Vertical distribution of size-fractionated bacterial communities in the water column of the Atacama Trench

Variation of vertical distribution of bacterial communities through the whole water column of the trench regions has received more and more attention. However, it is still unclear whether there are unique microbial communities in trenches. The study investigated the bacterial composition and diversity in three size-fractions (0.3-0.7μm, 0.7–2.7μm, and >3μm) from the surface to the hadal zone (7500 m) at one site in the Atacama Trench. The results showed that the alpha diversity changed with depth and was the highest at 6000 m and lowest at 1000 m. For different size fractions, the bacterial diversity was higher in 0.7-2.7μm and >2.7μm fractions than 0.3-0.7μm in the surface water, but the diversity in 0.3-0.7μm fraction was the highest in the hadalpelagic, likely reflecting the depth changes of quantity and bioavailability of organic matter. Firmicutes were the most abundant bacteria (36.3 ± 15.6%) in the water depth, while Alphaproteobacteria dominated in the samples at 7500 m for the 0.3–0.7μm fraction (36.3%), and 7000–7500 m for the 0.7–2.7μm fraction (30.6 ± 1.2%). PCoA analysis indicated that the hadalpelagic bacterial communities were more like those in the overlying deep water, but were distinct from the epipelagic in the study area. Though no significant differences among size fractions of bacterial community were observed in the pelagic Atacama Trench, lifestyle preferences existed that some relied on particulate matter, and some preferred to live freely. This suggested the important influence of surface primary production and export production on the bacterial communities in the deep ocean.