sp. 'CSMB_57', isolation and genomic insights from the most abundant bacterial taxon in eastern Australian coals.

One of the most abundant and ubiquitous taxa observed in eastern Australian coal seams is an uncultured species and part of the Coal Seam Microbiome dataset assigned as 'CSMB_57'. Despite this abundance and ubiquity, knowledge about this taxon is limited. The present study aimed to generate an enrichment culture of sp. 'CSMB_57' using culturing strategies that exploit its sulphur-reducing capabilities by utilizing a polysulfide solution in a liquid medium. Using dilution to extinction methods, a highly enriched culture was successfully generated. The full-length 16S rRNA sequence revealed that all closely related taxa were observed in subsurface environments suggesting that . sp. 'CSMB_57' may be a subsurface specialist. Subsequently, the DNA from the enrichment culture was sequenced and the genome of . sp. 'CSMB_57' was assembled. Genomic annotation revealed a high number of CRISPR arrays for viral defence, a large array of ABC transporters for amino acid and peptide uptake, as well as genes likely associated with syntrophy such as genes associated with type-IVa pilus, often used for direct interspecies electron transfer, and multiple hydrogenases capable of producing hydrogen. From the various genomic observations, a conceptual ecological model was developed that explores its possible syntrophic roles with hydrogenotrophic methanogens and acetogenic bacteria within coal-seam environments.