Microorganisms transform mercury (Hg) in ocean water to methylmercury (MeHg), which is very toxic. Because MeHg concentrates in living organisms, it eventually builds up to very dangerous concentrations in fish and higher animals in sea food webs, damaging both human and ecosystem health. However, we do not know exactly how the methylation occurs, which microorganisms do it and especially which fraction of Hg is available to them. We call this part the bioavailable fraction of Hg, it represents a small percentage of total Hg and its chemical identity is largely unknown. Bioavailable Hg in water can be elemental or ionic Hg species, associated with a great variety of other particles or dissolved organic in water column. Hg has also seven stable isotopes and their distribution in Hg species is very informative about the processes.
We will focus on this bioavailable fraction of Hg, the strength with which it is bonded, the processes that control its abundance and transformations, and the likelihood that Hg gets methylated. This will tell us in what conditions methylation happens so that we can in the future discover microbes that do it. To that end, we will be the first to measure Hg stable isotopes of bioavailable fraction, collected with Diffusive Gradient in Thin-films samplers onboard an automatic submarine in Adriatic Sea. With this extremely sensitive method we can describe conditions that support Hg methylation and predict MeHg stress in ecosystems.
Understanding how iHg bioavailability and MeHg toxicity are shifting with respect to ongoing ocean changes (Sonke and Heimbürger, 2012) facilitates responsible and effective governance of marineecosystems and mitigation of ecological impacts. Unfortunately, much empirical data is missing (Kritee et al., 2013, Blum et al., 2014). In the clear absence of a mechanistic and phylogenetic understanding of water column methylation, new methodological approach is needed to address bioavailable speciation at biological interfaces in the environment. Our work will provide foundation needed to begin understanding aerobic microbial methylation.
This project is divided in two parts; (1) validation of new methods (WPs 1 and 2) and (2) field application (WPs 3 and 4). WPs 5 and 6 relate to data delivery and dissemination. Our approach will integrate DGTs and Hg stable isotopes in an unprecedented and innovative way to provide a superior alternative to current methods. Validation of potential isotopic fractionation during DGT diffusion may be an analytical challenge, but we have versatile analytical infrastructure at our disposal and the result is well worth the effort. Simultaneously, DGT diffusive layer will be fundamentally augmented with reactive properties that significantly digress from its conventional use, thus becoming unique investigation technique into bioavailability of various complexes using competitive ligand exchange principle. The new method will become a powerful high-quality tool for in situ investigation of bioavailable Hg fractions and transformations, because it avoids analytical caveats and because in vitro experiments provide mechanistic explanations for field observations.
Our hypothesis is that biogeochemical transformations of bioavailable fraction of iHg and MeHg leave a clear spatiotemporal signal in their stable isotopic composition. Simultaneously and because passive diffusion of DGT resembles microbial passive uptake of neutral species in aerobic conditions, these results can further constrain ligand and speciation controls over bioavailable fraction. Together, niche-specific environmental properties of methylation can be defined; leading ultimately to the identification of water-borne methylating microorganisms.
Two main project goals are (1) to describe biogeotransformations acting upon Hg bioavailable fraction using stable isotopes; and (2) to characterize likely methylation donors and methylatable iHg complexes in situ. We will achieve that through (1) validation of DGT-stable isotope technique; (2) DGT diffusive layer augmentation; and (3) in situ deployments. Ultimately, joined results +will be integrated in 3-D maps representing distribution of methylatable iHg complexes, bioavailable species and stable isotope signals of both. This will be a completely new perspective in Hg biogeochemistry research.