Isolation and Genomic Study of Marine SAR202 Bacteria

초록

The ocean covers 71% of the Earth's surface and is a habitat for enormously diverse microbes, including “the uncultured bacterial majority.” Although culture-independent studies such as metagenomics and single-cell genomics have revealed diverse genomic repertoires of uncultured marine bacteria, cultivation of the uncultured is important to understand their phenotypes and physiology. In this study, a total of 610 strains were isolated by the high-throughput culturing method (HTC) based on dilution-to-extinction using seawater samples collected off the coast of Garorim bay. Many of the HTC isolates included “the most wanted bacterial groups to be cultured” such as SAR202 and SAR86 clades that do not have any cultured representatives. In this presentation, we focus on the SAR202 clade of the phylum Chloroflexi, which is known to be abundant in the mesopelagic and bathypelagic ocean and play an important role in the global carbon and sulfur cycling. Four representative strains selected from 24 SAR202 isolates exhibiting over 99% 16S rRNA gene sequences to each other showed an extremely slow growth with 3.3 days of doubling time and did not form colonies on agar plates. Whole genome sequences of the four SAR202 strains were 3.08 Mbp with 51.8% G + C ratio in average. The genomes of the strains contained typical gene sets involved in chemoheterotrophic metabolism, but also had two copies of heliorhodopsin gene. Moreover, the genomes exhibited huge gene expansion of the enolase superfamily proteins (COG4948), suggesting that the SAR202 strains have a metabolic potential to use recalcitrant dissolved organic matters with resolving chiral complexity. From the growth experiment using candidate substrates for COG4948, the strains showed a significantly enhanced growth pattern in rhamnonate- or fucose-supplemented artificial seawater medium. Further transcriptomic studies during the growth under the light/dark conditions or various carbon substrates are currently