Iron and silicic acid effects on phytoplankton productivity, diversity and chemical composition in the central equatorial Pacific Ocean

A microcosm nutrient-amendment experiment using central equatorial Pacific Ocean (0u, 140uW) mixed-layer waters was conducted to determine biogeochemical controls on phytoplankton with an emphasis on post-iron enrichment nutrient uptake dynamics and species composition. The addition of either Fe (termed Fe-only) or Fe and Si(OH)4 (termed FeSi) to on-deck incubations resulted in growth primarily of pennate diatoms, with statistically equivalent increases relative to the control in maximum photochemical efficiency, chlorophyll a (Chl a) concentrations, particulate organic carbon and nitrogen concentrations, and dissolved inorganic carbon uptake rates. In contrast, at peak Chl a concentrations, there was a 3.4-fold higher abundance of large diatoms and a 3.9- fold lower abundance of small pennate diatoms in FeSi relative to Fe-only, which translated into a 3.5-fold higher Si(OH)4 uptake rate and a 2.1-fold higher biogenic silica concentration. Fourier transform infrared spectroscopy indicated that relative to cells from Fe-only, cells from FeSi possessed the lowest protein : carbohydrate ratios, and ratios of lipids, proteins, and carbohydrates relative to silica, consistent with differences in diatom C allocation or increased silicification or both. Our results suggest that after Fe addition, diatom organic matter accumulation rates (i.e., C and N uptake rates) are enhanced but the low, ambient [Si(OH)4] retards cell division rates, resulting in fewer large diatoms with relatively high C and N contents. After the simultaneous addition of Fe and Si(OH)4, enhanced rates of diatom organic matter accumulation and cell division results in more large, heavily silicified diatoms with relatively low C and N contents.