Response of Micromonas sp. to iron-limitation sampled aboard R/V Roger Revelle RR1813 in the Subarctic North Pacific near Station PAPA (50 N, 144.8 W) from August to September 2018

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 Micromonas sp., recently isolated from the iron-limited subarctic North Pacific Ocean, responds to iron-limitation and quantify its potential contribution to new production. Iron-limited Micromonas exhibits reductions in cellular growth rate, volume, and carbon and nitrogen quotas along with a restructuring of cellular metabolism as inferred through gene expression. Gene expression and pathway analyses show evidence of cellular strategies to mitigate iron limitation but conservation 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 as a function of iron status.