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 (0°, 140°W) 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 aconcentrations, 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)4uptake 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.