Redox gradient shapes the abundance and diversity of mercury-methylating microorganisms along the water column of the Black Sea
Authors: Cabrol Léa , Capo Eric , van Vliet Daan M. , von Meijenfeldt F. A. Bastiaan , Bertilsson Stefan , Villanueva Laura , Sánchez-Andrea Irene , Björn Erik , G. Bravo Andrea , Heimburger Boavida Lars-Eric 
Published: August 2023

In the global context of seawater deoxygenation triggered by climate change and anthropogenic activities, changes in redox gradients impacting biogeochemical transformations of pollutants, such as mercury, become more likely. Being the largest anoxic basin worldwide, with high concentrations of the potent neurotoxic methylmercury (MeHg), the Black Sea is an ideal natural laboratory to provide new insights about the link between dissolved oxygen concentration and
hgcAB
gene-carrying (
hgc
+
) microorganisms involved in the formation of MeHg. We combined geochemical and microbial approaches to assess the effect of vertical redox gradients on abundance, diversity, and metabolic potential of
hgc
+
microorganisms in the Black Sea water column. The abundance of
hgcA
genes [congruently estimated by quantitative PCR (qPCR) and metagenomics] correlated with MeHg concentration, both maximal in the upper part of the anoxic water. Besides the predominant
Desulfobacterales
,
hgc
+
microorganisms belonged to a unique assemblage of diverse—previously underappreciated—anaerobic fermenters from
Anaerolineales
,
Phycisphaerae
(characteristic of the anoxic and sulfidic zone),
Kiritimatiellales,
and
Bacteroidales
(characteristic of the suboxic zone). The metabolic versatility of
Desulfobacterota
differed from strict sulfate reduction in the anoxic water to reduction of various electron acceptors in the suboxic water. Linking microbial activity and contaminant concentration in environmental studies is rare due to the complexity of biological pathways. In this study, we disentangle the role of oxygen in shaping the distribution of Hg-methylating microorganisms consistently with MeHg concentration, and we highlight their taxonomic and metabolic niche partitioning across redox gradients, improving the prediction of the response of marine communities to the expansion of oxygen-deficient zones.

IMPORTANCE

Methylmercury (MeHg) is a neurotoxin detected at high concentrations in certain marine ecosystems, posing a threat to human health. MeHg production is mainly mediated by
hgcAB
gene-carrying (
hgc
+
) microorganisms. Oxygen is one of the main factors controlling Hg methylation; however, its effect on the diversity and ecology of
hgc
+
microorganisms remains unknown. Under the current context of seawater deoxygenation, mercury cycling is expected to be disturbed. Here, we show the strong effect of oxygen gradients on the distribution of potential Hg methylators. In addition, we show for the first time the significant contribution of a unique assemblage of potential fermenters from
Anaerolineales
,
Phycisphaerae
, and
Kiritimatiellales
to Hg methylation, stratified in different redox niches along the Black Sea gradient. Our results considerably expand the known taxonomic diversity and ecological niches prone to the formation of MeHg and contribute to better apprehend the consequences of oxygen depletion in seawater.