Prevalence of Heterotrophic Methylmercury Detoxifying Bacteria across Oceanic Regions

Microbial reduction of inorganic
divalent mercury (Hg2+) and methylmercury (MeHg) demethylation
is performed by the mer operon, specifically by merA and merB genes, respectively, but
little is known about the
mercury tolerance capacity of marine microorganisms and its prevalence
in the ocean. Here, combining culture-dependent analyses with metagenomic
and metatranscriptomic data, we show that marine bacteria that encode mer genes are widespread and active in the global ocean.
We explored the distribution of these genes in 290 marine heterotrophic
bacteria (Alteromonas and Marinobacter spp.) isolated from different oceanographic
regions and depths, and assessed their tolerance to diverse concentrations
of Hg2+ and MeHg. In particular, the Alteromonas sp. ISS312 strain presented the highest tolerance capacity and a
degradation efficiency for MeHg of 98.2% in 24 h. Fragment recruitment
analyses of Alteromonas sp. genomes
(ISS312 strain and its associated reconstructed metagenome assembled
genome MAG-0289) against microbial bathypelagic metagenomes confirm
their prevalence in the deep ocean. Moreover, we retrieved 54 merA and 6 merB genes variants related
to the Alteromonas sp. ISS312 strain
from global metagenomes and metatranscriptomes from Tara Oceans. Our findings highlight the biological reductive MeHg degradation
as a relevant pathway of the ocean Hg biogeochemical cycle.