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 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.

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

“…Sphingobacteriales (Bacteroidetes) and Clostridiales (Firmicutes) were only detected in the merB sequence variants suggesting that some prokaryotes could potentially present only the merB gene as recently reported . Like for merA genes, all the merB -containing taxonomic groups detected in our study were previously described as merB -coding bacteria, ,, and Alteromonadales include members that have been experimentally proven in the laboratory to be able to degrade MeHg at reasonably high concentrations . Once again, the absence of archaeal sequences was observed among the merB genes, and similar factors explained for the merA gene detection may account for the absence of merB sequences.…”

“…However, a recent study reported that merA and merB of Alteromonas and Marinobacter spp. bacterial isolates, capable of demethylating MeHg and reducing Hg II forms, were widespread in surface, the deep chlorophyll maximum, and mesopelagic waters of different oceanographic regions, 47 suggesting the relevance of these processes, although rarely reported. Nevertheless, the global diversity, biogeography, activity, and especially the environmental drivers of the microorganisms involved in Hg II reduction and MeHg demethylation in the bathypelagic realm remain largely unknown.…”

Unravelling the Portuguese Coastal and Transitional Waters’ Microbial Resistome as a Biomarker of Differential Anthropogenic Impact

“…Another highly cosmopolitan sequence in terms of carrier genus is the well-known alkylmercury lyase sequence, also known as the mer B gene, which encodes an enzyme that degrades the highly toxic methyl-Hg form into lesser toxic ones. A recent study shows that mercury resistance genes are widely distributed in the marine realm [ 58 ]. Nevertheless, it is also known that its abundance responds to pollution gradients [ 59 ] and, therefore, considering its ubiquity, this resistome sequence can also provide a good candidate biomarker.…”

Biomineralization of struvite induced by indigenous marine bacteria of the genus Alteromonas

“…From a different perspective, marine microorganisms possess unique properties owing to the need to adapt to disadvantageous environmental conditions caused by ions, such as alkaline and acidic water, and high osmotic stress. For example, microorganisms can change the valence, existence form, and solubility of the ion (Beam et al, 2018;Vigliaturo et al, 2020;Sanz-Saez et al, 2022;van de Velde et al, 2022).…”