Methylmercury concentrations of up to a few hundred femtomoles have been measured in the oxic water column of various ocean basins [1-15]. Maximum concentrations are found in the oxygen minimum zones. Oxygen is consumed here by bacteria during the remineralization of sinking organic matter. Yet only anaerobic bacteria and archaea have been shown to be capable of methylating mercury [16].
Refs:
1. Mason, R.P. and W.F. Fitzgerald, Alkylmercury species in the Equatorial Pacific. Nature, 1990. 347(6292): p. 457-459.
2. Mason, R.P. and W.F. Fitzgerald, Mercury speciation in open ocean waters. Water Air & Soil Pollution, 1991. 56(1): p. 779-789.
3. Mason, R.P. and W.F. Fitzgerald, The distribution and biogeochemical cycling of mercury in the Equatorial Pacific-Ocean. Deep-Sea Research Part I-Oceanographic Research Papers, 1993. 40(9): p. 1897-1924.
4. Cossa, D., J.M. Martin, K. Takayanagi, and J. Sanjuan, The distribution and cycling of mercury species in the western Mediterranean. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 1997. 44(3-4): p. 721-740.
5. Mason, R.P., K.R. Rolfhus, and W.F. Fitzgerald, Mercury in the North Atlantic. Marine Chemistry, 1998. 61(1–2): p. 37-53.
6. Mason, R.P. and K.A. Sullivan, The distribution and speciation of mercury in the South and equatorial Atlantic. Deep Sea Research Part II: Topical Studies in Oceanography, 1999. 46(5): p. 937-956.
7. Monperrus, M., E. Tessier, D. Amouroux, A. Leynaert, P. Huonnic, and O.F.X. Donard, Mercury methylation, demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea. Marine Chemistry, 2007. 107(1): p. 49-63.
8. Cossa, D., B. Averty, and N. Pirrone, The origin of methylmercury in open Mediterranean waters. Limnology and Oceanography, 2009. 54(3): p. 837-844.
9. Sunderland, E.M., D.P. Krabbenhoft, J.W. Moreau, S.A. Strode, and W.M. Landing, Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models. Global Biogeochemical Cycles, 2009. 23: p. 14.
10. Heimbürger, L.E., D. Cossa, J.-C. Marty, C. Migon, B. Averty, A. Dufour, and J. Ras, Methyl mercury distributions in relation to the presence of nano- and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean). Geochimica Et Cosmochimica Acta, 2010. 74(19): p. 5549-5559.
11. Cossa, D., L.E. Heimbürger, D. Lannuzel, S.R. Rintoul, E.C.V. Butler, A.R. Bowie, B. Averty, R.J. Watson, and T. Remenyi, Mercury in the Southern Ocean. Geochimica Et Cosmochimica Acta, 2011. 75(14): p. 4037-4052.
12. Lehnherr, I., V.L. St. Louis, H. Hintelmann, and J.L. Kirk, Methylation of inorganic mercury in polar marine waters. Nature Geosci, 2011. 4(5): p. 298-302.
13. Cossa, D., M. Harmelin-Vivien, C. Mellon-Duval, V. Loizeau, B. Averty, S. Crochet, L. Chou, and J.F. Cadiou, Influences of Bioavailability, Trophic Position, and Growth on Methylmercury in Hakes (Merluccius merluccius) from Northwestern Mediterranean and Northeastern Atlantic. Environmental Science & Technology, 2012. 46(9): p. 4885-4893.
14. Hammerschmidt, C.R. and K.L. Bowman, Vertical methylmercury distribution in the subtropical North Pacific Ocean. Marine Chemistry, 2012. 132–133(0): p. 77-82.
15. Wang, F., R.W. Macdonald, D.A. Armstrong, and G.A. Stern, Total and Methylated Mercury in the Beaufort Sea: The Role of Local and Recent Organic Remineralization. Environmental Science & Technology, 2012.
16. Parks, J.M., A. Johs, M. Podar, R. Bridou, R.A. Hurt, S.D. Smith, S.J. Tomanicek, Y. Qian, S.D. Brown, C.C. Brandt, A.V. Palumbo, J.C. Smith, J.D. Wall, D.A. Elias, and L. Liang, The Genetic Basis for Bacterial Mercury Methylation. Science, 2013.
Researchers from the University of Alberta reported, based on an investigation across the Arctic Ocean, that relatively harmless inorganic mercury, released from human activities (like coal burning) undergoes methylation and becomes deadly monomethylmercury. They believe that the methylation process happens in oceans all over the world and that the conversion is carried out by microbial life forms in the ocean.
Here in the link:
http://www.nature.com/ngeo/journal/v4/n5/full/ngeo1134.html
It can be abiotic methylation promoted by light. And biotic methylation promoted by phytoplankton and planktonic bacteria. Sulfate-reducing bacteria can cope with oxygen to a certain point. They can also form aggregates with organic matter and protect themselves from high oxygen concentrations inside it.
At least sulfate-respiring bacteria may be involved under certain nearly oxic conditions in sediments, viz. when pathways alternative to sulfate respiration are preferred at elevated (positive) redox potential values (Piker & Reichardt, 1996. Proc. 13 th Symp. Baltic Marine Biologists, p.235-240).
Besides, it should also be kept in mind that sulfate reducing bacteria are even capable of aerobic respiration (Dilling & Cypionka, 1990. FEMS Microbiol. Lett. 71:123). Hence, in principle, oxic environments would not exclude metabolic activities of sulfate-reducing bacteria that are linked to methylation of mercury.
88: 55-60,1992 MARINE ECOLOGY PROGRESS SERIES Published October 20 Mar. Ecol. Prog. Ser.
Strictly aerobic and anaerobic bacteria associated with sinking particulate matter and zooplankton fecal pellets
Micheline Bianchil, et al.
anaerobic microhabitats are everywhere!
Anaerobic chemosynthetic bacteria and photo-catalyzed chemical reactions by aerobic bacteria have been implicated.
Methylmercury can readily be released to the ocean due to anthropogenic activities. However, some group SRB and anaerobic bacteria are responsible for the conversion of inorganic form to organic form irrespective of saline environment or terrestrial habitats. As Hg is having a high residence time, where ever it may form it can be transported a much longer distance and oceanic environment is the greatest example of it. For more details you can refer to http://www.sciencedirect.com/science/article/pii/S0964830512002739
That's a great question. So far we have the following hypotheses:
1. photochemically-driven abiotic methylation
2. methylation in anoxic microenvironments
3. methylation by phyto/bacterio plankton
4. methylation by anaerobic bacteria tolerant of oxic conditions
I think that we can probably rule out #1, as a number of papers (e.g., Black et al.) have shown that photochemical processes result in net demethylation and not net methylation. I think that #2 is a sound hypothesis...for example, particles can provide all the right conditions to promote Hg(II) methylation, including anoxic microhabitats, a source of Hg(II) and a source of organic C to fuel microbial metabolism. Someone above ruled out #3, but I would be curious to find out why. It has been previously suggested that Hg(II) methylation could be linked to the sulfur cycle and plankton-produced DMS, as there are a number of methyl transfer reactions linking DMSP, DMS and MeSH which could potentially result in Hg(II) becoming methylated (see for example, Larose et al 2010). I don't know enough about #4 to comment, but I would think that methylation of Hg(II) in marine waters is probably the result of a combination of hypotheses #2 and #3, although I would love to hear more people's opinions on this topic.
A humble argument from a student of chemistry where no excellence is proclaimed:
Chemically, the reactive form of Hg in auatic environments is Hg(II)[ Already very well established].
Methylation means the addition of methyl carbanion [ CH3(-)] so as to form [Hg(CH3)]^+.
In the aqutic conditions, SULPHATE REDUCING BATERIA have been identified as the most important Hg- METHYLATING ORGANISMS.
The COBALMIN BEING THE PRINCIPAL SOURCE OF METHYYL GROUP.
But Pls. let me continue with the argument.
It is also reported that all not all sulphate reducing bacteria methylate mercury. Studies show that more methyl mercury is produced when these bateria are growing fermentively than when they are reducing sulphate.
HOWEVER, SULPHATE ADDITION IS ALSO KNOWN TO STIMULATE METHYLATION UNDER LIMITING CONDITIONS.
So there can be a possibility that:
[I] At low sulphate, methylation is stimulated by sulphate addition as this enhances the activity of sulphate reducers but
[II] At high SULPHIDE concentrations, Hg bioavailability becomes limited by SULPHIDE COMPLEXATION which decreases the methylation rate. In many culture experiments, HgS precipitation occurs when the concentration of Hg is quite high.
A lack of genes encoding Hg methylation and anaerobic bacteria in OMZ suggest that Hg methylation in OMZ is dependent on mechanisms that differ from those in anoxic freshwater.If there are labile Hg(II) species in OMZ, Hg(II) may be methylated by extracellular methyl donors, e.g. methyl cobalamin.
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