Picoplankton contribution to biogenic silica stocks and production rates in the Sargasso Sea

Picocyanobacteria in the Sargasso Sea accumulate significant amounts of Si, a finding which questions how we interpret previous regional measurements of biogenic silica (bSi) production and the role of diatoms in the open ocean. The picoplankton (<3 μm cells) contributed a measurable, and at times significant, proportion of both the total bSi standing stock and its rate of production. The 100 m integrated bSi stock and bSi production rate in the <3 μm size fraction averaged 14% and 16%, respectively, of the total. At some stations, specific rates of bSi production in the <3 μm cells were up to threefold higher than for larger cells. But among all stations and depths, the two size fractions had statistically indistinguishable specific production rates (~0.35 day⁻¹). The estimated contributions of Synechococcus alone to the 100 m integrated bSi stock and bSi production in cells <3 μm were 15% and 55%, respectively, suggesting that half of bSi production in this small size fraction could be sustained by Synechococcus, but a majority of the bSi was not associated with living Synechococcus. Our results suggest that picoplankton have a small but persistent regional contribution to bSi stocks, which is masked by a dynamic bSi pool driven by larger cells. While a significant fraction of bSi production is attributable to picoplankton, their contributions are likely to have been included in previous analyses, making prior regional budgets still relevant. However, our understanding of the factors controlling regional bSi production and our interpretations of particulate matter elemental ratios (e.g., Si:C) may require revision.