The effects of iron limitation on the small chlorophyte <i>Micromonas</i> from the Northeast Pacific Ocean

Small eukaryotic phytoplankton can account for a considerable amount of biomass and primary production in high nutrient, low chlorophyll (HNLC) regions of the ocean where iron limitation is pronounced. However, the physiological and metabolic strategies these cells invoke to cope under low iron conditions and the extent to which they are responsible for new production (i.e., the fraction of primary production supported by nutrients from outside of the euphotic zone) are unclear. Here, we examined how a representative picoeukaryote-the chlorophyte Micromonas sp., recently isolated from the iron-limited subarctic Northeast Pacific Ocean-responded to iron limitation when grown on nitrate as a nitrogen source. Iron-limited Micromonas exhibited reductions in growth rate, cell volume, and elemental quotas along with a restructuring of cellular metabolism. Gene expression and metabolic pathway analyses showed evidence of strategies to mitigate iron limitation with constitutive expression of genes related to nitrogen uptake and utilization. Additionally, cellular carbon and nitrogen quotas were 20-70 fmol C · cell^-1 and 3.3-20 fmol N · cell^-1, respectively, as a function of iron status. Based on the measured cellular quotas, we have estimated that representative picoeukaryotes (<2 μm), such as Micromonas, in HNLC Northeast Pacific waters can account for a significant proportion of new production, supporting the need for a reconsideration of the role small eukaryotic phytoplankton play in the global carbon cycle.