Using enriched stable isotopes to elucidate marine phytoplankton cellular uptake mechanisms and bioavailability of MMHg bound to different natural DOM composition

Understanding the mechanisms governing mercury (Hg) bioavailability to marine phytoplankton is crucial for evaluating its ecological and biogeochemical impacts. Dissolved organic matter (DOM) plays a central role in modulating the mobility, complexation, and bioavailability of monomethylmercury (MMHg), a highly toxic Hg species. This study investigates how the origin of DOM (terrestrial, marine, and phytoplankton-derived exudates) affects MMHg complexation and cellular uptake by marine phytoplankton under controlled laboratory conditions.

Using a single phytoplankton culture and enriched stable Hg isotopes, we simultaneously tracked the uptake dynamics of MMHg complexed with different DOM sources as well as uncomplexed MMHg ions. Our results revealed that total MMHg uptake decreased with increasing dissolved organic carbon (DOC) concentrations, regardless of DOM origin. Notably, no significant differences were observed in MMHg uptake dynamics when comparing DOM of different origins at similar MMHg concentrations. However, we observed increased internalization of MMHg when complexed with both terrestrial and marine DOM. Specifically, organic complexation with DOM from both terrestrial and marine sources reduced MMHg's ability to bind to ligands on the algal surface (phycosphere) but did not hinder its passage across the cell membrane, ultimately enhancing its bioavailability.

These findings offer novel insights into the interaction between DOM composition and MMHg bioavailability, advancing our understanding of mercury dynamics in marine ecosystems. The study underscores the critical role of DOM characteristics in influencing MMHg bioaccumulation and provides valuable data for refining models of mercury cycling in aquatic environments.