The photic-aphotic divide is a strong ecological and evolutionary force determining the distribution of ciliates (Alveolata, Ciliophora) in the ocean

Effects of temporal, spatial, and environmental factors on ciliates community in northeastern South China Sea, with notes on co-occurrence patterns of environment, phytoplankton, and ciliate

Little is known about temp-spatial variations and determining mechanism of microplankton community from continental shelf to deep basin. Here, the distribution and determinants of ciliates in the northeastern South China Sea were investigated in summer and winter. We found that the alpha diversity were generally similar, but community composition showed clear difference in summer and winter. For spatial dimension, the alpha diversity significantly decreased from neritic water to deep, and the variations of community composition were only detected between upper and deep water. The determinants of community varied depending on the research scale. For entire community, the variations were higher explained by environmental factors than by temp-spatial factors. Specially, physical factors were the main determinants, which may attribute to the effects of physical processes occurred in the research area. For the three spatial subareas, season showed more significant influences on communities than other factors. Network analyses revealed that the ciliate exhibited more relationships with phytoplankton than with environment factors. Diatom and physical factors accounted for most of the ciliate-phytoplankton-environment co-occurrence relationships, suggesting their significant influences to the community stability of the microbial food web. This study improves our understanding for the mechanisms of community structuring of microzooplankton in the open sea across both space and time.

IMPORTANCE

Ciliates are important components of microplanktons and play a crucial role in global nutrient cycling. There is still large knowledge gap on their structuring mechanisms from continental shelf to deep basin. We revealed the distribution patterns of ciliate community in neSCS and assessed the relative contribution of temporal, spatial, and environmental factors for their community structuring. Compared with temp-spatial factors, environmental factors take more responsibilities for the ciliate distribution. Among the environmental factors, physical factors showed the largest contribution to community variation. Moreover, the ciliate-environment co-occurrence pattern showed that physical factors contributed more for their relationships. These findings suggested that the physical processes play key roles in temp-spatial dynamics of ciliate community in open ocean.

Parasitic taxa are key to the vertical stratification and community variation of pelagic ciliates from the surface to the abyssopelagic zone

BACKGROUND

An increase in upper-ocean thermal stratification is being observed worldwide due to global warming. However, how ocean stratification affects the vertical profile of plankton communities remains unclear. Understanding this is crucial for assessing the broader implications of ocean stratification. Pelagic ciliates cover multiple functional groups, and thus can serve as a model for studying the vertical distribution and functional strategies of plankton in stratified oceans. We hypothesize that pelagic ciliate communities exhibit vertical stratification caused by shifts in functional strategies, from free-living groups in the photic zone to parasitic groups in deeper waters.

RESULTS

306 samples from the surface to the abyssopelagic zone were collected from 31 stations in the western Pacific and analyzed with environmental DNA (the V4 region of 18 S rDNA) metabarcoding of pelagic ciliates. We found a distinct vertical stratification of the entire ciliate communities, with a boundary at a depth of 200 m. Significant distance-decay patterns were found in the photic layers of 5 m to the deep chlorophyll maximum and in the 2,000 m, 3000 m and bottom layers, while no significant pattern occurred in the mesopelagic layers of 200 m - 1,000 m. Below 200 m, parasitic Oligohymenophorea and Colpodea became more prevalent. A linear model showed that parasitic taxa were the main groups causing community variation along the water column. With increasing depth below 200 m, the ASV and sequence proportions of parasitic taxa increased. Statistical analyses indicated that water temperature shaped the photic communities, while parasitic taxa had a significant influence on the aphotic communities below 200 m.

CONCLUSIONS

This study provides new insights into oceanic vertical distribution, connectivity and stratification from a biological perspective. The observed shift of functional strategies from free-living to parasitic groups at a 200 m transition layer improves our understanding of ocean ecosystems in the context of global warming.

Role of Syndiniales parasites in depth-specific networks and carbon flux in the oligotrophic ocean

Microbial associations that result in phytoplankton mortality are important for carbon transport in the ocean. This includes parasitism, which in microbial food webs is dominated by the marine alveolate group, Syndiniales. Parasites are expected to contribute to carbon recycling via host lysis; however, knowledge on host dynamics and correlation to carbon export remain unclear and limit the inclusion of parasitism in biogeochemical models. We analyzed a 4-year 18S rRNA gene metabarcoding dataset (2016-19), performing network analysis for 12 discrete depths (1-1000 m) to determine Syndiniales-host associations in the seasonally oligotrophic Sargasso Sea. Analogous water column and sediment trap data were included to define environmental drivers of Syndiniales and their correlation with particulate carbon flux (150 m). Syndiniales accounted for 48-74% of network edges, most often associated with Dinophyceae and Arthropoda (mainly copepods) at the surface and Rhizaria (Polycystinea, Acantharea, and RAD-B) in the aphotic zone. Syndiniales were the only eukaryote group to be significantly (and negatively) correlated with particulate carbon flux, indicating their contribution to flux attenuation via remineralization. Examination of Syndiniales amplicons revealed a range of depth patterns, including specific ecological niches and vertical connection among a subset (19%) of the community, the latter implying sinking of parasites (infected hosts or spores) on particles. Our findings elevate the critical role of Syndiniales in marine microbial systems and reveal their potential use as biomarkers for carbon export.