Metagenomic assemblage genomes analyses reveal the polysaccharides hydrolyzing potential of marine group II euryarchaea.

Marine group II euryarchaea (MGII) dominates the planktonic archaeal community in global surface seawater and is associated to particulate organic matters mainly composed of polysaccharides. However, the polysaccharides metabolism of MGII euryarchaea is unclear. In this study, the distribution and polysaccharides metabolism potential of MGII euryarchaea in the estuary were investigated. High-throughput sequencing of 16S rRNA genes showed that MGII euryarchaea was the predominant archaeal group in the Pearl River Estuary (PRE), and the relative abundance of MGII euryarchaea in particle-attached fraction was higher than that in free-living fractions. A total of 19 metagenome-assembled genomes (MAGs) were successfully reconstructed from metagenomic data, of which 10 MAGs were grouped as MGII euryarchaea according to phylogenomic analysis. Genes encoding a variety of carbohydrate-active enzymes (CAZymes) were found in MAGs/genomes of MGII euryarchaea. These CAZymes annotated in MAGs were capable of hydrolyzing many polysaccharides, including α-glucans, β-glucans, xylans, nitrogen-containing polysaccharides, and some insoluble galactans. The results also indicated that MGII euryarchaea has some unique enzymes that can hydrolyze starch, β-1,3-glucans, complex xylans, carrageenan, and agarose. Collectively, our results demonstrated that MGII euryarchaea has great polysaccharides hydrolysis potential and could play an important role in the carbon cycle of marine ecosystem.