Small eukaryotic phytoplankton can play an outsized role in carbon export in iron-limited regions

Small eukaryotic phytoplankton can account for considerable amounts of biomass and primary production in high nutrient, low chlorophyll (HNLC) regions of the ocean where iron-limitation is pronounced. However, the 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) is unclear. Here, we examine how a representative picoeukaryote, the chlorophyte Bathycoccus sp ., recently isolated from the iron-limited subarctic North Pacific Ocean, responds to iron-limitation and quantify its potential contribution to carbon export. Iron-limited Bathycoccus exhibits reductions in cellular growth rate, volume, and carbon and nitrogen quotas along with a restructuring of cellular metabolism. Gene expression and pathway analyses show evidence of cellular strategies to mitigate iron limitation but constitutive expression of genes related to nitrogen uptake and utilization. Additionally, when grown on nitrate, cellular carbon and nitrogen quotas ranged from 0.02 – 0.17 pmol C cell ^-1 and 3.3 x 10 ^-3 – 2.2 x 10 ^-2 pmol N cell ^-1 , respectively, as a function of iron status. Based on the measured cellular quotas, we estimate that representative picoeukaryotes, such as Bathycoccus, in HNLC North Pacific waters can account for a majority of new production under ambient iron-limited conditions, supporting the need for a paradigm shift in our thinking of the role small eukaryotic phytoplankton play in the global carbon cycle.