Cellular iron contents of plankton during the Southern Ocean Iron Experiment (SOFeX)

Iron (Fe) availability limits phytoplankton biomass and production in large regions of the Southern Ocean and influences community composition and size structure, which may affect C export and other system-level functions. To improve our understanding of Fe partitioning within communities and the response of different components to fertilization, we assessed the cellular Fe contents of individual diatoms, autotrophic flagellates, and heterotrophic flagellates during the recent Southern Ocean Fe Experiment using synchrotron-based X-ray fluorescence (SXRF). Dual ⁵⁵Fe/¹⁴C radioisotope incubations were also conducted to estimate Fe:C ratios in size-fractionated plankton. Cellular Fe quotas determined by the two techniques were in close agreement when low amounts of ⁵⁵Fe (0.2 nM) were added, but ⁵⁵Fe additions of 2 nM resulted in 2–3-fold higher quotas. SXRF assessments of cellular Fe quotas (normalized to C) were generally in good agreement with prior bulk analyses of natural assemblages, but revealed compositional differences among protistan taxa not previously detected. Mean Fe:C ratios for diatoms, autotrophic flagellates, and heterotrophic flagellates from unfertilized waters were 6.0, 8.7, and 14.1 μmol mol C⁻¹, respectively. Smaller cells had higher Fe:C ratios than larger cells. Fertilization enhanced Fe quotas in all cell types, with mean Fe:C ratios increasing approximately 4-fold (from about 10 to about 40 μmol mol C⁻¹) after two Fe additions. This study provides some of the first measurements of Fe quotas in phytoplankton cells from natural communities and the first measurements of Fe quotas in natural protozoa.