Possible involvement of extracellular polymeric substrates of Antarctic cyanobacterium Nostoc sp. strain SO-36 in adaptation to harsh environments

Cyanobacteria are some of the primary producers in extremely cold biospheres such as the Arctic, Antarctic, and vast ice sheets. Many genera of cyanobacteria are identified from these harsh environments, but their specific mechanisms for cold adaptation are not fully understood. Nostoc sp. strain SO-36 is a cyanobacterium isolated in Antarctica more than 30 years ago and regarded as a psychrotolelant species. To determine whether the strain is psychrotolelant or psychrophilic, it was first grown at 30 degrees C and 10 degrees C. The cells grew exponentially at 30 degrees C, but their growth stopped at 10 degrees C, indicating that the strain is only psychrotolerant. Microscopic analysis revealed that the morphology of the cells grown at 30 degrees C was filamentous and differentiated heterocysts, which are specialized cells for gaseous nitrogen fixation under nitrogen-deprived conditions, indicating that the strain can grow diazotrophically. The cells grown at 10 degrees C have a smaller size, shortened filament length and decreased chlorophyll content per cell. At 10 degrees C, the cells are aggregated with extracellular polymeric substrates (EPSs), which is a common mechanism to protect cells from ultraviolet light. These results imply that segmentation into short filaments was induced by photodamage at low temperatures. To fully understand the adaptation mechanisms of Nostoc sp. strain SO-36 for low-temperature conditions, next-generation sequencing analyses were conducted. Complete genome sequence of the strain revealed that it has one main chromosome of approximately 6.8 Mbp with 4 plasmids, including 6855 coding sequences, 48 tRNA genes, 4 copies of rRNA operons, and 5 CRISPR regions. Putative genes for EPS biosynthesis were found to be conserved in Nostocaceae regardless of their habitat. These results provide basic information to understand the adaptation mechanisms at low temperatures, and the strain can be a model organism to analyze adaptation to extreme environments.

Automated summary

Cyanobacteria strain Nostoc sp. SO-36, isolated from Antarctica, was found to be psychrotolerant and filamentous when grown at 30 degrees Celsius, with the ability to grow diazotrophically. At 10 degrees Celsius, the cells showed reduced growth, smaller size, and increased aggregation with extracellular polymeric substrates (EPSs) to protect from UV damage. The complete genome sequencing revealed the presence of one main chromosome with plasmids and conserved genes for EPS biosynthesis.