Insights into the physiological and metabolic features of , a novel genus of with the potential to drive the carbon cycle.

UNLABELLED

are widely distributed across various habitats but are difficult to culture. Some previous multiomics analyses reported that have outstanding metabolic capacity for organic matter degradation and are able to degrade and synthesize polysaccharides, two activities that could contribute significantly to the Earth's carbon cycle. Here, we isolated from marine sediment two novel strains, SDUM461003 and SDUM461004, that represent a new genus of the difficult-to-culture phylum . Genome analysis, functional annotation, and experimental verification revealed that these two strains degrade polysaccharides and antibiotics, including some complex sulfated polysaccharides (SPs), primarily fucoidan and chondroitin sulfate. Moreover, electron microscopy images revealed that these bacteria can synthesize and store large amounts of glycogen. These polysaccharide degradation and synthesis capacities also exist but differ under nitrogen-deficient conditions, indicating that may have the potential to maintain their normal metabolism by nitrogen fixation under aerobic conditions. Given that polysaccharides and their degradation products are particularly crucial carbon sources for marine microorganisms, are thought to be important contributors to biogeochemical cycling in the ocean.

IMPORTANCE

are widely distributed and able to utilize a variety of difficult-to-biodegrade polysaccharides, which have a significant impact on the marine carbon cycle. However, there are not enough pure culture strains of , as hard-to-cultivate bacteria, for us to study. Here, our study reports a new genus in the phylum and investigates their ability to degrade and synthesize a variety of polysaccharides as well as the mechanism of utilizing difficult-to-degrade polysaccharides. We also explored their special performance on carbon utilization in marine nitrogen-deficient environments. This contributes to deepening our understanding of the involvement of marine microorganisms in the marine carbon cycle.