Abstract
Little is known about diversity distribution and community structure of ciliates in mesopelagic waters, especially how they are related to spatial and temporal changes. Here, an integrative approach, combining high-throughput cDNA sequencing and quantitative protargol stain, was used to analyze ciliate communities collected temporally along a transect from coastal to oceanic regions at depths ranging from the surface to 1000 m. The mesopelagic zone exhibited comparable alpha diversity to surface water which was consistent over temporal variation, with high diversity occurring at the interface with the euphotic zone. Comparison with the northeastern and the western Pacific Ocean revealed consistency of this vertical distribution of ciliates across oceanic basins. Mesopelagic ciliates harbored distinct community structure without significant seasonal differences, with the vertical variations driven largely by members of the classes Spirotrichea and Oligohymenophorea. Operational taxonomic units (OTUs) affiliated with Scuticociliatia, Astomatida and Apostomatida, members of which are known to be bacterivorous and/or commensal/parasitic species, were more abundant in mesopelagic waters than above, implying they are an important component of food webs in the mesopelagic zone. A combination of depth, geographic distance and environment shaped the ciliate communities, with depth being the most influential factor. Phylogenetic null modeling analysis further indicated that 57.1 and 33.3% of mesopelagic community variation was governed by dispersal limitation and heterogeneous selection, respectively, probably due to the marked biochemical and physical gradients down the water column. This suggests that ciliate community structure in the mesopelagic zone is mainly controlled by stochastic processes. Collectively, this study reports mesopelagic ciliates exhibited high diversity and distinct community structure across spatiotemporal scales and informs the processes mediating ciliate assembly in the mesopelagic zone. These should be fully considered in future studies to build a more comprehensive understanding of mesopelagic microbial assemblages.
Highlights
The mesopelagic realm is a vast, dimly lit region that spans the globe between about 200–1,000 m beneath the ocean’s surface
The ciliate alpha-diversity declined with depth less steeply than biomass and abundance below the euphotic zone in the Amundsen and Bellingshausen Seas (Wickham et al, 2011)
A lower abundance of ciliate OTUs was found in the waters below than above the euphotic zone in North Pacific Ocean, with alpha-diversity decreasing with increasing depth (Countway et al, 2010)
Summary
The mesopelagic realm is a vast, dimly lit region that spans the globe between about 200–1,000 m beneath the ocean’s surface It is the zone where 90% of the organic carbon exported from epipelagic waters is remineralized (Robinson et al, 2010). Despite limited studies, contrasting findings have been reported in terms of ciliate diversity distribution (Wickham et al, 2011; Grattepanche et al, 2016) Both microscopy- and 18S rRNA gene clone library-based studies showed that ciliate abundance and species richness decrease with the increasing water depth, the low abundance of prokaryotes as a food resource in the waters below the euphotic zone being cited as the most likely cause (Tanaka and Rassoulzadegan, 2002; Countway et al, 2010; Wickham et al, 2011). In order to resolve this discrepancy, studies that employ a combination of both morphology- and molecular-based methods to analyze ciliate diversity down the water column over spatial and temporal scales are required
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