Photosynthetic adaptation to low temperatures and phylogenetic analysis of Antarctic cyanobacteria

While the ecological interactions of the terrestrial microbial communities in Antarctica have been well-studied, adaptation of photosynthesis at cold temperatures has not received the same scrutiny. The isolation and characterization of cyanobacteria capable of low temperature carbon fixation may clarify the adaptations involved. Cyanobacterial communities from mats found in Antarctic ponds and from within sandstone rocks (cryptoendoliths) were able to conduct photosynthesis at low temperatures with no photoinhibition at high light levels. RuBisCO (a key enzyme involved in photosynthetic carbon fixation) from three different cryptoendolithic cyanobacterial strains was able to fix significant amounts of carbon at low temperatures ({dollar}-{dollar}3C), and the temperature of maximum activity was lowered by 8 degrees in two of these strains, relative to a mesophilic cyanobacterium. RuBisCOs from thermophilic cyanobacteria had activity curves shifted to higher temperatures, relative to the same mesophilic cyanobacterium. Comparison of RuBisCO gene sequences amplified from one Antarctic cyanobacterium, two thermophilic cyanobacteria, and several closely related mesophilic cyanobacteria offered support to current theories of specific molecular changes conferring activity at extreme temperatures. Low temperature activity of RuBisCO in psychrotrophic organisms may be due to the replacement of charged amino acids by small and neutral amino acids, allowing the protein to become more flexible or stabilizing the folded enzyme, maintaining catalytically active conformations. High temperature activity of RuBisCO in thermophiles may be due to the substitution of amino acids with smaller side chains, allowing the protein to adopt a conformation which increases internal hydrogen bonding and salt bridges, shifting the equilibrium toward folding at elevated temperatures. The Antarctic isolates were identified as a member of the Anabaena subgroup and a member of the genus Phormidium, based on 16S rRNA sequences. The RuBisCO gene sequences for the psychrotrophic and thermophilic cyanobacteria examined do not seem to be the result of gene transfer events from taxa outside the cyanobacterial clade. Comparison of a 16S rRNA-based phylogenetic assignment with that obtained from morphological characters led to the conclusion that morphology is a polymorphic character in cyanobacteria, and of little use as a taxonomic tool.