Dynamics of silicon metabolism and silicon isotopic discrimination in a marine diatom as a function of pCO2

Opal accumulation rates in sediments have been used as a proxy for carbon flux, but there is poor understanding of the factors that regulate the Si quota of diatoms. Natural variation in silicon isotopes (d30Si) in diatom frustules recovered from sediment cores are an alternative to opal mass for reconstructing diatom Si use and potential C export over geological timescales. Understanding the physiological factors that may influence the Si quota and the d30Si isotopic signal is vital for interpreting biogenic silica as a paleoproxy. We investigated the influence of pCO2 on the Si quota, fluxes across the cell membrane, and frustule dissolution in the marine diatom Thalassiosira weissflogii and determined the effect that pCO2 has on the isotopic fractionation of Si. We found that our Si flux estimates mass balance and, for the first time, describe the Si budget of a diatom. The Si quota rose in cells grown with low pCO2 (100 ppm) compared with controls (370 ppm), and the increased quota was the result of greater retention of Si (i.e., lower losses of Si through efflux and dissolution). The ratio of efflux : influx decreased twofold as pCO2 decreased from 750 to 100 ppm. The efflux of silicon is shown to significantly bias measurements of silica dissolution rates determined by isotope dilution, but no effect on the Si isotopic enrichment factor («) was observed. The latter effect suggests that silicon isotopic discrimination in diatoms is set by the Si transport step rather than by the polymerization step. This observation supports the use of the d30Si signal of biogenic silica as an indicator of the percentage utilization of silicic acid. Diatoms play an important role in the export of carbon to the deep sea and control the reverse weathering of silicon in the oceans. This is caused by their high productivity and absolute requirement for silicon in cell wall formation. Silica production has been hypothesized to be a useful proxy of current and past carbon export production (Dugdale and Wilkerson 1998; Ragueneau et al. 2000). The usefulness of this proxy is dependent on understanding the factors that influence the Si : C ratio of the phytoplankton community as 1 Present address: Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901 ([email protected]). 2 Present address: Department of Biology & School of Earth and Ocean Sciences University of Victoria, Victoria, British Columbia V8W 3N5, Canada.