Despite numerous studies on marine prokaryotes, the vertical distribution patterns of bacterial community, either on the taxonomic composition or the functional structure, remains relatively unexplored. Using HiSeq-derived 16S rRNA data, the depth-related distribution patterns of taxonomic diversity and functional structure predicted from diversity data in the water column and sediments of the Western Pacific Ocean were explored. The OTU richness declined along the water column after peaking between 100 to 200 m deep. Relative abundance of Cyanobacteria and SAR11 decreased significantly with depth, while Actinobacteria and Gammaproteobacteria increased. This clearly mirrors the vertical distribution pattern of the predicted functional composition with the shift between phototrophic to chemoheterotrophic groups from the surface to the deeper layers. In terms of community composition and functional structure, the epipelagic zone differed from other deeper ones (i.e., meso-, bathy-, and abyssopelagic zones) where no obvious differences were detected. For the epipelagic zone, temperature, dissolved oxygen, and salinity were recognized as the crucial factors shaping both community composition and the functional structure of bacteria. Compared with water samples, benthic sediment samples harbored unexpectedly higher read abundance of Proteobacteria, presenting distinguishable taxonomic and functional compositions. This study provides novel knowledge on the vertical distribution of bacterial taxonomic and functional compositions in the western Pacific.

As decomposers of organic materials as well as pathogens or symbionts of other organisms (Richards et al., 2012), fungi play important roles in nutrient cycling in marine ecosystems (Amend et al., 2019;Hassett et al., 2019;Orsi et al., 2013). Seasonal synchronizations between chytrid blooms and diatom blooms have been observed, indicative of the parasitism between marine chytrids and diatoms in the euphotic zone (Hassett & Gradinger, 2016;Taylor & Cunliffe, 2016). The nontrivial role of fungi in processing marine organic matter in upwelling ecosystems was also demonstrated through the seasonal co-occurrence