SILICON USE BY PHYTOPLANKTON: COMPARISONS BETWEEN FRESHWATER AND MARINE SYNECHOCOCCUS AND A FRESHWATER DIATOM

Abstract

Silicon (Si) plays a crucial role in aquatic ecosystems, primarily through its uptake by diatoms for formation of cell walls, or frustules. Observations over recent decades in Lake Michigan show rising dissolved silica (dSi) levels along with declining diatom biomass and increasing picoplankton, which suggest shifts in Si cycling that may involve changes in Si use by phytoplankton. This study aimed to examine Si accumulation, uptake and cellular storage in a common freshwater cyanobacteria, Synechococcus elongatus and compare with a marine cyanobacteria, Synechococcus bacillaris, known to accumulate Si, and a freshwater diatom, Cyclotella meneghiniana. Laboratory cultures were used to measure growth in Si-replete and Si-free conditions, biogenic silica (bSi) accumulation, dSi depletion, Si uptake kinetics, and intracellular Si storage. Results showed that freshwater S. elongatus exhibited growth dependence on Si, with growth stimulation and bSi accumulation only happening after Si was supplied into previously Si-free cultures, indicating a requirement for Si. In contrast, the marine S. bacillaris grew equally well with or without Si but still accumulated bSi and depleted dSi in Si-replete cultures. C. meneghiniana showed the highest growth rates and Si uptake with significantly higher bSi content per cell than the cyanobacteria and Michaelis-Menten uptake kinetics. However, bSi per biovolume was higher in the smaller cells of S. elongatus than in the diatom suggesting significant Si accumulation relative to its smaller cell size. Internal Si analysis showed substantial pools of intracellular dSi in both cyanobacteria, although the diatom had a higher bSi to internal dSi ratio, due to structural Si in frustules. Scanning Electron Microscopy with Energy Dispersive Spectroscopy confirmed elemental Si composition in all three species while the fluorescent dye PDMPO labeled silica only in the diatom, Cyclotella. The roles of Si in cyanobacteria cells are not clear from this study but these findings demonstrate that freshwater cyanobacteria such as S. elongatus not only accumulate bSi but may require it for growth, indicating a previously underrecognized potential role for picophytoplankton in freshwater Si cycling, particularly in Lake Michigan undergoing ecological changes.