The haptophyte Phaeocystis antarctica is endemic to the Southern Ocean, where iron supply is sporadic and its availability limits primary production. In iron fertilization experiments, P. antarctica showed a prompt and steady increase in cell abundance compared to heavily silicified diatoms along with enhanced colony formation. Here we utilized a transcriptomic approach to investigate molecular responses to alleviation of iron limitation in P. antarctica. We analyzed the transcriptomic response before and after (14 h, 24 h, and 72 h) iron addition to a low-iron acclimated culture. After iron addition, we observed indicators of a quick reorganization of cellular energetics, from carbohydrate catabolism and mitochondrial energy production to anabolism. In addition to typical substitution responses from an iron-economic towards an iron-sufficient state for flavodoxin (ferredoxin) and plastocyanin (cytochrome c6 ), we found other genes utilizing the same strategy involved in nitrogen assimilation and fatty acid desaturation. Our results shed light on a number of adaptive mechanisms that P. antarctica uses under low iron, including the utilization of a Cu-dependent ferric reductase system and indication of mixotrophic growth. The gene expression patterns underpin P. antarctica as a quick responder to iron addition.