Abstract
The unicellular green alga Chlamydomonas sp. ICE-L thrives in polar sea ice, where it tolerates extreme low temperatures, high salinity, and broad seasonal fluctuations in light conditions. Despite the high interest in biotechnological uses of this species, little is known about the adaptations that allow it to thrive in this harsh and complex environment. Here, we assembled a high-quality genome sequence of ∼542 Mb and found that retrotransposon proliferation contributed to the relatively large genome size of ICE-L when compared to other chlorophytes. Genomic features that may support the extremophilic lifestyle of this sea ice alga include massively expanded gene families involved in unsaturated fatty acid biosynthesis, DNA repair, photoprotection, ionic homeostasis, osmotic homeostasis, and reactive oxygen species detoxification. The acquisition of multiple ice binding proteins through putative horizontal gene transfer likely contributed to the origin of the psychrophilic lifestyle in ICE-L. Additional innovations include the significant upregulation under abiotic stress of several expanded ICE-L gene families, likely reflecting adaptive changes among diverse metabolic processes. Our analyses of the genome, transcriptome, and functional assays advance general understanding of the Antarctic green algae and offer potential explanations for how green plants adapt to extreme environments.
•The first whole-genome sequence of an Antarctic psychrophilic green alga•Retrotransposon proliferation results in the large genome of Chlamydomonas sp. ICE-L•Gene families related to extremophilic lifestyle are distinct from temperate alga•Acquisition of genes by HGT contributes to the origin of the psychrophilic lifestyle
Zhang et al. present the genome of an Antarctic psychrophilic green alga and reveal that retrotransposon proliferation results in its relatively large genome. This genome provides evidences that support its extremophilic lifestyle, including expanded repertoire of genes for diverse metabolic processes and genes gained by horizontal gene transfer.