Spatial Variability of Picoeukaryotic Communities in the Mariana Trench

Abstract

Picoeukaryotes play prominent roles in the biogeochemical cycles in marine ecosystems. However, their molecular diversity studies have been confined in marine surface waters or shallow coastal sediments. Here, we investigated the diversity and metabolic activity of picoeukaryotic communities at depths ranging from the surface to the abyssopelagic zone in the western Pacific Ocean above the north and south slopes of the Mariana Trench. This was achieved by amplifying and sequencing the V4 region of both 18S ribosomal DNA and cDNA using Illumina HiSeq sequencing. Our study revealed: (1) Four super-groups (i.e., Alveolata, Opisthokonta, Rhizaria and Stramenopiles) dominated the picoeukaryote assemblages through the water column, although they accounted for different proportions at DNA and cDNA levels. Our data expand the deep-sea assemblages from current bathypelagic to abyssopelagic zones. (2) Using the cDNA-DNA ratio as a proxy of relative metabolic activity, the highest activity for most subgroups was usually found in the mesopelagic zone; and (3) Population shift along the vertical scale was more prominent than that on the horizontal differences, which might be explained by the sharp physicochemical gradients along the water depths. Overall, our study provides a better understanding of the diversity and metabolic activity of picoeukaryotes in water columns of the deep ocean in response to varying environmental conditions.

Figure 1

Map of the sampling stations located on the North, Center and South slopes of the Mariana Trench, where samples were collected during a cruise in 2016.

Figure 2

The Shannon index for the samples collected from the different depths, which are grouped according to the three stations (North, Center and South), as revealed by DNA and cDNA.

Figure 3

Picoeukaryotic quantification obtained by qPCR from the three stations in the Mariana Trench with (A) the DNA and (B) the cDNA datasets. ‘Deepest’ represents the deepest water at three stations; thus 5908 m, 5405 m and 5900 m at the North, South and Center stations, respectively.

Figure 4

The relative abundance of the various picoeukaryotic communities in the water column at the three stations, as revealed by the (A) DNA and (B) cDNA datasets. DN, DC and DS indicate DNA at the North, Center and South stations, respectively, while CN, CC and CS indicate cDNA at the North, Center and South stations, respectively. In each case, the numbers indicate the water depth.

Figure 5

UPGMA cluster diagram of the Bray-Curtis similarities calculated from square-root transformed relative OTU abundances for the (A) DNA and (B) cDNA datasets.

Figure 6

Correspondence canonical analysis (CCA) biplots showing the variable composition of picoeukaryotes in relation to important environmental factors at the three stations in the Mariana Trench in the (A) DNA and (B) cDNA datasets.