Microalga–bacteria Community with High Level Carbon Dioxide Acclimation and Nitrogen-fixing Ability

•Microalga–bacteria community was obtained from biofilm of river cobbles.•The community could grow under up to 20% CO2 and nitrogen salts-free conditions.•Microalgal strain MP5 was identified as Coelastrella sp.•Bacteria in the community were implied to fix atmospheric nitrogen. Microalgal conversion of high-level CO2 in industrial flue gas to value-added products is attractive technology for mitigating global warming. However, reduction of microalgal production costs for medium ingredients, particularly nitrogen salts, is essential. The use of atmospheric nitrogen as a nitrogen source for microalgal cultivation will dramatically reduce its production costs. We attempted to enrich a microalga–bacteria community, which fixes both CO2 and atmospheric nitrogen under high level CO2. By cultivating biofilm recovered from the surface of cobbles in a riverbank, a microalgal flora which grows in a nitrogen salts-free medium under 10% CO2 was enriched, and the coccoid microalgal strain MP5 was isolated from it. Phylogenetic analysis revealed that the strain MP5 belongs to the genus Coelastrella, and the closest known species was C. terrestris. With PCR–DGGE analysis, it was found that the enriched microalgal community includes bacteria, some of which are suggested diazotrophs. The addition of bactericides in culture medium inhibited MP5 growth, even though the strain MP5 is eukaryotic. Growth of bacteria-free MP5 was stimulated by addition of Agrobacterium sp. isolates in nitrogen salts-free medium, suggesting that MP5 and the bacteria have responsibility for photosynthetic carbon fixation and nitrogen fixation, respectively.