Quantifying the biogenic silica production of picoplankton in the oligotrophic ocean: A case study in the South China Sea.

Silicon (Si) utilization is not limited to eukaryotes. Recent research has suggested that the pattern of a large contribution of picocyanobacteria to biogenic silica (bSi) stocks might be widespread in the oligotrophic open ocean. We are the first to measure the size-fractionated bSi standing stocks and production rates in the oligotrophic South China Sea (SCS), which has obvious characteristics of oligotrophic waters. The 150 m integrated bSi standing stocks in the pico-sized fractions averaged 23 % of the total; the contribution of picoplankton to the total bSi production rate was 44 %. Interestingly, our estimated contributions of Synechococcus alone to the <2 μm bSi standing stock and < 2 μm bSi production rates averaged 14 % and 66 %, respectively, indicating that the significant and persistent contribution of bSi was strongly associated with marine picocyanobacteria. Furthermore, the dynamic changes in nutrient concentrations, especially in DIN and DIP, also potentially affected the variability in picoplankton bSi stocks and production rates, while the effects of temperature and salinity were not obvious. In this study, we have provided new information on measurable bSi in the picoplankton size fraction and its production rate in the SCS. We have demonstrated that picoplankton contributes a measurable, and at times significant, proportion to both the total bSi standing stock and its production rate in the SCS. A high silicon content within picocyanobacteria has important implications for understanding both their ecology and their contribution to biogeochemistry.