Abstract
BackgroundBenthic ciliates and the environmental factors shaping their distribution are far from being completely understood. Likewise, deep-sea systems are amongst the least understood ecosystems on Earth. In this study, using high-throughput DNA sequencing, we investigated the diversity and community composition of benthic ciliates in different sediment layers of a seamount and an adjacent abyssal plain in the tropical Western Pacific Ocean with water depths ranging between 813 m and 4566 m. Statistical analyses were used to assess shifts in ciliate communities across vertical sediment gradients and water depth.ResultsNine out of 12 ciliate classes were detected in the different sediment samples, with Litostomatea accounting for the most diverse group, followed by Plagiopylea and Oligohymenophorea. The novelty of ciliate genetic diversity was extremely high, with a mean similarity of 93.25% to previously described sequences. On a sediment depth gradient, ciliate community structure was more similar within the upper sediment layers (0-1 and 9-10 cm) compared to the lower sediment layers (19-20 and 29-30 cm) at each site. Some unknown ciliate taxa which were absent from the surface sediments were found in deeper sediments layers. On a water depth gradient, the proportion of unique OTUs was between 42.2% and 54.3%, and that of OTUs shared by all sites around 14%. However, alpha diversity of the different ciliate communities was relatively stable in the surface layers along the water depth gradient, and about 78% of the ciliate OTUs retrieved from the surface layer of the shallowest site were shared with the surface layers of sites deeper than 3800 m. Correlation analyses did not reveal any significant effects of measured environmental factors on ciliate community composition and structure.ConclusionsWe revealed an obvious variation in ciliate community along a sediment depth gradient in the seamount and the adjacent abyssal plain and showed that water depth is a less important factor shaping ciliate distribution in deep-sea sediments unlike observed for benthic ciliates in shallow seafloors. Additionally, an extremely high genetic novelty of ciliate diversity was found in these habitats, which points to a hot spot for the discovery of new ciliate species.
Highlights
Benthic ciliates and the environmental factors shaping their distribution are far from being completely understood
On a water depth gradient, the proportion of unique Operational taxonomic unit (OTU) was between 42.2% (Fig. 1 f ) and 54.3% (Fig. 1 e) and that of shared OTUs only around 14%. 29 OTUs (78%) of the surface layer at the shallow site sample and accounted for 30.1% (S-M) were shared with the surface layers at sites deeper than 3800 m (Fig. 1 e)
Partitioning of diversity revealed that the ciliate community structure was more similar within the upper sediment layers (0-1 and 9-10 cm) compared to the lower sediment layers (19-20 and 29-30 cm) at each site
Summary
Benthic ciliates and the environmental factors shaping their distribution are far from being completely understood. Molecular techniques and sequencing the 18S rDNA as a taxonomic marker were used to investigate the diversity of protists in deep-sea sediments These studies did reveal high protistan diversities in various deep-sea habitats [7, 8], and numerous undescribed taxa and even several early branching eukaryotic lineages [9]. In some of these habitats, ciliates were the predominant and often the most diverse microeukaryotic group [10,11,12,13] Knowledge on their distribution patterns and the major factors enforcing their dispersal in deep-sea sediments are far from complete
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