Distribution, abundance, and ecogenomics of the , a new cosmopolitan thiamine-producing order within the phylum.

The phylum comprises mainly uncultured microorganisms that inhabit different environments such as soils, freshwater lakes, marine sediments, sponges, or corals. Based on 16S rRNA gene studies, the group PAUC43f is one of the most frequently retrieved in marine samples. However, its physiology and ecological roles are completely unknown since, to date, not a single PAUC43f isolate or metagenome-assembled genome (MAG) has been characterized. Here, we carried out a broad study of the distribution, abundance, ecotaxonomy, and metabolism of PAUC43f, for which we propose the name of . This group was detected in 4,965 16S rRNA gene amplicon datasets, mainly from marine sediments, sponges, corals, soils, and lakes, reaching up to 34.3% relative abundance, which highlights its cosmopolitan character, mainly salt-related. The potential metabolic capabilities inferred from 52 MAGs recovered from marine sediments, sponges, and saline soils suggested a facultative aerobic and chemoorganotrophic metabolism, although some members may also oxidize hydrogen. Some species might also play an environmental role as NO consumers as well as suppliers of serine and thiamine. When compared to the rest of the phylum, the biosynthesis of thiamine was one of the key features of the . Finally, we show that polysaccharide utilization loci (PUL) are widely distributed within the so that they are not restricted to , as previously thought. Our results expand the knowledge about this cryptic phylum and provide new insights into the ecological roles of the in the environment. IMPORTANCE Despite advances in molecular and sequencing techniques, there is still a plethora of unknown microorganisms with a relevant ecological role. In the last years, the mostly uncultured phylum is attracting scientific interest because of its widespread distribution and abundance, but very little is known about its ecological role in the marine ecosystem. Here we analyze the global distribution and potential metabolism of the marine group PAUC43f, for which we propose the name of order. This group presents a saline-related character and a chemoorganoheterotrophic and facultatively aerobic metabolism, although some species might oxidize H. Given that is potentially able to synthesize thiamine, whose auxotrophy is the second most common in the marine environment, we propose as a key thiamine supplier to the marine communities. This finding suggests that could have a more relevant role in the marine environment than previously thought.