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The fate of Hg in terrestrial isopod Porcellio scaber and its environment
Nataša Nolde1, Vesna Jereb1, Damjana Drobne2, Milena Horvat1
'Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana,
Slovenia; E-mail: natasa.nolde@ijs.si PDepartment of Biology, University of Ljubljana, Večna pot 111, 1000 Ljubljana, Slovenija
Abstract: In our work reduction and methylation of inorganic mercury in the Porcellio scaber (Isopoda, Crustacea) and its environment were studied using radiotracer 203Hg. Total mercury (T203Hg) and monomethylmercury (Me203Hg) in the whole animals, gut, digestive glands (hepatopancreas), food (hazelnut leaves) and excrement were measured in order to: (I) to obtain the distribution of T203Hg and Me203Hg in animals, (2) to investigate the origin of Me203Hg and the site of its accumulation, and finaly, (3) to assess the mass balance of mercury in our experimental system. After two weeks of the experiment majority of mercury in animals and their environment remained as inorganic 203Hg2+. The net formation of elemental mercury (203Hg°j and Me203Hg was detected at much lower concentrations as 203Hg2+. Approximately 3 % of consumed mercury was assimilated by the animals and the majority of Hg was excreted by feaces. Approximately 20 % of T203Hg was detected in hepatopancreas, SS % in gut and 2S % in residue of animal. About 2S % of Me203Hg was found in hepatopancreas, IS % in gut and 6S % in animal residue. Concentrations of Me203Hg were higher on leaves and in faeces compared to the animals. Also, the amounts of Me203Hg found in animals were lower than expected. This suggests that demethylation of Me203Hg could prevail over mercury methylation in the digestive system of the animal.
Key words: Mercury transformations, reduction, methylation, radiotracer 203Hg, Porcellio scaber
Introduction
In the framework of studies on mercury biogeochemistry in contaminated and polluted sites due to past mercury mining in Slovenia a study on the uptake, distribution and transformation of mercury in terrestrial isopod Porcellio scaber (Isopoda, Crustacea) was initiated. In our work reduction and methy-lation of inorganic mercury in the Porcellio scaber and its environment was followed. For this purpose an experimental set up was build (Figure I) where Hg uptake, distribu-
tion, retention and transformation was followed using a 203Hg tracer.
During experiment daily reduction of 203Hg2+ to 203Hg° was measured. Elemental mercury was trapped on activated carbon traps and the radiotracer 203Hg°was detected by gamma counting. In animals, animal organs, food (hazelnut leaves) and excrement 203Hg2+ and Me203Hg were measured. For Me203Hg and 203Hg2+ determination, a radiochemical method with specific separation of 203Hg2+-dithizonate and Me2H3Hg - dithizonate by thin
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layer chromatography described by Jereb et al.[1], and gamma counting was used.
Results and discussion
An the end of experiment majority of mercury in animals, their food and feaces remained as 203Hg2+. 203Hg° and Me203Hg were detected at much lower amounts as 203Hg2+. Approximately 0.60 - 1.88 % of 203Hg2+ added to the system reduced to 203Hg° (Table 1). Results from daily reduction of 203Hg2+ to 203Hg° from the experiment with animals showed that the reduction in the vessels containing leaves with 5 p,g 203Hg2+/g dry weight was higher than reduction on leaves with 0,5 ^g 203Hg2+/g of 203Hg2+ (Figure 2), indicating that 203Hg° formed as a function of initial concentration of 203Hg2+. Similar results were reported by Ludwicki[2].No differences in reduction of inorganic mercury between vessels containing animals and control vessels without animals were observed in 18 days, indicating that reduction of
Airflow direction
—Activated charcoal
Coarse plastic net
Figure I. Experimental set up RMZ-M&G 2004, SI
203Hg2+due to bacteria in digestive system of animals (Porcellio scaber) was negligible and mercury was most probably reduced due to humidity in the experimental set up and microorganisms on the leaves and feaces.
9 10 11 12 13 14 15 16 17 18 days
Figure 2. Daily reduction of P
l3Hg° under
normal conditions, where animals were present. Concentrations of 203Hg2+ were in two vessels 0,S |ig203Hg2+/ g dry weight of leaf and in other two vessels 5 ^g203Hg2+/g dry weight of leaf. In the vessels marked with 0,5bl and 5bl were no animals and were used as controls.
About 3 % of consumed T203Hg was assimilated by the animals, majority (60 to 100 %) of Hg was excreted by feaces (Table 1). The assimilated mercury was distributed in the animal as follows: hepatopancreas 12.2 to
36.8	%, the gut about 27.0 to 75.0 % and in the residue of the animals about 6.5 to
39.9	% of T203Hg. For MeHg the following distribution was abserved: 25 % of Me203Hg was detected in hepatopancreas, 15 % in gut and 65 % in residue (Figure 3). The percentages of Me203Hg compared to T203Hg in organs were very low, the highest, about 5 %, in animals residue (Figure 4). The results showed that the animal accumulated Me203Hg in some selective parts, which is in agreement with the data from the literature where accumulation of MeHg in nevtral nerve cord and gills in grass shrimp (Palaemonetes pugio)[6] was found.
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Table I. Mass balance of T203Hg in ng. O - amount of T203Hg (ng) offered on a food, C - amount of consumed T203Hg (ng) by animals, U - amount of unconsumed T203Hg (ng), F - amount of T203Hg in the feaces, % F - % of feaces production with regard to consumed food , A - amount of T203Hg assimilation from consumed food, % A - % of T203Hg assimilated from consumed food, R - amount of reduced 203Hg2+ to 203Hg0, % R...% of 203Hg0 in the system. Factor (F+A)/C indicates mass balance of T203Hg in animal. Factor (U+F+A+R)/O indicates mass balance of T203Hg in experimental system.
Cone. Hg in leaves (ng/g)	O	C	U	F	% F	A	% A	R 203Hg°	% R	(F+A) /C	(U+F+A +R)/0
0,5	153	103	50	97	94.2	2.5	2.43	0.92	0.60	0.97	0.98
0,5	152	66	86	66	100	2.31	3.50	1.22	0.80	1.04	1.02
5	1524	432	1092	248	57.4	13.3	3.08	28.6	1.88	0.60	0.91
5	1514	496	1018	311	62.7	15.6	3.14	13.6	0.90	0.66	0.90
80 -,
70
60
50
40
o> 30 20 ^ 10 0
-THg	_	-
AMeHg	■	A
		
_		
		
S	—	|
■	A	
—		
hep	gut	residue
Figure 3. Distribution of T203Hg and Me203Hg in the animals exposed to S |ig203Hg2+/g of dry weight of leaf.
10000.
s 1000
o>
o> 100 X
O) 10
o
AMeHg		A ■	h "
=		A	
	—	k A	A -
i A	A	A	A ""
18 16 14 12 10
8 I
R I
6 O
4 S
2 0
cd
O)
hep gut residue leaves feaces
Figure S. T203Hg and Me203Hg concentrations in composite samples of animal organs (nl=8, n2=9) and individual samples of leaves (n=6) and excrement (n=4) after acid digestion and extraction in Dz-tl. Anials were exposed to 5^g203Hg2+/g dry weight of leaf. For calculations of T203Hg and Me203Hg concentrations in organs liophylized mass l,23 mg for hepatopancreas, 0,98 mg for gut and 12 mg for residue were used.
Figure 4. % Me203Hg regarding T203Hg calculated from TLC (Thin Layer Chromatography) in different organs of animals, feaces and food (hazelnut leaves).
Precise values of T203Hg and Me203Hg concentrations in hepatopancreas, gut and residue are shown in Figure 5. Generaly, concentrations of Me2H3Hg in organs were very low, compared to T2H3Hg concentrations.
Concentrations of Me2H3Hg were higher in leaves and feaces compared to the animals (Figure 5), which shaws that the methyla-tion already occurs on hazelnut leaves. With regard to quantity of Me2H3Hg in consumed food and knowing that greater part (95 %[5], 70-80 %[«]) of consumed MeHg should be absorbed in the gut of the animal, our experimental animals assimilated relatively low amounts of Me2H3Hg (app. 4 %). This sug-
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gested that demethylation of Me203Hg could prevail over mercury methylation in the digestive system of the animal, leading to increased excretion of ingested mercury.
Loss of T203Hg from mass balance of system and animal were detected by higher concentrations of 203Hg2+on leaves (Table I). Some additional experiment (data not shown), where Hg reduction was measured immediately after 203Hg2+was aplied on leaves has shown that about 80 % of total reduced Hg formed during the first day of experiment. Therefore, most probably loss of Hg occured during leaves preparation (radiotracer aplication, dryig overnight), due to micro-bial reduction of inorganic mercury.
Conclusions
Our results have shown that 203Hg° formed as a function of initial concentration of 203Hg2+, and that reduction of 203Hg2+due to bacteria in digestive system of Porcellio scaber was negligible compared to reduction in its environment. Based on consumed Me203Hg, relatively low amounts of Me203Hg in animals were found, therefore demethylation of Me203Hg could prevail over mercury methylation in the digestive system of the animal. Further experiments will therefore be needed with 14CH3Hg in order to verify this hypothesis. In the future, transformation processes of mercury in feaces should be addressed as part of the experiment.
Acknowledgements
This work was supported by the Slovenian Ministry of Education, Science and Sport.
References
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[2]	Ludwicki, J. K. (1990): In vitro Methylation and
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[3]	Drobne, D., Hopkin, S. P. (1995): The toxicity of
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