Synechococcus silicon accumulation in oligotrophic oceans

Abstract

Si accumulation by Synechococcus, small unicellular marine picocyanobacteria that dominate in nutrient-depleted waters, is significant due to its potential to alter our understanding of the global Si cycle. To date, however, there are only a few studies on Si accumulation in individual Synechococcus cells. To determine the magnitude and variability of Synechococcus cellular Si content in oligotrophic oceans, we analyzed 1348 discrete cells collected from the understudied eastern Indian Ocean (EIO) and western Pacific Ocean (WPO). Synechococcus cellular Si quotas varied widely across both regions, ranging from 0 to 4651 amol Si cell−1. While highly variable in range, average Si quotas were ~ 2.5-fold lower in Synechococcus cells from the EIO (172 ± 304 amol Si cell−1) than in those from the WPO (438 ± 737 amol Si cell−1). The estimated contributions of Synechococcus to total and picoplankton biogenic Si stocks were < 1–8% and 1–11%, respectively, suggesting that Synechococcus has a small but persistent regional contribution to biogenic Si stocks. Here, we also made a first-order estimate of the Synechococcus biogenic Si stock for the global ocean, that is, 0.06–0.09 Tmol Si, accounting for 2–3% of the global ocean diatom Si stock. No significant effects of temperature and dissolved inorganic N and P on cellular Si quotas were observed within these oligotrophic ecosystems. However, salinity, ambient dissolved Si, and N/P ratio were closely correlated with the variable Si quotas with different relationships. In addition, Synechococcus cellular Si/P and Si/S ratios associated with physiological processes were significantly influenced by environmental and geographic variables. We thus suggest that the variance in cellular Si quotas may be indirectly driven by ambient variables via some unknown independent or strain-dependent physiological processes.