Illuminating the Relative Contribution of Different Diatom Genera to Whole-Assemblage Silica Production in the Surface Ocean

Diatoms are one of the largest contributors to primary productivity in the modern ocean with an absolute requirement for silicon (Si) to form their cell walls. They can therefore control the cycling of Si and contribute significantly to the downward flux of silica, and other nutrients in most oceanic regions. The production rate of silica by a diatom assemblage can be determined with radioactive and stable Si isotopic tracers in enrichment experiments or simply by measuring the accumulation of biogenic silica over time during field incubations. While the measurement of total diatom production is critically valuable for understanding the productivity of a marine system, it does not provide information about the contribution of different diatom taxa to the total production of an assemblage. Diatom genera are present in surface waters in different numerical abundance, but is the contribution of each genus to total silica production correlated to its numbers? To tease this out, we employed an improved widefield epifluorescence microscopy technique using the fluorescent dye PDMPO, which acts as a tracer for Si during uptake by diatoms. The accuracy of this technique was confirmed using 2-photon laser scanning microscopy. This method allowed the imaging and quantification of diatom fluorescence per cell, and identification of genera after incubation of water samples with PDMPO. We also measured total silica production rates with the radioactive tracer ³²Si and the accumulation of biogenic silica in shipboard incubation experiments. We present data for subarctic waters in the NE Pacific Ocean and arctic waters of the Bering and Chukchi Seas that show a poor correlation between the relative abundance of diatom genera and their contribution to total silica production. Our results imply that the abundant genera are not always the ones doing the heavy lifting, and those less numerically dominant could have a larger contribution to whole-assemblage silica production.