Aerobic and Anaerobic Bacterial Mercury Uptake is Driven by Algal Organic Matter Composition and Molecular Weight

The biological mobilization of mercury (Hg) into microbes capable of Hg methylation is one of the limiting steps in the formation of the neurotoxin methylmercury (MeHg). Although algal dissolved organic matter (DOM) has been associated with increased MeHg production, the relationship between bacterial Hg uptake and algal DOM remains unexplored. In this study, we aimed to address how the quantity and quality of DOM, freshly harvested from several algae, affected the bacterial uptake of Hg with the use of a biosensor capable of functioning both aerobically and anaerobically. We combined biosensor measurements with high-resolution mass spectrometry and field-flow fractionation to elucidate how DOM composition and molecular weight influenced microbial Hg uptake. We showed that freshly harvested DOM from Chlorophyte and Euglena mutabilis strongly inhibited aerobic and anaerobic Hg uptake, whereas DOM harvested from Euglena gracilis did not exhibit this same pronounced effect. Once fractionated, we found that amino acids and polyamines, most abundant in Euglena gracilis DOM, were positively correlated to increase Hg uptake, suggesting that these molecules are potentially underappreciated ligands affecting Hg bioavailability. As water quality is affected by eutrophication, algal community assemblages will change, leading to variations in the nature of autochthonous DOM released in aquatic systems. Our results highlight that variations in the emergent properties of DOM originating from varying algal species can have a profound effect on bacterial Hg uptake and thus methylation.