Prevalence of Heterotrophic Methylmercury Detoxifying Bacteria across Oceanic Regions

Abstract

Microbial reduction of inorganic divalent mercury (Hg²⁺) and methylmercury (MeHg) demethylation is performed by the <i>mer</i> operon, specifically by <i>merA</i> and <i>merB</i> 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 <i>mer</i> genes are widespread and active in the global ocean. We explored the distribution of these genes in 290 marine heterotrophic bacteria (<i>Alteromonas</i> and <i>Marinobacter</i> spp.) isolated from different oceanographic regions and depths, and assessed their tolerance to diverse concentrations of Hg²⁺ and MeHg. In particular, the <i>Alteromonas</i> sp. ISS312 strain presented the highest tolerance capacity and a degradation efficiency for MeHg of 98.2% in 24 h. Fragment recruitment analyses of <i>Alteromonas</i> 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 <i>merA</i> and 6 <i>merB</i> genes variants related to the <i>Alteromonas</i> sp. ISS312 strain from global metagenomes and metatranscriptomes from <i>Tara</i> Oceans. Our findings highlight the biological reductive MeHg degradation as a relevant pathway of the ocean Hg biogeochemical cycle.