Recent advances in microbial mercury methylation: A review on methylation habitat, methylator, mechanism, and influencing factor

Mercury (Hg) is a global pollutant that affects human and ecosystem health. We synthesize understanding of sources, atmosphere-land-ocean Hg dynamics and health effects, and consider the implications of Hg-control policies. Primary anthropogenic Hg emissions greatly exceed natural geogenic sources, resulting in increases in Hg reservoirs and subsequent secondary Hg emissions that facilitate its global distribution. The ultimate fate of emitted Hg is primarily recalcitrant soil pools and deep ocean waters and sediments. Transfers of Hg emissions to largely unavailable reservoirs occur over the time scale of centuries, and are primarily mediated through atmospheric exchanges of wet/dry deposition and evasion from vegetation, soil organic matter and ocean surfaces. A key link between inorganic Hg inputs and exposure of humans and wildlife is the net production of methylmercury, which occurs mainly in reducing zones in freshwater, terrestrial, and coastal environments, and the subsurface ocean. Elevated human exposure to methylmercury primarily results from consumption of estuarine and marine fish. Developing fetuses are most at risk from this neurotoxin but health effects of highly exposed populations and wildlife are also a concern. Integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.

During attempts to obtain novel, human-associated species of the domain Archaea, a coccoid micro-organism, designated strain B10(T), was isolated in pure culture from a sample of human faeces collected in Marseille, France. On the basis of its phenotypic characteristics and 16S rRNA and mcrA gene sequences, the novel strain was classified as a methanogenic archaeon. Cells of the strain were non-motile, Gram-staining-positive cocci that were approximately 850 nm in diameter and showed autofluorescence at 420 nm. Cells were lysed by 0.1% (w/v) SDS. With hydrogen as the electron donor, strain B10(T) produced methane by reducing methanol. The novel strain was unable to produce methane when hydrogen or methanol was the sole energy source. In an atmosphere containing CO(2), strain B10(T) could not produce methane from formate, acetate, trimethylamine, 2-butanol, 2-propanol, cyclopentanol, 2-pentanol, ethanol, 1-propanol or 2,3-butanediol. Strain B10(T) grew optimally with 0.5-1.0% (w/v) NaCl, at pH 7.6 and at 37 °C. It required tungstate-selenite for growth. The complete genome of the novel strain was sequenced; the size of the genome was estimated to be 2.05 Mb and the genomic DNA G+C content was 59.93 mol%. In phylogenetic analyses based on 16S rRNA gene sequences, the highest sequence similarities (98.0-98.7%) were seen between strain B10(T) and several uncultured, methanogenic Archaea that had been collected from the digestive tracts of a cockroach, a chicken and mammals. In the same analysis, the non-methanogenic 'Candidatus Aciduliprofundum boonei' DSM 19572 was identified as the cultured micro-organism that was most closely related to strain B10(T) (83.0% 16S rRNA gene sequence similarity). Each of the three treeing algorithms used in the analysis of 16S rRNA gene sequences indicated that strain B10(T) belongs to a novel order that is distinct from the Thermoplasmatales. The novel strain also appeared to be distinct from Methanosphaera stadtmanae DSM 3091(T) (72.9% 16S rRNA gene sequence similarity), another methanogenic archaeon that was isolated from human faeces and can use methanol in the presence of hydrogen. Based on the genetic and phenotypic evidence, strain B10(T) represents a novel species of a new genus for which the name Methanomassiliicoccus luminyensis gen. nov., sp. nov. is proposed. The type strain of the type species is B10(T) ( = DSM 24529(T) = CSUR P135(T)).