Abstract: Psychrophilic microalgae successfully survive in the extreme and highly variable polar ecosystems, which represent the energy base of most food webs and play a fundamental role in nutrient cycling. The success of microalgae is rooted in their adaptive evolution. Revealing how they have evolved to thrive in extreme polar environments will help us better understand the origin of life in polar ecosystems. We isolated a psychrophilic unicellular green alga, Microglena sp. YARC, from Antarctic sea ice which has a huge genome. Therefore, we predicted that gene replication may play an important role in its polar adaptive evolution. We found that its protein-coding gene number significantly increased and the duplication time was dated between 37 and 48 million years ago, which is consistent with the formation of the circumpolar Southern Ocean. Most duplicated paralogous genes were enriched in pathways related to photosynthesis, DNA repair, and fatty acid metabolism. Moreover, there were a total of 657 Microglena-specific families, including collagen-like proteins. The divergence in the expression patterns of the duplicated and species-specific genes reflects sub- and neo-functionalization during stress acclimation. Overall, key findings from this study provide new information on how gene duplication and their functional novelty contributed to polar algae adaptation to the highly variable polar environmental conditions.