Microbial population succession along the water columns and sediments in the Diamantina and Kermadec trenches

Microbes are widespread from the marine surface to the hadal zone, physicochemical properties of the environment and microbial community profiles that change with depth, which in turn influences their biogeochemical cycling potential, genomic features and evolutionary adaptations. However, the microbial communities, ecological functions and evolutions along the surface water down to the sediments by metagenomics in the Diamantina and Kermadec trenches are rarely studied. The euphotic waters had a significant higher diversity than the deep-sea waters and sediments. Inter-trench and intra-trench exchange of microbial communities were showed, the exchange of prokaryotic species among different water layers appeared to occur more frequently than that between seawaters and sediments. Moreover, the Diamantina Trench exhibited a remarkable vertical connectivity from the surface waters to deep sea via sinking particles of microbial communities than the Kermadec Trench. Cooperative relationship existed among different microbial groups by the co-occurrence network. Microbial groups have adapted to specific ecological niches at various depths, showing distinct dominant metabolic pathways in carbon fixation, nitrogen, and sulfur cycles. Notably, Rhodobacterales in different habitats exhibited distinct evolutionary adaptation and genomic properties, low ratios of non-synonymous substitutions to synonymous substitutions and high carbon atoms per residue side chain of Rhodobacterales genome in deep-sea sediments, suggested a pronounced purifying selection that was critical for the survival of Rhodobacterales. Our study sheds light on the microbial connectivity in vertical and multiple trenches, metabolic versatility and Rhodobacterales evolutionary adaptation related to genomic features in the deep-sea trench environments