Vertical distribution pattern, preferred life strategies and environmental response of prokaryotic microbiome in the...

The eastern tropical Indian Ocean (ETIO) features strong water stratification that forms typical oligotrophic conditions, making it an ideal region to study the environmental response of microbes. However, there is still a lack of understanding the vertical distribution patterns of prokaryotic microbiome with different lifestyles. In this study, we investigated the abundance, diversity, and distribution characteristics of free-living (FL) and particle-associated (PA) microorganisms along depth in the ETIO. Microbial abundance and alpha diversity indices decreased significantly with depth, dropping from 7.45 ± 1.20 to 6.35 ± 0.42 lg copies/L (FL) and from 5.72 ± 0.43 to 5.17 ± 0.43 lg copies/L (PA), respectively. The community composition and network complexity showed significantly vertical differentiation. Several bacteria become depth-specific microbial indicators, i.e., Cyanobacteria and Rhodospirillaceae in the mixed layer (<100m; SL), SAR324 clade and Salinisphaeraceae enriched in the mesopelagic zone (100-1,000m; mesopelagic zone, ML), whereas Acidobacteriota, Gemmatimonadota and Alteromonadaceae in the deep layer (>1,000m; bathypelagic layer, BL). The clustered patterns of microbes were mainly affected by temperature, salinity and dissolved oxygen (DO) in SL, and shifted toward dominance by stochastic processes with increasing depth. Our findings reveal strategies of microbes with different lifestyles to adapt to stratified marine environments through community restructuring, providing a theoretical basis for assessing their ecological roles in biogeochemical cycles.