Accumulation and speciation of Si by Synechococcus examined by single-cell synchrotron x-ray fluorescence and bulk x-ray spectroscopy

The widely distributed marine cyanobacterium Synechococcus is thought to exert an influence on the marine silica cycle through its recently demonstrated accumulation of Si. Cellular Si quotas were measured in individual Synechococcus cells collected during three cruises in the western north Atlantic Ocean at different times of the year. Cell quotas varied widely, from 1 to 4700 amol/cell, although at most stations quotas generally ranged between 10 and 400 amol/cell, with station-specific geometric least-square mean quotas varying between 33 and 88 amol/cell. Significant differences in Si quotas were not observed across cruises or between stations. No effect of ambient silicic acid on quotas was observed, however only relatively small gradients in silicic acid were encountered. Cells collected from the deep-chlorophyll maximum had, on average, 50% less Si than cells collected from surface waters. The form of Si accumulated by Synechococcus was investigated using bulk x-ray near edge spectroscopy (XANES) on several cultured Synechococcus strains isolated from a range of environments (ie, coastal, oligotrophic, cosmopolitan). Spectroscopy indicated that the form of Si accumulated by Synechococcus is distinct from the amorphous biogenic silica (a-SiO₂) deposited in diatoms. Synechococcus Si spectra resemble a SiO₂ polymorph more common in marine sedimentary opals (opal C and opal-CT), also referred to as cristobalite. Each of the three tested strains resembled opal-CT when grown at high silicic acid concentrations, but spectra varied between strains when grown at lower silicic acid concentrations.