Microorganisms Involved in Methylmercury Demethylation and Mercury Reduction are Widely Distributed and Active in the Bathypelagic Deep Ocean Waters

The ocean’s mercury (Hg) content has tripled due
to anthropogenic
activities, and although the dark ocean (>200 m) has become an
important
Hg reservoir, concentrations of the toxic and bioaccumulative methylmercury
(MeHg) are low and therefore very difficult to measure. As a consequence,
the current understanding of the Hg cycle in the deep ocean is severely
data-limited, and the factors controlling MeHg, as well as its transformation
rates, remain largely unknown. By analyzing 52 globally distributed
bathypelagic deep-ocean metagenomes and 26 new metatranscriptomes
from the Malaspina Expedition, our study reveals the widespread distribution
and expression of bacterial-coding genes merA and merB in the global bathypelagic ocean (∼4000 m depth).
These genes, associated with HgII reduction and MeHg demethylation,
respectively, are particularly prevalent within the particle-attached
fraction. Moreover, our results indicate that water mass age and the
organic matter composition shaped the structure of the communities
harboring merA and merB genes living
in different particle size fractions, their abundance, and their expression
levels. Members of the orders Corynebacteriales, Rhodobacterales, Alteromonadales, Oceanospirillales, Moraxellales, and Flavobacteriales were the main taxonomic players containing merA and merB genes in the deep ocean.
These findings, together with our previous results of pure culture
isolates of the deep bathypelagic ocean possessing the metabolic capacity
to degrade MeHg, indicated that both methylmercury demethylation and
HgII reduction likely occur in the global dark ocean, the
largest biome in the biosphere.