Silicic acid supplied to coastal diatom communities influences cellular silicification and the potential export of carbon

Microcosm experiments were conducted along the Washington and Oregon coasts in May 2009, May 2010, and July 2010 to determine whether variation in the supply of silicic acid from the Columbia River could influence the silicification and sinking potential of coastal diatom blooms. The chlorophyll a concentration increased similarly in communities incubated with added nitrate or both nitrate and silicic acid, indicating that growth was limited by nitrate availability. Communities that grew in the treatment with added silicic acid and nitrate were more silicified than communities in the treatment with only nitrate added. No difference in community composition was detected between these treatments in three out of four experiments. Isolates of Minutocellus, Cylindrotheca, Thalassiosira, and Odontella were obtained from the microcosm experiment conducted in May 2010 and were maintained in the laboratory in 20 µmol L⁻¹ silicic acid. All four diatom isolates contained ∼ 2.5 times more silica per cell when silicic acid concentration in the media was increased to 80 µmol L⁻¹. The intensity of a fluorescent cellular stain of newly precipitated silica (2-(4-pyridyl)-5{[4-dimethylaminoethyl-aminocarbamoyl)-methoxy]phenyl}oxazole) strongly correlated with silica content among species, but was a less sensitive indicator of changing silicification within a single species. Changes in silicification were not correlated with changes in the transcript abundance of silicic acid transporters. Sinking rates increased roughly 2-fold for cells that contained ∼ 2.5 times more silica. Variation in silicic acid supply alters the silicification of nitrate-fueled coastal diatom blooms and the potential sink of carbon from coastal zones.