Opening the black box of iron quotas in natural phytoplankton communities

A key challenge for the proposed BioGeoSCAPES program is to integrate the static ‘snap shots’ provided by ‘omics’ techniques and cellular elemental quotas with dynamic marine biogeochemical cycling to more fully understand how the various trajectories of the ocean’s metabolism are revealed across time and space. In iron-limited regions, such as the Southern Ocean, the relationship between photosynthetic carbon fixation and phytoplankton iron requirements are linked by the cellular iron to carbon ratio. Biogeochemical models used to predict how changes in iron supply influence ocean carbon biogeochemistry rely heavily on phytoplankton iron to carbon ratios (i.e., quotas) derived from lab-culture studies, which are relatively simple to interpret. In natural phytoplankton communities the interpretation of iron quotas is more complicated: quotas are influenced by, and integrate, the adaptative traits, physiology and composition of the entire microbial community; there is no conspicuous markers for ‘new’ versus ‘regenerated’ forms of iron; and there is often a mismatch between cellular quotas and uptake ratios. Here we will animate these static quotas, using examples drawn from a series of perturbation incubations and in situ observations conducted on natural populations over multiple GEOTRACES process studies that examined the effects of physiological acclimation, community composition, and microbially-mediated cycling, on iron to carbon uptake ratios. In the context of the BioGeoSCAPES program, we offer these findings as an illustrative guide to improve the design of future studies that link ‘omics’, physiological, and biogeochemical datasets.