Cultivation and genomic characterization of the first SAR202 bacterioplankton, isolated in the epipelagic zone of the Yellow Sea, Korea

초록

The SAR202 clade of the phylum Chloroflexota accounts for up to >20% of?bacterioplankton in the marine mesopelagic zones, but has never been cultured, remaining as one of “most-wanted in culture” microbial groups. Here, we report the first successful cultivation of the SAR202 clade. A total of 24 SAR202 group Ia bacterial strains were cultivated from coastal surface waters of the Yellow Sea using dilution-to-extinction method. These strains showed very slow but steady growth, with ~80 hours of doubling time and ~2×108 cells mL-1 of maximum cell density in low-nutrient media. The cultured cells exhibited various morphologies, including short rod, cocci, and biconcave disc shape. All strains were isolated only under the dark condition initially, and subsequent experiments showed that light had a negative effect on the growth of SAR202 strains with an intensity-dependent manner, ranging from growth retardation to cell death. Genome sequences of four selected strains showed the nearly same sizes (~3.1 Mbb) and G+C ratio (51.8%), with ANIb values among the genomes being ≥99.9%. Two copies of heliorhodopsin genes were found in the genomes. In accordance with previous studies, a large number of COG4948 (enolase superfamily) proteins were predicted. The SAR202 group I (o__UBA1151; GTDB) showed the hugest expansion of COG4948 across all prokaryotes. ?-Fucose and ?-rhamnonate, the utilization of which is predicted to require COG4948, promoted the SAR202 growth. These strains would be a resource for further ecophysiological studies on the SAR202 clade, a ubiquitous and abundant marine bacterial group.