Mercury methylation in oxygen deficient zones of the oceans: No evidence for the predominance of anaerobes

The source of the methylmercury (MeHg) that accumulates in pelagic fish is unknown. Since it is known that methylmercury is produced by anaerobic bacteria in freshwaters, we examined if this might also be the case in the open ocean and if oxygen deficient zones (ODZ) might be important sources of MeHg. Incubations carried out in the ODZs of the Equatorial Pacific with amendments to favor the growth of anaerobes showed modest and variable extent of Hg methylation. Although methylation appeared to be stimulated by the addition of organic carbon, it was not correlated with the redox state of the samples. 16S rRNA analysis of the microbial community in the incubations did not detect the presence of sulfate or iron-reducing bacteria, two groups known to methylate Hg. Laboratory experiments with pure cultures of a variety of marine bacteria, including representatives of taxa found to be abundant in the incubations, did not show evidence of Hg methylation, except for a sulfate reducer isolated from a hydrothermal vent. An additional cruise revealed low concentrations of methylmercury (up to 0.05 pM) in the surface layer and ODZ of the Arabian Sea, with no apparent pattern or relation to dissolved oxygen. We conclude that anaerobic bacteria in ODZs are not important sources of MeHg to the open ocean. This is in accord with recent results showing a correlation between MeHg and the decomposition of organic matter in the oxic water column throughout the subsurface.