Alternative Cold Response Modes in Chlorella (Chlorophyta, Trebouxiophyceae) from Antarctica

H. Hu, H. Li and X. Xu. 2008. Alternative cold response modes in Chlorella (Chlorophyta, Trebouxiophyceae) from Antarctica. Phycologia 47: 28–34. DOI: 10.2216/07-28.1Chlorella was known to show enhanced antifreeze capability after cold hardening. We isolated Chlorella strains NJ-7 and NJ-18, which display alternative cold response modes from rock surfaces in Antarctica. On the basis of 18S ribosomal (rRNA) sequences, NJ-7 is an Antarctic type of Chlorella vulgaris; NJ-18 is also a ‘true’ Chlorella species but differs from any previously reported species in structure. NJ-7 partially retained the enhancing effects of low temperature cultivation on freeze tolerance, which correlates with an increase of C18:3-fatty acid content and up-regulation of two antifreeze protein genes. NJ-18, however, showed stable freeze tolerance regardless of the pre-cultivation temperature. We propose that cold response modes vary widely in Chlorella and that the adaptation of C. vulgaris to Antarctica may serve as a model system for the evolution of antifreeze mechanisms in a single species of photosynthetic microorganism.