A central goal in marine microecology is to understand the ecological factors shaping spatiotemporal microbial patterns and the underlying processes. We hypothesized that abiotic and/or biotic interactions are probably more important for explaining the distribution patterns of marine bacterioplankton than environmental filtering. In this study, surface seawater samples were collected about 7000 miles from the Mediterranean Sea, transecting the North Atlantic Ocean, to the Brazilian marginal sea. In bacterial biosphere, SAR11, SAR86, Rhodobacteraceae, and Rhodospiriaceae were predominant in the Mediterranean Sea; Prochlorococcus was more frequent in Atlantic Ocean; whereas in the Brazilian coastal sea, the main bacterial members were Synechococcus and SAR11. With respect to archaea, Euryarchaeota were predominant in the Atlantic Ocean and Thaumarchaeota in the Mediterranean Sea. With respect to the eukaryotes, Syndiniales, Spumellaria, Cryomonadida, and Chlorodendrales were predominant in the open ocean, while diatoms and microzooplankton were dominant in the coastal sea. Distinct clusters of prokaryotes and eukaryotes displayed clear spatial heterogeneity. Among the environmental parameters measured, temperature and salinity were key factors controlling bacterial and archaeal community structure, respectively, whereas N/P/Si contributed to eukaryotic variation. The relative contribution of environmental parameters to the microbial distribution pattern was 45.2%. Interaction analysis showed that Gammaproteobacteria, Alphaproteobacteria, and Flavobacteriia were the keystone taxa within the positive-correlation network, while Thermoplasmata was the main contributor in the negative-correlation network. Our study demonstrated that microbial communities are co-governed by environmental filtering and biotic interactions, which are the main deterministic driving factors modulating the spatiotemporal patterns of marine plankton synergistically at the regional or global levels.

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The introduction of oysters to a waterbody is an efficient method for decreasing levels of eutrophication. Oysters affect sedimental environments and benthic microbes via their roles in nutrient cycling. However, little is known about how long-term oyster culturing affects benthic microbial community assembly. In the present study, top and bottom sediments from an oyster-culture area and non-culture area, in a eutrophic bay with a long history of oyster culturing, were obtained for environmental parameter measurement and microbe identification. Deterministic and stochastic processes in microbial community assembly were assessed. In particular, keystone species identification through network analysis was combined with measured environmental parameters to determine the factors related to community assembly processes. Our results suggest that oyster culturing relates to greater variation in both biological and non-biological sediment profiles. In benthic communities, Proteobacteria and Chloroflexi were the most abundant phyla, and community compositions were significantly different between sample groups. We also found that community assembly was more affected by deterministic factors than stochastic ones, when oysters were present. Moisture, or water content, and pH were identified as affecting deterministic and stochastic processes, respectively, but only water content was a driver associated with oyster culturing. Additionally, although keystone species presented a similar pattern of composition to peripheral species, they responded to their environments differently. Furthermore, model selection, fitting keystone species to community assembly processes, indicates their role in shaping microbial communities.

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