Ciliates in different types of pools in temperate, tropical, and polar climate zones – implications for climate change

Small water bodies are typically characterized by high diversity of various groups of microorganisms. Moreover, these ecosystems react very quickly to even the slightest climate changes (e.g. a temperature increase or water level fluctuations). Thus far, studies of planktonic ciliates in small water bodies having different origins and located in various climate zones have been scarce. Our study aimed to verify the following hypotheses: planktonic ciliate assemblages exhibit higher diversity in pools with higher concentrations of biogenic compounds; pools in warmer climates have higher biodiversity of planktonic ciliates than those in the polar climate zone; individual functional groups of ciliates demonstrate considerable diversity, both between individual pool types and between climate zones. The study was conducted in 21 small pools in temperate, tropical, and polar climate zones. While the type of pool clearly influenced the makeup of microbial communities, the influence of climate was stronger. The factors with the greatest influence on the occurrence of these microorganisms were temperature, total organic carbon, and nutrients. Our results show that in warmer climates the abundance of bacterivorous ciliates is higher, while that of mixotrophs is lower. This has consequences for modelling of climate change and assessment of its influence on the carbon cycle in small water body ecosystems.

Spatiotemporal distribution of planktonic ciliates in the Kuybyshev water reservoir in the area of Klimovskaya narrowing

The paper attempts to link changes in quantitative characteristics of the community of free-living planktonic ciliates and the nature of their vertical distribution with the operation mode of the hydroelectric power station in the reservoir. Low values of the abundance (8–1,560 thousand cells / m3) and biomass (0.1–66.9 mg / m3) of ciliates in the water column, and a fairly wide range of their variation in the reservoir are probably due to the oscillatory nature of the hydrological regime in the regulated river conditions. The vertical distribution of free-living planktonic ciliates of the near-dam section of the Kuibyshev reservoir was studied at all times of the day on all days of the week. The species were distributed throughout the water column to the bottom with some preference for the surface (0–2 m) and medium (4–10 m) layers. There was no clear attachment of individual species to certain layers in the conditions of homothermy. The preference for certain horizons is due to some biological characteristics of the species (for example, peritriches on the coenobia of algae in the surface layers), departure from “blooming spots”, etc. No data in favor of vertical migrations or “turbulent transport” has yet been revealed. The influence of the weekly regime of regulation of the Zhigulevskaya HPS, in which at the end of the working week, the fluctuation amplitude of many hydrological parameters probably decreases, is manifested in an increase in the abundance and biomass of planktonic ciliates at this time.The nature of the daily change in the average ciliates abundance in the water column was the same for a variant of the community in working days and was in antiphase with the change in the community abundance on weekends, namely: the maximum number on working days and on weekends was recorded in the evening (20.00) and in the daytime (14.00), respectively.

Genetic Diversity in Marine Planktonic Ciliates (Alveolata, Ciliophora) Suggests Distinct Geographical Patterns – Data From Chinese and European Coastal Waters

Unraveling geographic distribution patterns of planktonic protists is a central goal in marine microbial ecology. Using a novel combination of recently developed phylogenetic and network analyses on a V4 18S rDNA metabarcoding dataset, we here analyzed the genetic diversity of marine planktonic ciliate communities in Chinese and European coastal waters. Thereby, our approach provided an unprecedented perspective on geographic patterns inferred from ciliate genetic diversity and accomplished a very fine resolution down to single nucleotides within operational taxonomic units (OTUs). While most OTUs (87%) exclusively contained sequences of either Chinese or European origin, those OTUs detected in both regions comprised the vast majority of reads (84%). Phylogenetic analyses of OTUs belonging to the same taxon revealed genetically distinct clades that were geographically restricted to either Chinese or European coastal waters. The detection of signature nucleotides emphasized this genetic distinction of Chinese and European clades. Second-level clustering of OTUs and reference sequences in two selected taxa (the oligotrichid Spirotontonia and the tintinnid Tintinnidium) revealed the presence of several potentially new species or ones lacking genetic reference data. Geographic patterns were also discovered by network analyses within 700 widespread and abundant OTUs; in 77 of these OTUs, European and Chinese sequences formed significantly assortative groups. These assortative groupings indicated a higher genetic similarity among sequences from the same region than between sequences from different regions. Our results demonstrate that detailed analyses of metabarcoding data down to single nucleotide differences expand our perception of geographical distribution patterns and provide insights into historic and ongoing effective dispersal in protists. The congruent discovery of geographic patterns at different levels of resolution (between and within OTUs) suggests that cosmopolitan distribution in marine planktonic ciliates is less common than previously postulated.

Temporal Variability of Planktonic Ciliates in a Coastal Oligotrophic Environment: Mixotrophy, Size Classes and Vertical Distribution

The temporal variability of planktonic ciliates was studied in an extreme oligotrophic environment with special focus on trophic modes and size classes. Abundance, biomass, size classes, mixotrophy vs. heterotrophy, and species composition of planktonic ciliates were investigated focusing on temporal (samples collected on a monthly basis during 2019) and vertical (7 depth layers in the euphotic zone, from surface to 120 m) distribution at a coastal station in the oligotrophic Eastern Mediterranean. Abundance was in general very low (20 to 1150 cells L–1), except for September, which presented the highest abundance and biomass. Aloricate species dominated the ciliate community in all months and depths (% contribution from 77% in September to 99% in April). In general, oligotrichs presented maximum abundance at 2–10 m (except for June, July, and November: 100–120 m) whereas choreotrichs were more homogeneously distributed [and showed maxima at deep chlorophyll maximum (DCM)]. Small heterotrophs dominated the ciliate community at all depths and months, on average by 76% (they were 3 times more abundant than mixotrophs in terms of abundance and 2.5 times in terms of biomass). They were equally distributed both vertically and seasonally (and also in terms of size classes). In contrast, mixotrophs were found mainly at the surface layer to 20 m throughout the year, except for June and July (max at 100, 50 m). On average, 63% of integrated aloricate abundance was species <30 μm, of which 25% were mixotrophs. During the stratification period of May to November, the very small (<18 μm) and small (18–30 μm) mixotrophic species were distributed throughout the water column whereas the >50 μm mixotrophic species were found only above the DCM. In contrast, during the mixing period of December to May, mixotrophic ciliates were very few and were dominated by small and medium-sized species. It seems that mixotrophic and heterotrophic ciliates, on one hand, and the four size classes, on the other, are very distinct groups characterized by different distributions both vertically and seasonally. This may have important consequences for the structure and function of the microbial food web of which they are part, as well as for the carbon flow to the higher trophic levels of this oligotrophic environment in which ciliates are the main grazers.