Microbial eukaryotes are key components of the ocean plankton. Yet, our understanding of their community composition and activity in different water layers of the ocean is limited, particularly for picoeukaryotes (0.2–3 µm cell size). Here, we examined the picoeukaryotic communities inhabiting different vertical zones of the tropical and subtropical global ocean: surface, deep chlorophyll maximum, mesopelagic (including the deep scattering layer and oxygen minimum zones), and bathypelagic. Communities were analysed by high-tthroughput sequencing of the 18S rRNA gene (V4 region) as represented by DNA (community structure) and RNA (metabolism), followed by delineation of Operational Taxonomic Units (OTUs) at 99% similarity. We found a stratification of the picoeukaryotic communities along the water column, with assemblages corresponding to the sunlit and dark ocean. Specific taxonomic groups either increased (e.g., Chrysophyceae or Bicosoecida) or decreased (e.g., Dinoflagellata or MAST-3) in abundance with depth. We used the rRNA:rDNA ratio of each OTU as a proxy of metabolic activity. The highest relative activity was found in the mesopelagic layer for most taxonomic groups, and the lowest in the bathypelagic. Altogether, we characterize the change in community structure and metabolic activity of picoeukaryotes with depth in the global ocean, suggesting a hotspot of activity in the mesopelagic.

/pdf/marked-changes-in-diversity-and-relative-activity-of-zepvmkdy7e.pdf

Marked changes in diversity and relative activity of picoeukaryotes with depth in the world ocean.

Are We Overestimating Protistan Diversity in Nature

Free-living amoebae are protists frequently found in water and soils. They feed on other microorganisms, mainly bacteria, and digest them through phagocytosis. It is accepted that these amoebae play an important role in the microbial ecology of these environments. There is a renewed interest for the free-living amoebae since the discovery of pathogenic bacteria that can resist phagocytosis and of giant viruses, underlying that amoebae might play a role in the evolution of other microorganisms, including several human pathogens. Recent advances, using molecular methods, allow to bring together new information about free-living amoebae. This review aims to provide a comprehensive overview of the newly gathered insights into (1) the free-living amoeba diversity, assessed with molecular tools, (2) the gene functions described to decipher the biology of the amoebae and (3) their interactions with other microorganisms in the environment.

Free-living amoebae and squatters in the wild: ecological and molecular features.

Time-series can provide critical insights into the structure and function of microbial communities. The analysis of temporal data warrants statistical considerations, distinct from comparative microbiome studies, to address ecological questions. This primer identifies unique challenges and approaches for analyzing microbiome time-series. In doing so, we focus on (1) identifying compositionally similar samples, (2) inferring putative interactions among populations, and (3) detecting periodic signals. We connect theory, code and data via a series of hands-on modules with a motivating biological question centered on marine microbial ecology. The topics of the modules include characterizing shifts in community structure and activity, identifying expression levels with a diel periodic signal, and identifying putative interactions within a complex community. Modules are presented as self-contained, open-access, interactive tutorials in R and Matlab. Throughout, we highlight statistical considerations for dealing with autocorrelated and compositional data, with an eye to improving the robustness of inferences from microbiome time-series. In doing so, we hope that this primer helps to broaden the use of time-series analytic methods within the microbial ecology research community.

/pdf/a-primer-for-microbiome-time-series-analysis-22fs6fmtuy.pdf

A Primer for Microbiome Time-Series Analysis.

Remote deep-ocean sediment (DOS) ecosystems are among the least explored biomes on Earth. Genomic assessments of their biodiversity have failed to separate indigenous benthic organisms from sinking plankton. Here, we compare global-scale eukaryotic DNA metabarcoding datasets (18S-V9) from abyssal and lower bathyal surficial sediments and euphotic and aphotic ocean pelagic layers to distinguish plankton from benthic diversity in sediment material. Based on 1685 samples collected throughout the world ocean, we show that DOS diversity is at least threefold that in pelagic realms, with nearly two-thirds represented by abundant yet unknown eukaryotes. These benthic communities are spatially structured by ocean basins and particulate organic carbon (POC) flux from the upper ocean. Plankton DNA reaching the DOS originates from abundant species, with maximal deposition at high latitudes. Its seafloor DNA signature predicts variations in POC export from the surface and reveals previously overlooked taxa that may drive the biological carbon pump.

https://www.science.org/doi/pdf/10.1126/sciadv.abj9309?download=true

Patterns of eukaryotic diversity from the surface to the deep-ocean sediment

To study the diversity and metabolic activity of microbial eukaryotes in the water column of the South China Sea, genomic DNA and RNA were co-extracted from samples collected down to bathyal depth at two sites. V9 regions of both SSU rRNA gene and its transcript (cDNA) were amplified and sequenced using high throughput sequencing. Our study revealed: (1) DNA and RNA datasets showed significant differences in microbial eukaryote community composition, with the variability between the two datasets for the same sample exceeding that between samples within each dataset, indicating that nucleic acid source overrode environmental factors in determining the composition of microeukaryotes; (2) despite the differences in community composition between the two datasets, both DNA and RNA revealed similar depth-related distribution patterns of microbial eukaryotes; (3) using the ratio of RNA: DNA as a proxy of relative metabolic activity, a depth-related pattern was found for the relative metabolic activity of some but not all groups of microbial eukaryotes, with the highest activity for the groups with depth-related pattern usually found in the middle water layers; and (4) the presence of live and active photoautotrophic microbial eukaryotes in the deep ocean was confirmed, indicating that they play an important role in controlling the deep-sea organic carbon pool. Overall, our study sheds light on the diversity and activity of microbial eukaryotes in the water column of a tropical oligotrophic ocean and their potential contributions in the downward transportation of organic material from the surface ocean to the deep via the biological pump.

/pdf/microbial-eukaryote-diversity-and-activity-in-the-water-2gwft2udan.pdf

Microbial Eukaryote Diversity and Activity in the Water Column of the South China Sea Based on DNA and RNA High Throughput Sequencing.

Changes in community structure of active protistan assemblages from the lower Pearl River to coastal Waters of the South China Sea.

Phototrophic microbial eukaryotes dominate primary production over large oceanic regions. Due to their small sizes and slow sinking rates, it is assumed they contribute relatively little to the downward export of organic carbon via the biological pump. Therefore, the community structure of phototrophic cells in the deep ocean has long been overlooked and remains largely unknown. In this study, we used an integrative approach, including epifluorescence microscopy, sequencing of 18S rRNA and photosystem-II psbA gene transcripts, to investigate phototrophic microbial eukaryotes in samples collected from the tropical Western Pacific Ocean. It was found that: (i) pigmented nano-sized eukaryotes (PNEs) are ubiquitous in the deep Western Pacific Ocean down to 5000 m depth; (ii) the PNE community is dominated by cells 2-5 μm in size; (iii) their abundance is significant, averaging 4 ± 1 (± s.e.) cells ml-1 in waters below 1000 m which is comparable to that of heterotrophic nanoflagellates; (iv) the active pigmented microbial eukaryotes in the deep waters are highly diverse and dominated by Haptophyta followed by Chlorophyta and Bacillariophyta; (v) PNEs in deep waters were likely transported from surface ocean by various fast-sinking mechanisms, thus contributing to the biological pump and fuelling the deep-sea communities by supplying fresh organic carbon.

/pdf/pigmented-microbial-eukaryotes-fuel-the-deep-sea-carbon-pool-4d98v34sv7.pdf

Pigmented microbial eukaryotes fuel the deep sea carbon pool in the tropical Western Pacific Ocean

Marine protists are essential for globally critical biological processes, including the biogeochemical cycles of matter and energy. However, compared with their prokaryotic counterpart, it remains largely unclear how environmental factors determine the diversity and distribution of the active protistan communities on the regional scale. In the present study, the biodiversity, community composition, and potential drivers of the total, abundant, and rare protistan groups were studied using high throughput sequencing on the V9 hyper-variable regions of the small subunit ribosomal RNA (SSU rRNA) along an estuary to basin transect in the northern South China Sea. Overall, Bacillariophyta and Cercozoa were abundant in the surface water; heterotrophic protists including Spirotrichea and marine stramenopiles 3 (MAST-3) were more abundant in the subsurface waters near the heavily urbanized Pearl River estuary; Chlorophyta and Pelagophyceae were abundant at the deep chlorophyll maximum depth, while Hacrobia, Radiolaria, and Excavata were the abundant groups in the deep water. Salinity, followed by water depth, temperature, and other biological factors, were the primary factors controlling the distinct vertical and horizontal distribution of the total and abundant protists. Rare taxa were driven by water depth, followed by temperature, salinity, and the concentrations of PO43-. The active protistan communities were mainly driven by dispersal limitation, followed by drift and other ecological processes.

https://www.mdpi.com/2076-2607/9/2/351/pdf

Biogeographical Distribution and Community Assembly of Active Protistan Assemblages Along an Estuary to a Basin Transect of the Northern South China Sea.

Armophorean ciliates constitute an important component of microeukaryotic community in anaerobic or hypoxic environments. Yet, their diversity remains poorly known due to under-sampling or the scarcity of knowledge. In this study, three metopid ciliates, i.e., Metopus paraes sp. n., Metopus spiculatus sp. n., and Metopus parapellitus sp. n., collected from coastal sediments in Qingdao and Rizhao, China, were investigated using live observation, protargol staining, and molecular phylogenetic methods. M. paraes sp. n. can be distinguished by its long caudal cilia. M. spiculatus sp. n. resembles M. vestitus in many ways, but differs mainly in having a beak-like preoral dome end and a conspicuous tail. The most remarkable features of M. parapellitus sp. n. include an ovate body shape, caudal cilia located at the rear end and right posterior body, and an adoral zone that never extends onto the dorsal surface. Sequence divergences supported the species identification of these three species. Phylogenetic analyses confirmed that the Metopus is not monophyletic, and first revealed that all marine species of Metopus form a well-supported clade. The clustering of these marine forms with M. es (type species) is not rejected by the AU test, which infers that the marine clade represents the genus Metopus together with M. es.

/pdf/new-contribution-to-the-diversity-of-the-anaerobic-genus-q8sgwqaj.pdf

Ran Li

Papers

Microbial Eukaryote Diversity and Activity in the Water Column of the South China Sea Based on DNA and RNA High Throughput Sequencing.

Changes in community structure of active protistan assemblages from the lower Pearl River to coastal Waters of the South China Sea.

Pigmented microbial eukaryotes fuel the deep sea carbon pool in the tropical Western Pacific Ocean

Biogeographical Distribution and Community Assembly of Active Protistan Assemblages Along an Estuary to a Basin Transect of the Northern South China Sea.

New Contribution to the Diversity of the Anaerobic Genus Metopus (Ciliophora, Armophorea), With Descriptions of Three New Marine Species