Controls on phytoplankton iron quotas in natural systems

Dissolved iron (dFe) availability is usually assumed to be the primary control on cellular Fe contents, or quotas, in marine phytoplankton. However, phytoplankton taxa differ in their Fe requirements and abilities to compete for ambient Fe, and Fe quotas are likely also impacted by cellular growth rate (i.e., ‘biodilution’) and macronutrient availability (via biochemical machinery required for nutrient uptake and impacts on cell size). A global compilation of cellular Fe quotas indeed shows that diatoms – and pennates, in particular – contain more Fe than non-diatoms at the same ambient Fe concentration, matching the enhanced Fe storage abilities known for some diatoms. To explore controls on quotas, we measured taxon-specific phytoplankton Fe quotas from three eastern margin upwelling regions in the Pacific Ocean that span significant gradients in Fe, nitrate, and community composition. Measurements of ambient cells were supplemented with Fe addition and removal incubations to examine physiological plasticity of Fe quotas in individual diatom taxa from natural communities. Iron quotas decreased in all taxa moving offshore into the South Pacific from the shelf, with flagellate quotas matching expectations for low dFe conditions from N-replete cultures. However diatom Fe quotas in the oligotrophic gyre remained 5-fold above minimum levels, while diatom abundance decreased significantly. This suggests that diatom growth was primarily limited by N, allowing cellular Fe to accumulate despite <0.1 nM dFe. Along Line P in the North Pacific, pennate diatoms accumulated more Fe than centrics in offshore waters, although quotas were comparable between these taxa in high-Fe nearshore waters. Across the gradient, Fe quotas of individual diatom taxa dropped about 4-fold before taxa were replaced by competing taxa. This realized plasticity is less than physiological capabilities estimated from laboratory culture experiments. These results are the first measurements of iron plasticity in natural communities. Cellular Fe in natural communities may be more consistent than in individual laboratory cultures, with taxon-specific physiology and macronutrient availability placing important constraints on community micronutrient stoichiometries.