Vertical diversity and association pattern of total, abundant and rare microbial communities in deep-sea sediments

Microbial abundance and community composition in marine sediments have been widely explored. However, high-resolution vertical changes of benthic microbial diversity and co-occurrence patterns are poorly described. The ecological contributions of abundant and rare species in sediments also remain largely unknown. Here, by analysing microbial populations at 14 depth layers of 10 subseafloor sediment cores (water depth 1,250-3,530 m) obtained in the South China Sea, we provided the vertical profiles of microbial beta-diversity and co-occurrence influenced by subcommunities of different abundance. These 134 sediment samples were clustered into four groups according to sediment depth (1-2, 6-10, 30-90 and 190-790 cm) with obvious shifts in microbial community compositions. The vertical succession of microorganisms was consistent with redox zonation and influenced by terrestrial inputs. Partitioning of vertical beta-diversity showed extremely high species replacement between deep layers and the surface layer, indicating selection-induced loss of rare species and dispersal of dormant cells and spores. By contrast, for horizontal beta-diversity, richness of rare species became increasingly significant in deep sediments. Accompanying this beta-diversity profile were clear changes in the association pattern, with microorganisms being less connected in deeper sediment layers, probably reflecting reduced syntrophic interactions. Rare species accounted for an indispensable proportion in the co-occurrence network, and tended to form complex "small worlds." The rare subcommunity also responded differently to various environmental factors compared with the abundant subcommunity. Our findings expand current knowledge on vertical changes of marine benthic microbial diversity and their association patterns, emphasizing the potential roles of rare species.