Characterizing Ag uptake and storage in the marine diatom Thalassiosira pseudonana: Implications for Ag biogeochemical cycling

Silver is a non-nutrient element that is readily taken up by diatoms. It exists in the ocean in pM concentrations and is known to accumulate in marine sediments underlying surface waters that support diatom blooms. Laboratory studies and open-ocean water column profiles of Ag versus Si have led to the hypothesis that diatoms accumulate Ag within their silica frustules, subsequently delivering Ag to underlying sediments as the organisms die and sink to the seafloor. Through this delivery mechanism, sedimentary Ag concentrations might serve as a useful record of past export production. However, the actual partitioning of Ag between diatom organic and frustule parts is unknown, and as such represents a considerable source of uncertainty in the development of Ag as a proxy for diatom paleoproductivity and in fully describing Ag biogeochemical cycling. In this study, we use a synchrotron XRF microprobe to map the location of Ag within the laboratory-grown diatom Thalassiosira pseudonana and show that diatoms primarily store Ag in their soft tissue, not in their frustules. Intracellular Ag appears to be concentrated in vacuoles, although the majority of Ag is widely distributed and may be associated with the cell membrane and/or cytoplasm. These results imply an alternate mechanism for Ag delivery to sediments whereby Ag is associated with decomposing organic particles rather than skeletal remains. Silver therefore continues to show promise as a qualitative paleoproductivity proxy. Quantitative use is unlikely, however, given variable uptake rates by diatoms in response to environmental conditions and the significant potential for remineralization with organic matter in the water column.