Antarctic sea ice, the largest expanse of sea ice on Earth, includes a system of liquid brine channels characterized by extreme low temperatures, high salinity, and broad seasonal fluctuations in light conditions. Sea ice algae are important primary producers in these ecosystems, and green algae form an important component of these communities. Although the genomic underpinnings of adaptation to extreme Antarctic environments is understood in bacteria and animals, this knowledge is lacking for green algae. The unicellular green alga Chlamydomonas sp. ICE-L, a typical psychrophilic green alga, represents an emerging model system for studying adaptations of green algae to extreme Antarctic sea ice environments. 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. On July 2, 2020, the team led by Prof. Bojian Zhong from the College of Life Sciences, Nanjing Normal University has published their findings in the Current Biology (https://doi.org/10.1016/j.cub.2020.06.029).
Figure 1. The ICE-L genome characteristics and comparison of genome contents with other green algae.
The genome sequencing of ICE-L, and whole-genome comparisons between temperate and psychrophilic green algae will further our understanding of how psychrophilic organisms adapt to the polar sea ice environments. Here we assembled a high-quality genome sequence of ~542 megabases and found that retrotransposon proliferation contributed to the relatively large genome size of ICE-L when compared to other chlorophytes.
Figure 2: Schematic summary of adaptive strategies of ICE-L to various abiotic stresses.
The ecological success of ICE-L in Antarctic sea ice may have been facilitated by massive expansion of gene families with functions associated with unsaturated fatty acid biosynthesis, ionic homeostasis, osmotic homeostasis, reactive oxygen species detoxification, DNA repair, light harvesting, and photoprotection. 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 up-regulation, under abiotic stress, of several expanded ICE-L gene families, likely reflecting adaptive changes among diverse metabolic processes. These findings suggest that the expansion of specific gene families, the acquisition of foreign genes, and alterations in gene expression patterns in diverse metabolic processes, likely underpin the successful colonization of Antarctic sea ice by green algae. The 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.
Dr. Bojian Zhong from Nanjing Normal University and Dr. Jinlai Miao from Ministry of Natural Resources are the corresponding authors. Zhenhua Zhang from Nanjing Normal University, Changfeng Qu from Ministry of Natural Resources, Kaijian Zhang from Novogene Bioinformatics Institute, Yingying He from Ministry of Natural Resources, Xing Zhao from Novogene Bioinformatics Institute and Lingxiao Yang from Nanjing Normal University are joint first authors. This study is supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the National Key Research and Development Program of China, the China Ocean Mineral Resources R&D Association, the Jiangsu Province Key Project for Scientific Research, the Young Elite Scientists Sponsorship Program of Jiangsu Province, the Basic Scientific Fund for National Public Research Institutes of China, the Natural Science Foundation of Shandong, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the National Aeronautics and Space Administration and a NIFA-USDA Hatch grant.