutrophic reservoirs, Slovenia Zapomelova et al. (2011), L.Hoffmann and stagnant in temperate Spain Sukenik et al. (2013), J.Komârek waters (ponds, zones of both Database of Slovenian syn. Anabaena reservoirs) hemispheres, Environment Agency crassa up to (Lemmermann) subtropical Komark.-Legn. regions and Cronberg Dolichospermum hypertrophic 20 °C - 28 °C Freshwater, Britain Skuja (1948), flos-aquae fishponds, common Czech Republic Âlvarez-Cobelas (1982), (Brébisson mesotrophic- in plankton Denmark Alvarez-Cobelas and ex Bornet eutrophic of stagnant Germany Gallardo (1988), and Flahault) stagnant waters, Romania Kosi (1999), P.Wacklin, waters (ponds, cosmopolitan Slovenia Caraus (2002), L.Hoffmann and reservoirs) species with Spain Whitton (2002), J.Komârek exception Sweden Dean and Sigee (2006), syn. Anabaena of subpolar Sigee et al. (2007), flos-aquae regions; Täuscher (2011), Brébisson ex tropical Zapomelova et al. (2011), Bornet and populations Caraus (2012) Flauhault less frequent; often creates blooms Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 7 Taxa Trophic state Optimal Optimal Occurrence Additional Country Source of water body temperature light info intensity Dolichospermum hypertrophic 20 oc - 28 oc Freshwater, One of the Czech Republic Caraus (2002), lemmermannii fishponds, common in dominant Denmark Olli et al. (2005), (Ricter) P. Wacklin, mesotrophic- plankton of species in Finland Täuscher (2011), L.Hoffmann and eutrophic reservoirs cyanobacterial Germany Zapomelova et al. (2011), J.Komárek stagnant in whole mass Romania Caraus (2012) syn. Anabaena waters (ponds, temperate occurrences lemmermannii P. reservoirs) zone (distinct in boreal Richter water blooms); lakes, even never found in relatively in tropical oligotrophic regions lakes Dolichospermum hypertrophic 20 oc - 28 oc prefers Freshwater, Belgium Bruno et al. (1994), planctonicum fishponds, moderate common in Britain Caraus (2002), (Brunnth.) mesotrophic- light plankton of Czech Republic Whitton (2002), Wacklin, L.Hoffm. eutrophic intensities stagnant waters Germany Willame et al. (2006), and Komárek stagnant Italy Carrasco et al. (2007), syn. Anabaena waters (ponds, Luxembourg Täuscher (2011), planctonica reservoirs) Romania Zapomelova et al. (2011), Brunnthaler Slovenia Caraus (2012), Spain Database of Slovenian Environment Agency Dolichospermum oligotrophic 20 oc - 28 oc Freshwater, Czech Republic Willen and Mattsson solitarium stagnant common in Finland (1997), (Klebahn) waters plankton of Germany Caraus (2002), Wacklin, (mountain stagnant waters Romania Kastovsky et al. (2010), L.Hoffmann and lakes, Slovenia Täuscher (2011), Komárek quarries) and Caraus (2012), syn. Anabaena mesotrophic Database of Slovenian solitaria Klebahn stagnant Environment Agency waters, reservoirs Dolichospermum hypertrophic 20 oc - 28 oc prefers Freshwater, Belgium Skuja (1948), spiroides (Kleb.) fishponds, moderate common in Britain Alvarez-Cobelas and Wacklin, L.Hoffm. mesotrophic- light plankton of Czech Republic Gallardo (1988), and Komárek eutrophic intensities stagnant and France Kosi (1999), syn. Anabaena stagnant slowly running Germany Caraus (2002), spiroides Klebahn waters (ponds, waters, mainly Romania Whitton (2002), reservoirs) from May to Slovenia Ersanli and Gönülol October Spain (2006), Sweden Willame et al. (2006), Turkey (Europe) Täuscher (2011), Zapomelova et al. (2011), Caraus (2012) Gloeotrichia mesotrophic Freshwater, Britain Whitton (2002), echinulata stagnant common in Germany Täuscher (2011), P.Richter waters, plankton of Romania Caraus (2012) reservoirs stagnant and slowly running waters, sometimes creates blooms 102 Acta Biologica Slovenica, 60 (1), 2017 Taxa Trophic state Optimal of water body temperature Optimal light intensity Occurrence Additional info Country Source Limnothrix redekei mesotrophic- Freshwater, often Czech Republic Chorus and Bartram (van Goor) M.-E. eutrophic and planktic, together with Germany (1999), Meffert mesotrophic widely Planktothrix Poland Kastovsky et al. (2010), stagnant distributed in agardhii Romania Kokocinski et al. (2010), waters, temperate zone Slovenia Täuscher (2011), reservoirs, throughout Caraus (2012), also wetlands, the whole Database of Slovenian pools, year (distinct Environment Agency furrows, populations usually with occur in winter water plants season); common in Northern and Central Europe Microcystis eutrophic 28 °C - 32 °C Fresh and Britain Skuja (1948), aeruginosa water bodies brackish Germany Alvarez-Cobelas and (Kützing) Kützing (lakes, waters, Portugal Gallardo (1988), fishponds, planktic, Romania Kosi (1999), reservoirs) sometimes forming heavy water blooms, Slovenia Spain Sweden Nalewajko and Murphy (2001), Caraus (2002), common; Turkey (Europe) Carrillo et al. (2003), cosmopolitan Whitton (2002), with exception Martín et al. (2004), of polar and Bárbara et al. (2005), subpolar Ersanli and Gönülol regions (2006), Jöhnk et al. (2008), Paerl and Huisman (2008), Young et al. (2008), Leao et al. (2009), Metcalf et al. (2009), Pérez et al. (2009), Pérez et al. (2010), Täuscher (2011), Caraus (2012) Microcystis mesotrophic 28 °C - 32 °C Freshwater, Belgium Nalewajko and Murphy ichtyoblabe or slightly planktic, Czech Republic (2001), (Kunze) Kützing eutrophic, but sometimes Germany Willame et al. (2006), not polluted forming water Slovenia Jöhnk et al. (2008), lakes blooms; more in northern regions of the temperate zone, probably not occurring in tropical countries Paerl and Huisman (2008), Kastovsky et al. (2010), Täuscher (2011), Database of Slovenian Environment Agency Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 9 Taxa Trophic state Optimal Optimal of water body temperature light intensity Occurrence Additional info Country Source Microcystis flos- mesotrophic 28 °C - 32 °C Freshwater, Britain Nalewajko and Murphy aquae (Wittrock) and slightly planktic, Germany (2001), Kirchner eutrophic usually Romania caraus (2002), water bodies together with Turkey (Europe) Whitton (2002), other planktic Slovenia Ersanli and Gönülol algae and (2006), cyanoprokaryotes, Sigee et al. (2007), sometimes Jöhnk et al. (2008), part of water Paerl and Huisman blooms, (2008), cosmopolitan Täuscher (2011), in the whole Caraus (2012), temperate zone, Database of Slovenian particularly Environment Agency in northern regions Microcystis slightly 28 °C - 32 °C Freshwater, Germany Skuja (1948), viridis (A.Braun) eutrophic planktic, Romania Alvarez-Cobelas Lemmermann lakes and sporadical, Slovenia and Gallardo (1988), ponds sometimes Spain Nalewajko and Murphy forming water Sweden (2001), blooms; Caraus (2002), cosmopolitan Jöhnk et al. (2008), Paerl and Huisman (2008), Elersek (2009), Täuscher (2011), Caraus (2012), Scholz and Liebezeit (2012) Nodularia eutrophic 20 oc • - 30 °C high Mostly in Britain Guiry (1978), spumigena ponds, lakes tolerance of salty/brackish Ireland Alvarez-Cobelas and Mertens ex Bornet and reservoirs ultraviolet waters, also in Poland Gallardo (1988), and Flahault radiation fresh waters, planktonic, common, often forms blooms in lagoons and estuaries Romania Spain Turkey Calvo and Bárbara (2002), Caraus (2002), Moisander et al. (2002), Whitton (2002), Bárbara et al. (2005), Akcaalan et al. (2009), Jodlowska and Latala (2010), Caraus (2012) Planktothrix mesotrophic- 10 oc • • 25 °C prefers Freshwater, Never forms Belgium Chorus and Bartram agardhii (Gomont) eutrophic low light planktic in scums Germany (1999, Anagnostidis and stagnant intensities lakes and Luxembourg Kosi (1999), Komárek waters (ponds, inhibited ponds, often Poland Dokulil and Teubner reservoirs), above 180 forming water Romania (2000), hypertrophic ^E m"2s"' blooms, widely Slovenia Willame et al. (2006), fishponds distributed in temperate zones; less in tropical regions Spain Quesada et al. (2006), Willame et al. (2006), Carrasco et al. (2007), Oberhouse et al. (2007), López Rodríguez et al. (2009), Kokocinski et al. (2010), Täuscher (2011), caraus (2012), Kokocinski and Soininen (2012)_ 102 Acta Biologica Slovenica, 60 (1), 2017 Taxa Trophic state Optimal Optimal Occurrence Additional Country Source of water body temperature light info intensity Planktothrix mesotrophic cold water prefers Freshwater, Usually does France Kosi (1999), rubescens and eutrophic form (10 °C - low light planktic, in not form Germany Dokulil and Teubner (De Candolle large lakes 14 °C) intensities large lakes scums during Italy (2000), ex Gomont) and stagnant and stagnant the bathing Romania Almodovar et al. (2004), Anagnostidis and waters waters, forming season Slovenia Grach-Progrebinsky et Komarek red water blooms; in several regions in northern temperate zone with obligatory distribution, outside of these areas occasionally over the whole temperate zone Spain Switzerland al. (2004), Viaggiu et al. (2004), Willame et al. (2006), Carrasco et al. (2007), Holland and Walsby (2008), Täuscher (2011), Caraus (2012) Woronichinia eutrophic Freshwater, Britain Skuja (1948), naegeliana lakes and common Czech Republic Alvarez-Cobelas (Unger) Elenkin ponds in plankton Germany and Gallardo (1988), syn. of lakes Luxembourg Cronberg et al. (1999), Coelosphaerium and ponds, Romania Whitton (2002), naegelianum sometimes Slovenia Rajaniemi-Wacklin et al. Unger, forming water Spain (2006), Gomphosphaeria blooms, in Sweden Willame et al. (2006), naegeliana temperate Täuscher (2011), (Unger) zones, in Caraus (2012), Lemmermann Europe and Database of Slovenian North America Environment Agency up to northern regions Nitrogen and phosphorous Because CBs often develop in eutrophic lakes, it was originally assumed that they require high P and N concentrations. However, in late summer, when CBs mostly occur, concentrations of dissolved phosphate tend to be the lowest. Experimental data showed that the affinity of many cyanobacteria for N or P is higher compared to other photosynthetic organisms meaning that they can out-compete other phytoplankton organisms under conditions of P or N limitation (Chorus and Bartram 1999). In most freshwater systems P is considered to be prime limiting nutrient (Xu et al. 2010) and small changes in P levels may influence the growth and toxin production of cyanobacteria (Sivonen 1990, Chorus and Bartram 1999). Cyanobacteria usually uptake P in orthophosphate form (PO43-), however they are also able to uptake other phosphate forms like polyphosphates (Mukherjee et al. 2015). According to Downing et al. (2001) high concentration (30-100 ng/L) of TP promotes CB formation. Because of their high affinity for P, cyanobacteria can store substantial amount of P during P-sufficient conditions. Excess P-loading (luxury consumption) may facilitate growth of other phytoplankton groups leading to increased turbidity, which additionally favours cyanobacte-rial growth (Chorus and Bartram 1999). Besides P alone, P in combination with other nutrients may regulate cyanobacterial dominance in bloom environment. High TN:TP ratio is indicative for P limitation and vice-versa (Pinckney et al. 2001). However, according to Downing et al. (2001), TP is better predictor of cyanobacterial dominance than TN:TP ratios. Light intensity Turbid, low irradiance conditions promote growth of non-heterocystous cyanobacteria Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 11 (e.g. Oscillatoria, Lyngbia, Planktothrix rube-scens) causing their domination in the phyto-plankton community, which is attributed mainly to their ability to maintain net growth at low underwater irradiance (Havens et al. 2003). However, cyanobacteria which form surface blooms (e.g. Cylindrospermopsis raciborskii) have a higher tolerance for high light intensities most probably due to an increase in carotenoid production, which protects the cells from photoinhibition (Paerl et al. 1983, Wiedner et al. 2007, Carneiro et al. 2009). In moderately deep, stratified eutrophic lakes typically N2-fixing cyanobacteria such as Anabaena and Aphanizomenon (Paerl et al. 2001) are present. Temperature CBs in stagnant waters are correlated, to a considerable degree, with weather conditions and consequently with climate conditions in a given area (Zhang et al. 2012). Cyanobacteria usually dominate phytoplankton assemblages in temperate freshwater environments during the warmest period of the year, particularly in eu-trophic systems (Paerl 2008, Paerl and Huisman 2008). Optimum temperatures for cyanobacteria are in general higher than for green algae and diatoms (e.g. 25 °C or higher for species from genera Microcystis), which may explain why, in addition to the lower nutrient levels in epilimnion, in temperate and boreal water bodies most CBs occur during summer (Chorus and Bartram 1999, Dokulil and Teubner 2000, Paerl and Huisman 2008, Johnk et al. 2008, Mehnert el at. 2010). However, some species such as Planktothrix rubescens and Aphanizomenon flos-aquae have low temperature preference or tolerance and thus bloom during late autumn and winter (Tsujimura et al. 2001). According to Lurling et al. (2013) intensification of CBs in warmer climate is not attributed to their higher growth rates compared to other phytoplankton species, but rather to their ability to migrate vertically and prevent sedimentation in warmer and more strongly stratified waters and to their resistance to grazing. Water column stability CBs are promoted by calm, vertically stratified conditions with adequate nutrient supplies and weak wind mixing (Paerl and Millie 1996, Kanoshina et al. 2003, Huisman et al. 2004, Sharma et al. 2010). In the case of wind- or flow induced destratification cyanobacteria may lose their competitive advantage, which together with cell and filaments damages due to increased turbulence (Moisander et al. 2002) can cause, if such conditions persist, rapid degradation of CBs. However, when intermittent weak stratification occurs during favourable growth periods (summer), blooms can quickly re-emerge. Non-disruptive, low-level turbulence can promote localized nutrient cycling, alleviate certain forms of nutrient limitation (e.g. PO43, trace metals), and enhance cyanobacterial growth. pH Alkaline conditions favour CB formation (Havens 2008). Cyanobacteria capacity for photosynthesis in environments with low CO2 concentrations (by using bicarbonate (HCO3-) as their carbon source (Kaplan et al. 1991)) and high pH is an important characteristic giving cyanobacteria advantage over other phytoplankton organisms in water environments with high pH values, a general characteristic of eutrophic lakes (Dokulil and Teubner 2000, Kardinal and Visser 2005). Salinity Increased salination (e.g. summer droughts, rising sea levels, increased use of freshwater for agricultural irrigation) has major impacts on freshwater plankton communities with repercussions for water quality and use (Paerl and Huis-man 2009). One such impact is increased vertical density stratification, which benefit buoyant cyanobacteria (Walsby et al. 1997, Huisman et al. 2004). In addition, some species of the common cyanobacterial genera Anabaena, Anabaenopsis, Microcystis and Nodularia are sometimes more salt tolerant than eukaryotic freshwater phyto-plankton species (Moisander et al. 2002, Tonk et al. 2007). Thus, increased salination of freshwater and brackish waters can favour cyanobacteria over other freshwater phytoplankton species exposing other aquatic organisms and human users of these waters to elevated concentrations of cyanobacterial toxins (Paerl and Huisman 2009). The high salt 102 Acta Biologica Slovenica, 60 (1), 2017 tolerance of freshwater cyanobacteria is reflected by increasing numbers of CBs in brackish waters, for example, in the Baltic Sea in Scandinavia (Kanoshina et al. 2003, Suikkanen et al. 2007) and in the Kujukjekmece Lagoon in Turkey (Albay et al. 2005). Bioactive substances Cyanotoxins Healthy CBs produce little extracellular toxin, while cell-bound concentrations are several orders of magnitude higher (Li et al. 2009). Very often, different strains of the same cyanobacteria species with similar growth rate produce different amounts of the same types of toxins (Watanabe and Oishi 1985, Sivonen 1990). External factors, including chemical conditions, modify not only cyanobacteria growth and toxin production but also affect cell longevity and the leakage of toxins to the environment which, in natural conditions, occurs mainly as the result of cell damage, death, lysis and decomposition of the aging cells. Thus, concentration of dissolved toxins may be much higher in ageing or declining CBs compared to healthy young CBs. However, toxin excretion from the cells can be promoted also by high temperature, high salination, high light intensities, low concentrations of P and chemical treatment for the eradication of cyanobacteria (especially use of algicides) (Sivonen 1990, van Apeldoorn et al. 2007, Rapala et al. 1997). Not all toxigenic species or toxic CBs will be toxic at all times (Carmichael 2001). Factors influencing formation and toxicity of toxic CBs include i) genetics as there are distinct toxin and non-toxin producing strains, and ii) good growth conditions together with optimum conditions for toxin production. Toxicity of CBs depends also on the ratio of toxin to non-toxin producers and the factors that lead to surface scums production (Carmichael 2001). The freshwater cyanotoxins fall into three broad groups of chemicals: i) cyclic peptides (hepatotoxic microcystins and nodularins); ii) alkaloids (neurotoxic anatoxin-a, anatoxin-a(S), saxitoxins and hepatotoxic cylindrospermopsins); and iii) lipopolysaccharides (potentially irritant) (van Apeldoorn et al.2007). General features of the cyanotoxins occurring in freshwaters in Europe and their effects on human health are shown in Tab. 2. Hepatotoxic cyclic peptides are the most frequently found cyanobacterial toxins in CBs from fresh and brackish waters and pose a major challenge for the production of safe drinking water from surface waters containing cyanobacteria with these toxins. In mouse bioassays, which traditionally have been used to screen toxicity of field and laboratory samples, cyanobacterial hepatotoxins (liver toxins) cause death by liver haemorrhage within a few hours of the acute doses (Chorus and Bartram 1999). The cyclic peptide microcystins and nodularins are specific liver poisons in mammals. Following acute exposure to high doses, they cause death from liver haemorrhage or from liver failure, and they may promote the growth of liver and other tumours following chronic exposures to low doses (Chorus and Bartram 1999). Microcystins Microcystins (MC) are the most frequently reported cyanobacterial toxins. 248 MC analogues have been reported to date (Spoof and Catherine, 2017). The amount of MC production by a cyanobacterial population in culture is directly proportional to its growth rate, no matter what environmental factor is limiting the growth (van Apeldoorn et al. 2007). While variants of MC produced by a particular strain are rather constant, the ratio of individual MC may change with time, temperature and light intensity. According to van Apeldoorn et al. (2007) at high P levels hepato-toxic cyanobacterial strains produced more toxins. Non-nitrogen fixing species such as Microcystis and Oscillatoria produce more toxins under N rich conditions (van Apeldoorn et al. 2007). MC are intracellular toxins, and whilst contained only in living cells, they are degraded slowly. MC are only released into the water by senescence or cell death or through water treatment processes such as pre-chlorination or algicide application. The study of Zastepa et al. (2014) demonstrated that MCs can persist well beyond the disappearance of the bloom. Dissolved MC-LA declined more slowly and persisted longer than particulate (cell-bound) MC-LA with in situ half-lives (total 1.5-8.5 days) shorter than in vitro (total 6.8-60.0 days). Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 13 Table 2: General features of the cyanotoxins occurring in freshwaters in Europe and their effects on human health. Adapted from Chorus and Bartram (1999), Chorus et al. (2000) and van Apeldoorn et al. (2007). Tabela 2: Splošne značilnosti cianotoksinov, ki se pojavljajo v evropskih celinskih vodah, in njihovi učinki na zdravje ljudi. Povzeto po Chorus in Bartram (1999), Chorus in sod. (2000) in van Appeldoorn in sod. (2007). Toxin group Primary Reported effects on Taxa LD50 of pure toxin target organ in human health (mouse bioassay) mammals CYCLIC PEPTIDES Microcystins (MC) Liver Short term: gastroenteritis, Microcystis, MC in general: liver damage, acute liver Anabaena, 45-1000 ^g/kg failure, birth defect, Haff Planktothrix (Oscillatoria), MC-LR: disease, blistering of lips, Nostoc, 60 (25-125 ^g/kg) allergic reactions (contact Hapalosiphon, MC-YR: dermatitis, asthma, hay Anabaenopsis 70 ^g/kg fever, conjunctivitis), Woronichinia MC-RR: vomiting, diarrhoea, Limnothrix 300-600 ^g/kg abdominal pain, sore throat, Gloeotrichia pneumonia. Aphanizomenon Long term: hepatocellular carcinoma. Nodularins Liver no human poisonings Nodularia 30-50 ^g/kg recorded, only reports of skin rashes ALKALOIDS Anatoxin-a Nerve synapse no data till date Anabaena, 250 ^g/kg Planktothrix (Oscillatoria), Aphaniziomenon Dolichospermum Microcystis aeruginosa Anatoxin-a(S) Nerve synapse no data till date Anabaena 40 ^g/kg Dolichospermum Raphidiopsis mediterranea Cylindrospermopsins Liver hepatoenteritis, acute tender liver enlargement, constipation, vomiting and headache, diarrhoea, dehydration Cylindrospermopsis, Aphanizomenon, Umezakia Dolichospermum Cuspidothrix Chrysosporum 2100 ^g/kg/d 200 ^g/kg/5-6 d Saxitoxins Nerve axons no data till date of human Anabaena, 10-30 ^g/kg poisonings Aphanizomenon, Dolichospermum Lyngbya, Cylindrospermopsis LIPOPOLYSACCHARIDES Potentially can cause skin irritation All irritant; affects any exposed tissue 102 Acta Biologica Slovenica, 60 (1), 2017 Decline of MC was accelerated by higher temperature and irradiance, both of which are considered the most important environmental factors in MC degradation. MC can accumulate in aquatic organisms, such as zooplankton, phytoplankton, gastropods, mussels, clams and fish, and thus enter the food chain and pose possible threat to human health. The oral intoxication route is the most important as it involves not only the drinking of water containing cyanobacterial toxins but also the consumption of toxin-containing animal or plant tissues (Spoof 2005, van Apeldoorn et al. 2007). Many reported worldwide cases demonstrate that MC cause both acute and chronic effects on humans (Ueno et al. 1996, WHO 1998, Zhou et al. 2002). Acute intoxication by MC coincides frequently with the lysis of the bloom-forming cells (by natural senescence or water treatment processes) and liberation of toxins to the water. The inhalation of dry cyanobacteria cells or contaminated water is more dangerous than oral ingestion of contaminated water indicating the hazardous potential of practising aquatic sports in recreational waters that suffer a microcystin producing bloom (WHO 2003). Chronic exposure to low concentrations of microcystins in drinking water can be a serious problem to public health, contributing to promotion of cancer in humans. Epidemiological studies have already related the presence of MC in drinking water to an increase in the incidence of colorectal cancer (Zhou et al. 2002) and primary liver cancer (Ueno et al. 1996). Recent studies show that toxic responses of MC may also be seen in kidney, heart, reproductive system, brain and lungs (Milutinovič et al. 2006, Wang et al. 2008, Chen et al. 2016, McLellan and Manderville 2017). Nodularins Nodularins are cyclic pentapeptides found in Nodularia spumigena (Chorus and Bartram 1999, Spoof 2005). To date, approximately 10 variants have been discovered, among which nodularin-R is the most abundant (Chen et al. 2013). The occurrence of N. spumigena blooms is determined by water temperature, light intensity, and nutrients concentration, among which levels of N and P are critical (Mazur et al. 2003). Nodularins tend to accumulate in mussels, clams and fish (van Apel- doorn et al. 2007) and have been implicated in the deaths of wild and domestic animals (Chen et al. 2013). No guidelines have been set for nodularins by the World Health Organization (WHO), and their toxicity can currently only be estimated from MC, which have been reported to have similar toxicity to nodularins (Paerson et al. 2010). Since nodularins generally occurs in brackish waters, accidental swallowing of water during recreational activities and seafood consumption could be the major routes with regard to human exposure. Alkaloids Anatoxins Anatoxins are a group of neurotoxic alkaloids which includes anatoxin-a, homoanatoxin-a and anatoxin-a(S). Anatoxins exposure and effects on humans or aquatic biota have not been fully determined yet also no clear evidence of human poisoning from anatoxins exists (Osswald et al. 2007, van Apeldoorn et al. 2007, EPA 2015). Anatoxin-a (ANTX-a) is produced by certain species of Anabaena (A. planctonica, A. flos-aquae, A. spiroides and A. circinalis), Planktothrix (Oscil-latoria), Cylindrospermum, Aphanizomenon, and in minor amountsM. aeruginosa (e.g. Agnihotri, 2014). ANTX-a is a potent postsynaptic depolarizing neuromuscular blocking agent and causes death in laboratory animals within minutes to a few hours (Stevens and Krieger 1991, Fitzgeorge et al. 1994, van Apeldoorn et al. 2007). According to Chorus and Bartram (1999) P levels have no effects on ANTX-a production. ANTX-a differs from other cyanotoxins (like microcystins) in that it undergoes rapid photochemical degradation in sunlight even in the absence of cell pigments. Stevens and Krieger (1991) found that the degradation of ANTX-a is dependent on the light intensity and/or pH, with higher pH favouring degradation reactions. ANTX-a has been widely identified in surface waters in North America and Europe used for recreation, and hence a risk exists for ANTX-a poisoning of recreational water users (Chorus et al. 2000). Homoanatoxin-a was reported to be produced by Planktothrixformosa, by Norvegian strain of O. formosum (Phormidium formosum), some unidentified Anabaena species from Ireland and Raphidiopsis mediterranea (Chorus and Bartram 1999, Furey et al. 2003, Watanabe et al. Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 15 2003). ANTX-a(S), which chemical structure is un-related to ANTX-a, is produced by Anabaena flos-aquae, A. lemmermannii, A. spiroides and A. crassa (Chorus and Bartram 1999, Becker et al. 2010, de Abreu et al. 2013). Saxitoxins Saxitoxins (STX) are a group of carbamate alkaloid neurotoxins which are either non-sulphated (saxitoxins - STX), singly sulphated (gonyautoxins - GTX) or doubly sulphated (C-toxins). In addition, decarbamoyl derivatives (dc) and several new toxins (Lyngbya-wollei toxins, LWTXs) have been identified in some cyanobacterial species (van Apeldoorn et al. 2007). STX and its analogues are produced by Anabaena circinalis (Chorus and Bartram 1999); very low concentrations were detected also in two other Anabaena species: A pertubata and A. spiroides (Velzeboer et al. 2000). In a few Danish lakes containing STX, A. lemmermannii was the dominant cyanobacterium (Kass and Hendriksen 2000). Also A. flos-aquae from Portugal and Planktothrix sp. FP1 from Italy were reported to produce STX (Molica et al. 2002). All saxitoxins act in the same way: they block nervous transmission causing muscle paralysis (Briand et al. 2003). Till date no reports of human poisonings due to STX presence in freshwater environments are known (Chorus and Bartram 1999). Cylindrospermopsin Cylindrospermopsin (CYN) is a tricyclic alkaloid, possessing a tricyclic guanidine moiety combined with hydroxymethyluracil produced by Cylindrospermopsis raciborskii, Umezakia natans and Aphanizomenon ovalisporum (van Apeldoorn et al. 2007). CYN like microcystins, primarily affects the liver, although causes considerable damage also to other major organs e.g. kidneys, spleen, thymus and heart. CBs which caused both liver and kidney damage due to the CYN (and possibly related cyanotoxins) have been reported in Australia, Japan, Israel and Hungary (Chorus and Bartram 1999). Chorus and Bartram (1999) and Falconer (2001) reported health problems associated with presence of CYN in drinking water supplies in Australia. Patients suffered from an unusual hepatoenteritis, acute tender liver enlargement, constipation, vomiting and headache, followed by diarrhoea and dehydration. Chonudomkul et al. (2004) pointed out that C. raciborskii is not only an ongoing invasive species but also a species with different physiological strains or ecotypes and temperature tolerance. Volatile organic compounds and other bioactive substances Cyanobacteria can produce various compounds causing off-flavour, also known as volatile organic compounds (VOC). 2-Methylisoborneol (2-MIB) and geosmin are among the most important odorous compounds in cyanobacteria and are often cited as sources of unpleasant earth-like and musty odour, especially in various aquatic environments (Fujise et. al. 2010). VOC are primarily produced by different prokaryotic and eukaryotic benthic and pelagic aquatic microorganisms (e.g. Streptomyces, fungi). Many of the known cyanobacterial producers of VOC are nonplanktic (approx. 30%), while the remainder are benthic or epiphytic. According to Juttner and Watson (2007) geosmin and 2-MIB production is limited to filamentous cyanobacteria and is unknown among chorococcalean taxa. According to Milovanovic et al. (2015) growing conditions have significant impact on production of VOC in cyanobacteria, and altering these conditions may be useful in obtaining cyanobacterial biomass with favourable sensory properties for potential use in formulation of food and feed products. Beside toxins and VOC, cyanobacteria produce also other very heterogeneous biologically active substances, such as peptides, retinoids, alkaloids, lactones, phospholipids (Sychrova et al. 2012, Wu et al. 2012a). Some of these metabolites are also potentially toxic to mammals, as they can cause inhibition of enzymes in key metabolic pathways, skin irritation, signalling and hormonal disruption, cytotoxicity, reproductive disorders, and neurological damage, or act as a tumour promotors. Also, they can influence CB physiology and their blooming capacity (Sukenik et al. 2002, Schatz et al. 2007). Bioactive substances produced by cyanobacteria can be divided in following groups a) aeruginosins and spumigins (Ersmark et al. 2008, Fewer et al. 2009); b) anabaenopeptins (Harada et al. 1993, Bubik et al. 2008); c) biogenic amines (MLA 2001, EFSA 2011); d) depsipeptides (Blom et al. 2006, Bubik et al. 2008); e) endocrine 102 Acta Biologica Slovenica, 60 (1), 2017 disruptors and novel tumour promoters (Blaha et al. 2010, Novakova et al. 2013); f) microginins (Neumann et al. 1997); and g) microviridins (Murakami et al. 1995). Most common bloom-forming cyanobacterial taxa of European freshwaters Occurrence and reported observations of the most common potentially toxic bloom-forming cyanobacteria in European freshwaters are shown in Tab. 1. Ecology of the most common cyanobacterial taxa occurring in European freshwaters is shown in Tab. 3. Control of cyanobacterial blooms Several approaches are available to control CBs in water bodies such as minimizing nutrient load, using chemical, biological, and/or physical treatment. Nutrient removal can have positive long-term effects leading to the reduction in the trophic state of the water body and thus to the reduction of CBs, but is almost impossible for most areas across the world due to economical limitations (Jancula and Marsalek 2011). CBs can be efficiently reduced by addition of chemicals to water such as copper-based algae-cides, herbicides, photosensitizers, and chemical flocculants (e.g. Surosz and Palinska, 2004; Jancula and Marsalek 2011). However, chemical treatment has several disadvantages: (1) toxicity against non-target organisms; (2) generation of secondary pollutants; (3) introduction of heavy metals to the water and their accumulation in the environment (Jancula et al. 2014). Chemical treatment of CBs especially using potassium permanganate or chlorine may indirectly effect other organisms due to the sudden release of cyanotoxins from cyanobacteria cells as a consequence of cell lysis (Mahvi and Dehgani 2005). Dissolved cyanotoxins can enter water supplies and pose potential risk for human health (van Apeldoorn 2007, Rajasekhar et al. 2012). In such cases, additional treatment of water by activated carbon, powerful oxidants such as ozone and/or intense ultraviolet light are needed to inactivate or degrade dissolved toxins (Chorus and Bartram 1999; Jancula and Marsalek 2011). Copper-based algaecides and chemical flocculants are commonly used to control CBs, but may be harmful to aquatic life by generating secondary pollutants (Mahvi and Dehgani 2005, McNeary and Erickson 2013, Jancula et al. 2014) and large amount of algae sludge (Xu et al. 2006). More sustainable treatment method is using clay minerals as flocculation agents, where dense clay particles attach to the cyanobacteria cells and promote conglomeration and sinking of the cells, despite their buoyancy (McNeary and Erickson 2013). Also, low concentrations of hydrogen peroxide (HP) have shown promising potential to act as specific cyanocide for Planktothrix agardhii, Anabaena, Aphanizomenon and Microcystis, both in the laboratory and in whole-lake treatments. HP acts very fast and there are no lasting chemical traces of the added HP (sustainability), nor toxic substances including released cyanotoxins or particulate organic matter from dead cyanobacteria retained in the water body (Matthijs et al. 2016). Biological removal of CBs such as natural grazing by phytoplanktivorous fish (Jancula et al. 2008) or biomanipulation by introduction of new cyanobacteria eating species to the water body (Lacerot et al. 2013) is gaining importance due to its environmental friendliness compared to chemical treatment. Biomanipulation is faster than natural establishment of cyanobacteria eating communities and can selectively affect only target organisms (Guo et al. 2015). Hydrodynamic and acoustic cavitation are the main physical methods for CBs control. Although the effects of acoustic cavitation on CBs removal have been studied more extensively compared with hydrodynamic cavitation (Dular et al. 2016), both techniques are still in the research phase. According to Xu et al. (2006) hydrodynamic cavitation is causing the collapse of gas vesicles and the destruction of thylakoid together with the changes in the structures of phycocyanins and chlorophyll a inM. aeruginosa cells, eventually resulting in the death of the cells. Wu et al. (2012b) studied combined effects of hydrodynamic cavitation and ozone treatment on growth of M. aeruginosa assuming that mechanical forces affect the cyanobacteria by damaging the cell wall and make them more sensitive to ozone treatment. 99% reduction of cyanobacteria was achieved Jarni et al.: Bloom-forming cyanobacteria of European freshwaters 17 Table 3: Ecology ofthe most common cyanobacterial taxa occurring in European freshwaters. Chl a = chlorophyll a, N = nitrogen, P = phosphorus, PAR = photosynthetically active radiation, CB = cyanobacteria. Tabela 3: Ekologija najpogostejših cianobakterijskih taksonov, ki se pojavljajo v evropskih celinskih vodah. Chl a = klorofil a, N = dušik, P = fosfor, PAR = fotosintetsko aktivno sevanje, CB = cianobakterije. Species Blooms typically found in Advantages Importance Sources Microcystis spp. • Warm, turbid, slow- • Less sensitive to • M. aeruginosa one of Prasath et al. 2014, moving waters, high in high light intensities the most damaging Chorus and Bartram nutrients (capable of buoyancy species. Prevalence in 1999, Lehman et al. • Waters deeper than 3 regulation) bodies with varying 2005, m, but also in shallower nutrient loading. High Vezie et al. 2002 lakes (temperate toxicity to aquatic and regions) terrestrial organisms. • Bodies with chl a • Rapid reproduction concentrations of triggered the most by 20-50 ^g/L and Secchi P runoff. transparency of 1-2 m • High nutrient levels • Spring and summer favour the growth of toxic over nontoxic strains. Planktothrix • Turbulent, low radiance 1 • Ability to absorb ' • One of the most Budzynska et al. 2009, agardhii waters sufficient energy common toxic bloom- Scheffer et al. 1997, • First few meters of from the entire PAR forming species. Oberhaus et al. 2007, the water column in spectrum Dokulil and Teubner shallow waters • Resistance to 2000, • Greatly dependent photoinhibition Padisak and Reynolds on high-frequency • Tolerant to continuous 1998, phosphate availability mixing of water Bonilla et al. 2012, • Summer (temperate column Catherine et al. 2008, regions) • High P storage capacity Crossetti and Bicudo • Buoyancy regulation 2008, • Tolerant to shade and Kokocinski et al. 2010, temperature variation Aubriot et al. 2011 Aphanizome-nonflos-aquae Higher latitudes (less frequent at lower latitudes) Grows independently of dissolved N resources and also under P limitation Late autumn and winter Low temperature preference Ability of autonomous fixation of atmospheric N Able to grow in unfavourable conditions (forms akinetes) Substantial storage capacity for P May appear as plankton in eutrophic waters where other CB are almost undetectable. Its dynamic affected by co-occurring CB like Microcystis spp. Tsujimura et al. 2001, Yamamoto 2009, Preussel et al. 2006, Takano and Hino 2000 Anabaena spp. • Lake environment • N fixing abilities • Widely diversified Zapomelova et al. • Toxin production group with around 80 2010, morphospecies. Dean et al. 2008, • Dominant long term Agnihotri 2014, populations Paerl 1979 • A. flos-aquae usually appears during summer (max N starvation and PAR inputs). 102 Acta Biologica Slovenica, 60 (1), 2017 compared to less than 15% removal of cyano-bacteria by hydrodynamic cavitation and 35% by ozone treatment alone. According to Jancula et al. (2014) and Dular et al. (2016) hydrodynamic cavitation is more effective on removal of buoyant cyanobacteria by disintegrating their gas vesicles than other planktonic algae without gas vesicles (e.g. green microalgae), which indicates good potential of hydrodynamic cavitation for selective cyanobacterial removal from water bodies. Acoustic cavitation has similar effects on cyanobacteria as hydrodynamic cavitation (Jancula et al. 2014, Li et al. 2014), namely reducing the growth rate of cyanobacteria by collapsing the gas vesicles, inhibiting cell division, and/or inflicting immediate damage on photosynthetic activities (Nakano et al. 2001, Ahn et al. 2003, Mahvi and Dehgani 2005, Zhang et al. 2006a). Acoustic cavitation is known to cause cell lysis and thus releasing the intracellular materials in water column (Zhang et al. 2006b, Rajasekhar et al. 2012). On the other side it is also effective in degrading the cyanotoxins (Song et al. 2005). Acoustic cavitation has potential to reduce algal capacity to float and thus reducing their concentration near the surface of water bodies, which is consequently inhibiting their growth and survival (Mahvi and Dehgani 2005). Effects of acoustic cavitation on cyanobacteria removal depends on frequency, intensity and time of sonication (Rajasekhar et al. 2012). Beside acoustic cavitation, a low intensity ultrasound without cavitation can be used for CBs control; in fact several such technologies are already available on the market. Low intensity ultrasound is appropriate solution for aquaculture systems, natural ponds or drinking water reservoirs, since it is not damaging the cyanobacteria cells and the toxins are not released from the cells (Krivograd Klemencic and Griessler Bulc 2010). Furthermore, it is affecting cyanobacteria selectively by collapsing gas vacuoles causing cyanobacteria cells to sink at the bottom of the water body, where cells in deep water bodies die due to lack of light necessary for photosynthesis (Krivograd Klemencic and Griessler Bulc 2010). The disadvantage of low intensity ultrasound technology is relatively long contact time with cyanobacteria to affect their buoyancy (Williams 2014). In 2014 the European Commission funded the research project (7FP Dronic, http://dronicproject. com) developing the monitoring and ultrasonic treatment robotic system that can localize and treat hotpots of CBs in large water bodies. Because of the direct and localized treatment, the Dronic system is environmentally friendly, with a minimal impact on the ecology of the water body. The Dronic system is equipped with ultrasound acoustic system that uses two different types of ultrasound. The first type precipitates the cyanobacteria by directly affecting their buoyancy, while the second type is neutralizing the cyanotoxins by cavitation. The Dronic system is still in the research phase. However, if it proves successful it will be the first system that can autonomously locate and localized treat CBs only at the part of the water body, which is experiencing CB. Povzetek Ob ugodni temperaturi, svetlobnih pogojih in zadostni količini hranil v površinskih vodah lahko pride do cvetenja cianobakterij, ki je pogost problem evtrofnih vodnih teles v Evropi in po svetu. Pričakovano je, da se bo s klimatskimi spremembami problem še poglobil, kar prestavlja nove izzive za upravljalce voda. Cianobakterije v splošnem cvetijo pozno poleti in zgodaj jeseni, ko so rekreacijske aktivnosti na vodnih telesih v porastu. Cvetenje cianobakterij poveča motnost vode, zavira rast makrofitov in vpliva na nevretenčarje in ribe v vodnem okolju. Poleg tega nekatere cianobakterijske vrste proizvajajo toksine, škodljive ljudem in živalim. Cvetenje toksičnih vrst cianobakterij pogosto povzroči zmanjšanje biodiverzitete, porušenje trofičnih verig in ravnotežja v ekosistemih. Mehanizem pojavljanja cianobakterijskega cvetenja je kompleksen, ker do tega pride ob hkratnem delovanju več dejavnikov. 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