Interrelated influence of iron, light and cell size on marine phytoplankton growth

The sub-optimal growth of phytoplankton and the resulting persistence of unutilized plant nutrients (nitrate and phosphate) in the surface waters of certain ocean regions has been a long-standing puzzle1,2. Of these regions, the Southern Ocean seems to play the greatest role in the global carbon cycle3,4, but controversy exists as to the dominant controls on net algal production. Limitation by iron deficiency4,5, light availability1,6,7 and grazing by zooplankton2 have been proposed. Here we present the results from culture experiments showing that the amount of cellular iron needed to support growth is higher under lower light intensities, owing to a greater requirement for photosynthetic iron-based redox proteins by low-light acclimatized algae. Moreover, algal iron uptake varies with cell surface area, such that the growth of small cells is favoured under iron limitation, as predicted theoretically8. Phytoplankton growth can therefore be simultaneously limited by the availability of both iron and light. Such a co-limitation may be experienced by phytoplankton in iron-poor regions in which the surface mixed layer extends below the euphotic zone—as often occurs in the Southern Ocean6,7—or near the bottom of the euphotic zone in more stratified waters. By favouring the growth of smaller cells, iron/light co-limitation should increase grazing by microzooplankton, and thus minimize the loss of fixed carbon and nitrogen from surface waters in settling particles9,10.