Abstract
Candidate bacterial phylum CSP1-3 has not been cultivated and is poorly understood. Here, we analyzed 112 CSP1-3 metagenome-assembled genomes (MAGs) and showed they are likely facultative anaerobes, with three of five families encoding autotrophy through the reductive glycine pathway (RGP), Wood–Ljungdahl pathway (WLP), or Calvin-Benson-Bassham (CBB), with hydrogen or sulfide as electron donors. Chemoautotrophic enrichments from hot spring sediments and fluorescence in situ hybridization (FISH) revealed enrichment of six CSP1-3 genera, and both transcribed genes and DNA-stable isotope probing (DNA-SIP) were consistent with proposed chemoautotrophic metabolisms. Ancestral state reconstructions showed that the ancestors of phylum CSP1-3 may have been acetogens that were autotrophic via the RGP, whereas the WLP and CBB were acquired by horizontal gene transfer. Our results reveal that CSP1-3 is a widely distributed phylum with potential to contribute to cycling of carbon, sulfur, and nitrogen. The name Sysuimicrobiota phy. nov. is proposed.