Diversity patterns and ecological assembly mechanisms of bacterial communities in the northeastern Indian Ocean epipelagic waters during the northeast monsoon.

Disentangling microbial community diversity patterns and assembly mechanisms is critical for understanding ecological processes and evaluating biogeochemical cycling in ecosystems. However, the diversity patterns and assembly mechanism of the microbial communities in the epipelagic waters in the northeastern Indian Ocean (NEIO) on the spatial scale are still unclear. In this study, we investigated the spatial dynamics, geographic distribution pattern, and assembly process of the bacterial community using 532 samples collected from the epipelagic waters in the NEIO during the northeast monsoon. The results indicate that the bacterial richness and Bray-Curtis dissimilarity exhibited the strongest correlations with depth compared to the latitudinal and longitudinal scales. The dissolved oxygen was identified as the most important environmental factor affecting the bacterial richness and Bray-Curtis dissimilarity compared to temperature and salinity. The distance-decay relationship (DDR) of the bacterial community strengthened with increasing water depth. Turnover was the predominant ß-diversity component influencing the spatial changes in the whole bacterial community. The dispersal limitation of the stochastic process and homogeneous selection of the deterministic process governed the bacterial ecological assembly process of the whole bacterial community. Abundant and rare subcommunities differed in terms of the niche breath, composition changes. The abundant subcommunities exhibited a much wider niche breath than the rare subcommunities. Regarding the abundant subcommunity species changes, the contributions of the turnover and nestedness varied with the water depth and oceanic region. In contrast, turnover was the major ß-diversity component regarding the changes in the rare species. These data improve our understanding of the ecological processes of bacterial community assemblages in the NEIO.

Microbial community structures and important taxa across oxygen gradients in the Andaman Sea and eastern Bay of Bengal epipelagic waters.

In oceanic oxygen minimum zones (OMZs), the abundances of aerobic organisms significantly decrease and energy shifts from higher trophic levels to microorganisms, while the microbial communities become critical drivers of marine biogeochemical cycling activities. However, little is known of the microbial ecology of the Andaman Sea and eastern Bay of Bengal (BoB) OMZs. In the present study, a total of 131 samples which from the Andaman Sea and eastern BoB epipelagic waters were analyzed. The microbial community distribution patterns across oxygen gradients, including oxygenic zones (OZs, dissolved oxygen [DO] ≥ 2 mg/L), oxygen limited zones (OLZs, 0.7 mg/L < DO < 2 mg/L), and OMZs (DO ≤ 0.7 mg/L), were investigated. Mantel tests and Spearman's correlation analysis revealed that DO was the most important driver of microbial community structures among several environmental factors. Microbial diversity, richness, and evenness were highest in the OLZs and lowest in the OZs. The microbial community compositions of OZ and OMZ waters were significantly different. Random forest analysis revealed 24 bioindicator taxa that differentiated OZ, OLZ, and OMZ water communities. These bioindicator taxa included Burkholderiaceae, HOC36, SAR11 Clade IV, Thioglobaceae, Nitrospinaceae, SAR86, and UBA10353. Further, co-occurrence network analysis revealed that SAR202, AEGEAN-169, UBA10353, SAR406, and Rhodobacteraceae were keystone taxa among the entire interaction network of the microbial communities. Functional prediction further indicated that the relative abundances of microbial populations involved in nitrogen and sulfur cycling were higher in OMZs. Several microbial taxa, including the Thioglobaceae, Nitrospinaceae, SAR202, SAR406, WPS-2, UBA10353, and Woeseiaceae, may be involved in nitrogen and/or sulfur cycling, while also contributing to oxygen consumption in these waters. This study consequently provides new insights into the microbial community structures and potentially important taxa that contribute to oxygen consumption in the Andaman Sea and eastern BoB OMZ.