Can pelagic ciliates indicate vertical variation in the water quality status of western Arctic pelagic ecosystems?

Insight into the latitudinal gradient of biodiversity based on spatial variations in pelagic ciliate communities along the western Arctic Ocean

The latitudinal dynamics of biodiversity has been the focus of global attention. This study is based on the latitude gradient of biodiversity in the spatial changes of pelagic ciliate communities in the western Arctic Ocean. The gradient pattern of pelagic ciliate communities across four latitudes were investigated from the water surface at 22 sampling station in the northern Bering Sea of the western Arctic Ocean and Chukchi Sea from August 5 to August 24, 2016. Based on multivariate analyses, the results showed that (1) the spatial patterns of pelagic ciliates represented a significant latitudinal gradient along the western Arctic Ocean; (2) the species number and abundance of pelagic ciliate communities declined from 64°N to 80°N; (3) variations in the horizontal distribution of ciliates were significantly correlated with changes in physicochemical variables, especially water temperature and Chl a; Thus it is suggested that the expected latitudinal decline of biodiversity was evident along the western Arctic Ocean.

Full-depth vertical distribution of planktonic ciliates (Ciliophora) and a novel bio-index for indicating habitat suitability of tintinnid in the Arctic Ocean

Despite the planktonic ciliate importance in the microzooplankton compartment, their full-depth vertical distribution in the Arctic Ocean was poorly documented as well as the related variations in different water masses. The full-depth community structure of planktonic ciliates was investigated in the Arctic Ocean during summer 2021. The ciliate abundance and biomass decreased rapidly from 200 m to bottom. Five water masses were identified throughout the water column and each one exhibited a unique ciliate community structure. Aloricate ciliates were singled out as the dominant group with average abundance proportion to total ciliates at each depth >95%. Large (>30 μm) and small (10-20 μm) size-fractions of aloricate ciliates were abundant in shallow and deep waters, respectively, which revealed an anti-phase relationship in vertical distribution. Three new record tintinnid species were found during this survey. Pacific-origin species Salpingella sp.1 and Arctic endemic species Ptychocylis urnula occupied the top abundance proportion in the Pacific Summer Water (44.7%) and three water masses (≥38.7%, Mixed Layer Water, Remnant Winter Water, Atlantic-origin Water), respectively. The habitat suitability of tintinnid abundant species was characterised by the Bio-index revealing a distinct death-zone for each species. Variations in survival habitat of abundant tintinnids can be regarded as indicators for the future Arctic climate change. These results provide fundamental data on the microzooplankton response to the intrusion of Pacific waters into the Arctic Ocean upon its rapid warming.

Spatial dynamics of active microeukaryotes along a latitudinal gradient: Diversity, assembly process, and co-occurrence relationships

Recent global warming is profoundly and increasingly influencing the Arctic ecosystem. Understanding how microeukaryote communities respond to changes in the Arctic Ocean is crucial for understanding their roles in the biogeochemical cycles of nutrients and elements. Between July 22 and August 19, 2016, during cruise ARA07, seawater samples were collected along a latitudinal transect extending from the East Sea of Korea to the central Arctic Ocean. Environmental RNA was extracted and the V4 hypervariable regions of the reverse transcribed SSU rRNA were amplified. The sequences generated by high throughput sequencing were clustered into zero-radius OTUs (ZOTUs), and the taxonomic identities of each ZOTU were assigned using SINTAX against the PR2 database. Thus, the diversity, community composition, and co-occurrence networks of size fractionated microeukaryotes were revealed. The present study found: 1) the alpha diversity of pico- and nano-sized microeukaryotes showed a latitudinal diversity gradient; 2) three distinct communities were identified, i.e., the Leg-A, Leg-B surface, and Leg-B subsurface chlorophyll a maximum (SCM) groups; 3) distinct network structure and composition were found in the three groups; and 4) water temperature was identified as the primary factor driving both the alpha and beta diversities of microeukaryotes. This study conducted a comprehensive and systematic survey of active microeukaryotes along a latitudinal gradient, elucidated the diversity, community composition, co-occurrence relationships, and community assembly processes among major microeukaryote assemblages, and will help shed more light on our understanding of the responses of microeukaryote communities to the changing Arctic Ocean.

Hydrographic Feature Variation Caused Pronounced Differences in Planktonic Ciliate Community in the Pacific Arctic Region in the Summer of 2016 and 2019

Planktonic ciliates are an important component of microzooplankton, but there is limited understanding of their responses to changing environmental conditions in the Pacific Arctic Region. We investigated the variations of ciliate community structure and their relationships with environmental features in the Pacific Arctic Region in the summer of 2016 and 2019. The Pacific water was warmer and more saline in 2019 than in 2016. The abundance and biomass of total ciliate and aloricate ciliate were significantly higher in 2019 than those in 2016, while those of tintinnid were significantly lower. The dominant aloricate ciliate changed from large size-fraction (> 30 μm) in 2016 to small size-fraction (10-20 μm) in 2019. More tintinnid species belonging to cosmopolitan genera were found in 2019 than in 2016, and the distribution of tintinnid species (Codonellopsis frigida, Ptychocylis obtusa, and Salpingella sp.1) in 2019 expanded by 5.9, 5.2, and 8.8 degrees further north of where they occurred in 2016. The environmental variables that best-matched tintinnid distributions were temperature and salinity, while the best match for aloricate ciliate distributions was temperature. Therefore, the temperature might play a key role in ciliate distribution. These results provide basic data on the response of the planktonic ciliate community to hydrographic variations and implicate the potential response of microzooplankton to Pacification as rapid warming progresses in the Pacific Arctic Region.

Trophic Enrichment Factors of Carbon and Nitrogen Isotopic Ratios (Δ13C and Δ15N) in Four Marine Ciliates

Understanding the magnitude and causes of isotopic fractionation between organisms and their dietary resources is crucial for gaining knowledge on stable isotope ecology. However, little is known regarding the diet-tissue fractionation values of marine ciliates, which play a critical role in the reconstruction of microbial food webs. In the present study, we conducted experiments on two benthic (Pseudokeronopsis pararubra and Protocruzia labiata) and two pelagic (Strombidium sulcatum and Uronemella filificum) marine ciliates, where they were fed with isotopically constant foods (Chaetoceros calcitrans and Isochrysis galbana) under laboratory culture conditions to determine their carbon and nitrogen isotopic fractionation values (Δ¹³C and Δ¹⁵N). The stable isotope values (δ¹³C and δ¹⁵N) of ciliates for all experiments rapidly increased after the initial feeding, with half-lives ranging from 6.1 to 23.0h for δ¹³C and from 3.1 to 24.9h for δ¹⁵N. The Δ¹³C and Δ¹⁵N for all ciliates represented significantly positive enrichments, with overall mean fractionations of 0.6±0.2 and 1.2±0.4, respectively. Irrespective of the dietary type, both Δ¹³C and Δ¹⁵N were very similar for the same ciliate species. These results suggest that Δ¹³C and Δ¹⁵N for marine ciliates are similar to those found in common marine organisms with very little food-dependent variation. Overall, quantifying the specific isotopic fractionation of marine ciliates is expected to provide fundamental information on the trophic transfer of carbon, nitrogen, and energy flow through the microbial pathway in marine ecosystems.

Notes on the Occurrence of Tintinnid Ciliates, and the Nasselarian Radiolarian Amphimelissa setosa of the Marine Microzooplankton, in the Chukchi Sea (Arctic Ocean) Sampled each August from 2011 to 2020

Here we summarize the results from 10 cruises in the Chukchi Sea, in August, each year from 2011 to 2020. Samples for the qualitative analysis of the microzooplankton were obtained from stations located across the Chukchi Sea using a 20μm plankton net. Conditions encountered, in terms of sea ice coverage and chlorophyll concentrations, varied widely from year to year without any obvious relationship with the composition of the microzooplankton assemblage. Examining a total of 242 samples gathered, we found a total of 44 tintinnid species (morphologically distinct forms). Plotting cumulative number of tintinnid species encountered vs cumulative number of samplings gave a typical species accumulation curve showing no sign of saturation suggesting that continued sampling in the Chukchi Sea will likely yield increases in the tintinnid species catalogue. The tintinnid species found ranged widely in lorica opening diameters (LOD) from about 11 μm to 80 μm in diameter. However, the median size of the LOD of the tintinnid assemblages varied little from year to year ranging only from about 30 μm to 40 μm. Most of the forms encountered were found in samples from only 1 or 2 cruises. Very few forms were found every year throughout the 10 years of sampling. These were 5 species of tintinnids (Acanthostomella norvegica, Leprotintinnus pellucidus, Pytchocylis obtusa, Salpingella acuminata, Salpingella faurei) and the nasselarian radiolarian Amphimelissa setosa. Examples of the morphological variability observed among individuals of Acanthostomella norvegica and Pytchocylis obtusa within single samples are shown with some individuals easily confused with forms described as other species are shown. To our knowledge, our data are the most extensive data set on Chukchi Sea microplankton. We provide all of the data recorded, which may serve as a baseline from which to assess changes projected in Arctic Sea systems, in a supplementary data file.

Use of a broad β-diversity measure of pelagic ciliate communities for assessing vertical heterogeneity of water columns in the Pacific Arctic Ocean

Multivariate dispersion has proven to be a broad β-diversity measure that shows the heterogeneity of environmental conditions. The dispersion patterns of pelagic ciliate communities were investigated at eight water depths in the northern Bering Sea of the western Arctic Ocean and Chukchi Sea. Multivariate analysis indicated that (1) pelagic ciliates showed significant variability in multivariate dispersion on a vertical scale, (2) dispersion patterns were shaped by both the species composition and individual abundance, (3) vertical variation in species occurrence was significantly related to nutrients and chlorophyll a, and (4) the dispersion measures at both species occurrence and species abundance resolutions were significantly negatively related to salinity and dissolved oxygen. This suggests that multivariate dispersion measures driven by both species composition and the individual abundance of pelagic ciliates may be a useful indicator of environmental heterogeneity in marine ecosystems.