Heterotrophic Nanoflagellates Research Articles

Ecology of aerobic anoxygenic phototrophs on a fine-scale taxonomic resolution in Adriatic Sea unravelled by unsupervised neural network

BackgroundAerobic anoxygenic phototrophs are metabolically highly active, diverse and widespread polyphyletic members of bacterioplankton whose photoheterotrophic capabilities shifted the paradigm about simplicity of the microbial food chain. Despite their considerable contribution to the transformation of organic matter in marine environments, relatively little is still known about their community structure and ecology at fine-scale taxonomic resolution. Up to date, there is no comprehensive (i.e. qualitative and quantitative) analysis of their community composition in the Adriatic Sea.ResultsAnalysis was based on pufM gene metabarcoding and quantitative FISH-IR approach with the use of artificial neural network. Significant seasonality was observed with regards to absolute abundances (maximum average abundances in spring 2.136 ± 0.081 × 104 cells mL−1, minimum in summer 0.86 × 104 cells mL−1), FISH-IR groups (Roseobacter clade prevalent in autumn, other Alpha- and Gammaproteobacteria in summer) and pufM sequencing data agglomerated at genus-level. FISH-IR results revealed heterogeneity with the highest average relative contribution of AAPs assigned to Roseobacter clade (37.66%), followed by Gammaproteobacteria (35.25%) and general Alphaproteobacteria (31.15%). Community composition obtained via pufM sequencing was dominated by Gammaproteobacteria clade NOR5/OM60, specifically genus Luminiphilus, with numerous rare genera present in relative abundances below 1%. The use of artificial neural network connected this community to biotic (heterotrophic bacteria, HNA and LNA bacteria, Synechococcus, Prochlorococcus, picoeukaryotes, heterotrophic nanoflagellates, bacterial production) and abiotic environmental factors (temperature, salinity, chlorophyll a and nitrate, nitrite, ammonia, total nitrogen, silicate, and orthophosphate concentration). A type of neural network, neural gas analysis at order-, genus- and ASV-level, resulted in five distinct best matching units (representing particular environments) and revealed that high diversity was generally independent of temperature, salinity, and trophic status of the environment, indicating a potentially dissimilar behaviour of aerobic anoxygenic phototrophs compared to the general bacterioplankton.ConclusionThis research represents the first comprehensive analysis of aerobic anoxygenic phototrophs in the Adriatic Sea on a trophic gradient during a year-round period. This study is also one of the first reports of their genus-level ecology linked to biotic and abiotic environmental factors revealed by unsupervised neural network algorithm, paving the way for further research of substantial contribution of this important bacterial functional group to marine ecosystems.

Vertical and latitudinal distribution of bottom-up and top-down controls of heterotrophic prokaryotes in the Red Sea

Heterotrophic prokaryotes (HPs) represent the largest fraction of living biomass in the ocean. Comprehensively understanding the spatio-temporal variability of their controlling factors remains a challenge in microbial oceanography, especially in little explored low latitude regions such as the Red Sea, one of the hottest and saltiest basins on Earth. In this study, we assessed the vertical (5–1000 m) and latitudinal (16°-27° N) variations in HPs and their bottom-up (resource availability) and top-down controls (protistan grazing and viral lysis) at eight stations along the Red Sea, in three cruises carried out between 2017 and 2019. The decrease in HPs abundances with depth was less pronounced than that of heterotrophic nanoflagellates (HNFs) and viruses. We found that inorganic nutrient and dissolved organic carbon (DOC) concentrations do not vary significantly from north to south, thus suggesting a similar bottom-up control on HPs abundances along the latitudinal gradient. We found significant southward increase in the HP:HNF ratio (r = 0.56, p < 0.0001, n = 140), suggesting that HNFs have a lower impact on their HPs prey in the southern Red Sea. The preference of HNFs for larger HPs cells with depth was found only in the spring cruise. Viral abundances do not show any marked latitudinal gradient but show a significant positive relationship with HPs abundances in the water column in all seasons. The higher linear regression slope found in summer suggests that viruses are more important for HPs mortality in the warmer months. This study strengthens the importance of top-down controls in maintaining lower HPs stocks in the Red Sea and suggests that both latitudinal and seasonal variations have minor but measurable roles.

Do weathered microplastics impact the planktonic community? A mesocosm approach in the Baltic Sea

Microplastics (MPs) are ubiquitous pollutants of increasing concern in aquatic systems. However, little is still known about the impacts of weathered MPs on plankton at the community level after long-term exposure. In this study, we investigated the effects of weathered MPs on the structure and dynamics of a Baltic Sea planktonic community during ca. 5 weeks of exposure using a mesocosm approach (2 m3) mimicking natural conditions. MPs were obtained from micronized commercial materials of polyvinyl chloride, polypropylene, polystyrene, and polyamide (nylon) previously weathered by thermal ageing and sunlight exposure. The planktonic community was exposed to 2 μg L-1 and 2 mg L-1 of MPs corresponding to measured particle concentrations (10–120 μm) of 680 MPs L-1 and 680 MPs mL-1, respectively. The abundance and composition of all size classes and groups of plankton and chlorophyll concentrations were periodically analyzed throughout the experiment. The population dynamics of the studied groups showed some variations between treatments, with negative and positive effects of MPs exhibited depending on the group and exposure time. The abundance of heterotrophic bacteria, pico- and nanophytoplankton, cryptophytes, and ciliates was lower in the treatment with the higher MP concentration than in the control at the last weeks of the exposure. The chlorophyll concentration and the abundances of heterotrophic nanoflagellates, Astromoeba, dinoflagellate, diatom, and metazooplankton were not negatively affected by the exposure to MPs and, in some cases, some groups showed even higher abundances in the MP treatments. Despite these tendencies, statistical analyses indicate that in most cases there were no statistically significant differences between treatments over the exposure period, even at very high exposure concentrations. Our results show that weathered MPs of the studied conventional plastic materials have minimal or negligible impact on planktonic communities after long-term exposure to environmentally relevant concentrations.

Extracted Spectral Signatures from the Water Column as a Tool for the Prediction of the Structure of a Marine Microbial Community

In this communication, we present an innovative approach leveraging advanced Machine Learning (ML) and Artificial Intelligence (AI) techniques, specifically the Non-Negative Matrix Factorization (NMF) method, to analyze downward and upward light spectra collected by Hyperspectral Ocean Color Radiometer (HyperOCR, HOCR) sensors in the water column. Our work focuses on the development of a robust and efficient tool for unraveling the structure and activities of natural microbial assemblages in the ocean. By applying the NMF method to HyperOCR data, we successfully extracted five spectral signatures, representing unique patterns in the data. These signatures were instrumental in predicting the abundances of various microbial components, including bacteria, heterotrophic nanoflagellates, and picoeukaryotes, showcasing the potential of ML and AI in advancing oceanographic studies. To validate these methods, the study area included a shallow coastal area under the influence of freshwater inflow and an open offshore area with a depth of 100 m. The study sites in coastal and offshore waters (Kaštela Bay and Stončica Vis, respectively) had significantly different hydrographic and microbiological characteristics. Kaštela Bay had lower temperatures and salinity than the site on Vis. We have demonstrated prediction of the structure of the microbial community through application of different AI and ML methods with specific HOCR sensors.

Algae–zooplankton relationships during the year-round cyanobacterial blooms in a shallow lake

Cyanobacterial blooms have a strong impact on the food web structure, interactions and ecosystem functioning. The aim of this study was to describe the seasonal changes in composition and abundance of heterotrophic nanoflagellates, ciliates, rotifers, and crustaceans in relation to algae and nutrients in a shallow eutrophic lake (north-eastern Poland) dominated by cyanobacteria and exposed to the strong impact of cormorants. Our results showed that algae accounted for a small part of the total phytoplankton abundance (9–40%) and biomass (10–21%) and were dominated by diatoms and cryptophytes. All of the studied groups of planktonic organisms were quite rich in species (95 algal, 79 ciliate, 44 rotifer and 25 crustacean species) and relatively abundant. Copepods formed a substantial part (45–83%) of the total zooplankton biomass during all seasons. Relatively low algal to zooplankton biomass ratio (0.8–1.1) suggests that during spring, summer, and winter algae were not sufficient food resources for metazooplankton, which supplemented its diet with protists (heterotrophic nanoflagellates and ciliates). In a shallow lake dominated by cyanobacteria, winter (ice-covered period) may be a more favourable period for the growth of some groups of algae, ciliates and rotifers than other seasons due to decreasing abundance of cyanobacteria.

Morphological, phylogenetic and biogeographic characterizations of three heterotrophic nanoflagellates isolated from coastal areas of Korea

ABSTRACT Heterotrophic nanoflagellates (HNFs, 2–20 μm in size) are important bacterivores in aquatic environments. We isolated and distinguished three HNF strains from Korean coastal samples: Ancyromonas kenti strain KM086, Cafeteria mylnikovii strain JS001 and Multimonas media strain JS004. Their 18S rDNA sequences aligned with previously known counterparts. Under light microscopy, all strains match their previous respective species descriptions. However, under scanning electron microscopy, strain JS004 shows some features at odds with prior observations of M. media, including different putative extrusomes, an ‘unfrilled’ skirt margin, and a putative ‘tusk’. We also used the V4 region of 18S rDNA to study species distribution globally using Malaspina-2010 data from 122 surface stations and 13 various depth stations. Intriguingly, we found that A. kenti was most abundant in low-oxygen conditions of the bathypelagic zones (1000–4000 m deep) in the Pacific Ocean, suggesting a specialization for thriving in low-oxygen habitats. In contrast, we only found M. media represented at one surface site, and C. mylnikovii was not found at all. Thus, A. kenti may be one of the most important feeders on bacteria in the bathypelagic zones with low-oxygen concentrations, while the other two isolates appear to be rare species in marine systems.

Top-down and bottom-up control of plankton structure and dynamics in hypertrophic fishponds

We investigated the effects of strong top-down control by high fish stock on structure and seasonal dynamics of plankton in nine fishponds under conventional fishery management based on auxiliary feeding during two vegetation seasons. Mean concentrations of total nitrogen, phosphorus, and high densities of phytoplankton, bacteria, heterotrophic nanoflagellates, and ciliates indicated hypertrophic state of the fishponds, as well as a markedly reduced control of these microbial food web components by crustacean zooplankton. Mean seasonal densities of zooplankton varied within one order of magnitude for cladocerans, copepods, nauplii, and rotifers. Daphnia were found in most fishponds in densities up to 630 ind. l−1 (median: 53 ind. l−1). While TN and TP concentrations were high, dissolved inorganic N (median: 29 µg l−1) and reactive P (median: 11 µg l−1) indicated possible nutrient deficiency. The fish stock index (defined as the product of biomass and square root of densities) was used as a proxy for fish predation pressure. Multivariate analysis revealed that nutrients and high fish stocks (market carp, carp fry, and/or undesirable small planktivorous fishes) were the main driving forces shaping the fishpond plankton. The resulting trophic structure thus severely reduced the herbivorous zooplankton–fish link during a vegetation season.

Predation in a Microbial World: Mechanisms and Trade-Offs of Flagellate Foraging.

Heterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean and thus play a key role in ocean biogeochemistry. They are found in all major branches of the eukaryotic tree of life but are united by all being equipped with one or a few flagella that they use to generate a feeding current. These microbial predators are faced with the challenges that viscosity at this small scale impedes predator-prey contact and that their foraging activity disturbs the ambient water and thus attracts their own flow-sensing predators. Here, I describe some of the diverse adaptations of the flagellum to produce sufficient force to overcome viscosity and of the flagellar arrangement to minimize fluid disturbances, and thus of the various solutions to optimize the foraging-predation risk trade-off. I demonstrate how insights into this trade-off can be used to develop robust trait-based models of microbial food webs.

Identification of the heterotrophic nanoflagellate Bilabrum latius in the southern Adriatic (Mediterranean Sea)

Heterotrophic flagellates (HF) represent an important protist group in marine ecosystem functioning. Characterised by high taxonomic diversity, identification and classification of HF is often difficult using classical methods of light microscopy (LM). Complementing LM with molecular methods, such as barcoding, enables reliable taxonomic identification of even small size nanoflagellates that share similar or unnoticeable morphological features. The order Bicosoecida is a group of heterotrophic nanoflagellates that are important part of protist plankton and benthic communities of the world oceans. Recently, on the basis of high-resolution light microscopy and barcoding, a new bicosoecid genus, Bilabrum, was described with B. latius sp. as a type species. Our study reports on identification of B. latius from co-culture with the diatom species Chaetoceros affinis isolated from fresh plankton samples collected in the southern Adriatic. This detection of the Adriatic B.latius represents first record of this species outside itś up to now known and described habitat (deep-sea sediment of the South - East Atlantic Ocean) and in diatom co-culture.

Seasonality of top-down control of bacterioplankton at two central Red Sea sites with different trophic status.

The role of bottom-up (nutrient availability) and top-down (grazers and viruses mortality) controls on tropical bacterioplankton have been rarely investigated simultaneously from a seasonal perspective. We have assessed them through monthly samplings over 2 years in inshore and offshore waters of the central Red Sea differing in trophic status. Flow cytometric analysis allowed us to distinguish five groups of heterotrophic bacteria based on physiological properties (nucleic acid content, membrane integrity and active respiration), three groups of cyanobacteria (two populations of Synechococcus and Prochlorococcus), heterotrophic nanoflagellates (HNFs) and three groups of viruses based on nucleic acid content. The dynamics of bacterioplankton and their top-down controls varied with season and location, being more pronounced in inshore waters. HNFs abundances showed a strong preference for larger prey inshore (r = -0.62 to -0.59, p = 0.001-0.002). Positive relationships between viruses and heterotrophic bacterioplankton abundances were more marked inshore (r = 0.67, p < 0.001) than offshore (r = 0.44, p = 0.03). The negative correlation between HNFs and viruses abundances (r = -0.47, p = 0.02) in shallow waters indicates a persistent seasonal switch between protistan grazing and viral lysis that maintains the low bacterioplankton stocks in the central Red Sea area.

Analysis of the Influence of Seasonal Water Column Dynamics on the Relationship between Marine Viruses and Microbial Food Web Components Using an Artificial Neural Network

Artificial neural network analysis (ANN) is used to study the seasonal distribution of viruses and microbial food web (MFW) components in the open Adriatic Sea. The effect of viruses within the MFW is often overlooked, although viruses play an important role in microbial community dynamics. The results showed that the strongest influence is found in the nonlinear relationship between viruses and temperature. In addition, the algorithm showed that the number of viral populations in the P-limited open sea varies by season and according to the abundance of their main hosts, HB. A strong positive relationship between viruses and HB was found in more than 50% of the observed data. Moreover, this algorithm confirmed the association of the virus with the autotrophic part of the picoplankton and with heterotrophic nanoflagellates. The dynamics of the four resulting clusters, characterized by biological and environmental parameters, is described as a cyclic pattern in the water layer above the thermocline. Neural gas network analysis has been shown to be an excellent tool for describing changes in MFW components in the open Adriatic.

Distribution, Size-Morphological Structure and Production of Heterotrophic Bacterioplankton in Gorky Reservoir

The abundance, biomass, size-morphological structure, growth rate and production of bacterioplankton, the intensity of primary phytoplankton production and dark fixation of CO2, as well as the abundance and biomass of heterotrophic nanoflagellates were determined in a large plain eutrophic reservoir (Gorky Reservoir, Middle Volga). The abundance, biomass, and production of bacterioplankton were relatively high and averaged 7.6 × 106 cells/mL, 117.9 mg C/m3, and 59.2 mg C/(m3 × day), respectively. Heterotrophic nanoflagellates reached a high level of quantitative development – 6.9 × 103 cells/mL, 47.9 mg C/m3. Their biomass averaged 41.6 ± 18.4% of the bacterioplankton biomass, which indicates that, in addition to bacteria, nanoflagellates used other food sources. Small rods and cocci dominated among the size-morphological bacterioplankton groups and accounted for 36.3 and 33.3% of its total abundance, respectively. Small rods averaged more than a half (56.2%) of the total biomass and were the most stable component of the community. The growth rate and production of bacterioplankton increased in those parts of the reservoir where the medium-sized cocci and coccobacilli accounted for from 18.2 to 29.3% of the total abundance. The highest bacterial activity was recorded in the area affected by the warm waste waters of the Volgorechensk State District Power Plant. Based on the data obtained, the reservoir divided on the upper river section and the lower lake section.

Winter dynamics of abiotic and biotic parameters in eutrophic temperate lakes

Winter conditions in lakes of temperate climatic zones, especially the duration and thickness of ice and snow cover, are strongly influenced by climate changes. The aim of this study was to follow the dynamics of abiotic (organic carbon, total phosphorus, total nitrogen) and biotic (chlorophyll a, cyanobacteria, algae, nanoflagellates, ciliates, rotifers, crustaceans) parameters during two consecutive winters in three eutrophic lakes of different mixing regime (northeastern Poland). Our results showed that dissolved organic carbon was highly stable during both winters and practically did not differ among the studied lakes. Total phosphorus and chlorophyll a concentrations differed significantly between the dimictic and meromictic lakes. The concentrations of nutrients and chlorophyll a and the biomass of phytoplankton, protists, rotifers and crustaceans reached higher values in late winter than in early winter, depending on the lake type (morphometry, mixing regime) and the year of the study. The composition of phytoplankton and ciliates was more or less similar throughout the study in all lakes during both years, but the share of individual groups in the total biomass differed between early and late winter. A high cyanobacterial biomass in January under the ice in the meromictic lake was probably a legacy of the summer/autumn community. Small prostomatid ciliates were the most important group in late winter in the shallow lake. Cladocerans (Bosmina coregoni and B. longispina) were an important component of winter crustacean community on some occasions, especially in the shallow lake. Phosphorus was the major factor determining phytoplankton biomass. Heterotrophic nanoflagellates and rotifers were negatively related to ice cover, while ciliates were positively related to the water temperature. Our study demonstrated that the dynamics, abundance, and structure of the phytoplankton and zooplankton communities in eutrophic lakes changed from one winter to the next despite similar environmental conditions.

Cryptic and ubiquitous aplastidic cryptophytes are key freshwater flagellated bacterivores

Morphology-based microscopic approaches are insufficient for a taxonomic classification of bacterivorous heterotrophic nanoflagellates (HNF) in aquatic environments since their cells do not display reliably distinguishable morphological features. This leads to a considerable lack of ecological insights into this large and taxonomically diverse functional guild. Here, we present a combination of fluorescence in situ hybridization followed by catalyzed reporter deposition (CARD-FISH) and environmental sequence analyses which revealed that morphologically indistinguishable, so far largely cryptic and uncultured aplastidic cryptophytes are ubiquitous and prominent protistan bacterivores in diverse freshwater ecosystems. Using a general probe for Cryptophyceae and its heterotrophic CRY1 lineage, we analyzed different water layers in 24 freshwater lakes spanning a broad range of trophic states, sizes and geographical locations. We show that bacterivorous aplastidic cryptophytes and the CRY1 lineage accounted for ca. 2/3 and ¼ of total HNF, respectively, in both epilimnetic and hypolimnetic samples. These heterotrophic cryptophytes were generally smaller and more abundant than their chloroplast-bearing counterparts. They had high uptake rates of bacteria, hinting at their important roles in channeling carbon flow from prokaryotes to higher trophic levels. The worldwide ubiquity of Cryptophyceae and its CRY1 lineage was supported by 18S rRNA gene sequence analyses across a diverse set of 297 freshwater metagenomes. While cryptophytes have been considered to be mainly plastidic “algae”, we show that it is the aplastidic counterparts that contribute considerably to bacterial mortality rates. Additionally, our results suggest an undiscovered diversity hidden amongst these abundant and morphologically diverse aplastidic cryptophytes.

The influence of macrophyte ecological groups on food web components of temperate freshwater lakes

Aquatic macrophyte taxonomic composition, species abundance and cover determine the physical structure, complexity and heterogeneity of aquatic habitats – the structuring role of macrophytes. These traits influence richness, distribution, feeding and strength of the relationships between food web communities in lakes. The aim of this study was to determine how lakes with different dominating macrophyte ecological groups affect planktonic food web components, emphasising the influence on young of year (YOY) fish and large (≥1 +) fish community. We hypothesised that different dominating macrophyte ecological groups have different structural effects on food web components and YOY fish growth, abundance and feeding. Studied lakes categorised into three different macrophyte ecological groups – lakes dominated by emergent, floating+floating-leaved or submerged vegetation. We found that all dominating ecological groups had a strong influence on plankton communities (except heterotrophic bacterioplankton and nanoflagellates), YOY fish and large fish. Floating-leaved plant dominance was positively related to planktonic food web structure and YOY fish weight, length, abundance and the consumption of zooplankton as a prey of all major species of YOY fishes. Larger fish tended to favour the presence of emergent vegetation. This conclusion has important implications for local managers and conservationists in respect to the maintenance and protection of littoral habitats and fish resources.

Feeding by heterotrophic flagellates on marine archaea

Archaea live in diverse habitats including extreme environments. Although they are sometimes the dominant heterotrophic organisms in microbial communities, relatively little is known about their ecological roles in microbial food webs. To explore predator–prey relationships between archaea and heterotrophic protists, we determined feeding occurrence by the heterotrophic nanoflagellates Bodo sp., Goniomonas sp., and Pedospumella sp. the common heterotrophic dinoflagellates Cryptoperidiniopsis brodyi, Gyrodinium dominans, Oblea rotunda, Oxyrrhis marina, and Polykrikos kofoidii on ammonia-oxidizing archaea (AOA) by examining the food vacuoles of these heterotrophic flagellates under an epifluorescence microscope using fluorescently labeled AOA. In addition, we measured the ingestion rates of Goniomonas sp., C. brodyi, G.dominans, and O. marina on AOA as a function of prey concentration. Cells of Bodo sp., Goniomonas sp., Pedospumella sp., C. brodyi, G.dominans, and O. marina were able to feed on AOA, whereas O. rotunda and P. kofoidii were not. With increasing initial prey concentration up to 1.7 × 106 – 9.3 × 106 cells mL−1, the ingestion rates of Goniomonas sp., C. brodyi, G.dominans, and O. marina on AOA increased rapidly but increased slowly or became saturated at higher concentrations. Although the maximum ingestion rates of the smaller flagellate Goniomonas sp. (23.1 cells predator−1 h−1) on AOA was much lower than that of the larger flagellate O. marina (296.4 cells predator−1 h−1), AOA supported the positive growth of only Goniomonas sp. The results of the present study suggest that these small heterotrophic flagellates could be effective heterotrophic protist grazers of AOA in marine ecosystems.

Cafeteria in extreme environments: Investigations on C. burkhardae and three new species from the Atacama Desert and the deep ocean

The heterotrophic nanoflagellate genus Cafeteria has been found to be ubiquitously distributed in the marine realm. We could isolate and cultivate ten strains morphologically similar to Cafeteria from various types of environment, including the deep sea, brackish waters and also meso- to hypersaline inland waters. Molecular analyses (18S rDNA, 28S rDNA) of newly isolated strains from the marine realm resulted in four more Cafeteria burkhardae strains from the deep North Atlantic Ocean and one new species (C. baltica sp. nov.) isolated from brackish waters of the Baltic Sea. Two strains isolated from the Atacama Desert belong to two new species (C. atacamiensis sp. nov. and C. paulosalfera sp. nov.), one other strain could not yet be assigned. Morphological characterizations of these strains obtained by high resolution microscopy revealed only small differences to already described species. However, molecular analyses showed a clear separation of the different Cafeteria species. We exposed several strains to different salt concentrations (2–150 PSU) to investigate their salinity tolerance. Only the marine strains of C. burkhardae were able to survive at salinities up to 150 PSU, indicating the possibility to inhabit a broader spectrum of habitats including hypersaline environments besides the deep sea with its high hydrostatic pressure.

Bottom-Up Control of the Groundwater Microbial Food-Web in an Alpine Aquifer

Groundwater ecosystems are typically poor in organic carbon and productivity sustaining a low standing stock of microbial biomass. In consequence, microbial food webs in oligotrophic groundwater are hypothesized to be bottom-up controlled. To date, quantitative information on groundwater microbial communities, food web interactions, and carbon flow is relatively lacking in comparison to that of surface waters. Studying a shallow, porous alpine aquifer we collected data on the numbers of prokaryotes, virus-like particles and heterotrophic nanoflagellates (HNFs), the concentration of dissolved (DOC) and assimilable organic carbon (AOC), bacterial carbon production (BCP), and physical-chemical conditions for a 1 year hydrological cycle. The potential effects of protozoan grazing and viral lysis onto the prokaryotic biomass was tested. Flow of organic carbon through the microbial food web was estimated based on data from the literature. The abundance of prokaryotes in groundwater was low with 6.1 ± 6.9 × 104 cells mL–1, seasonally influenced by the hydrological dynamics, with higher densities coinciding with a lower groundwater table. Overall, the variability in cell numbers was moderate, and so it was for HNFs (179 ± 103 HNFs mL–1) and virus-like particles (9.6 ± 5.7 × 105 VLPs mL–1). The virus to prokaryotes and prokaryote to HNF ratios ranged between 2–230 and 33–2,084, respectively. We found no evidence for a viral control of prokaryotic biomass, and the biomass of HNFs being bottom-up controlled. First estimations point at carbon use efficiencies of 0.2–4.2% with prokaryotic production, and carbon consumed and recycled by HNFs and phages to be of minor importance. This first groundwater microbial food web analysis strongly hints at a bottom-up control on productivity and standing stock in oligotrophic groundwater ecosystems. However, direct measurement of protozoan grazing and phage mediated lysis rates of prokaryotic cells are urgently needed to deepen our mechanistic understanding. The effect of microbial diversity on the population dynamics still needs to be addressed.

Differences in the Composition of Abundant Marine Picoeukaryotes in the Marginal Sea Derived from Flooding

The transient impact of flooding on the community composition of marine picoeukaryotes (PEs, cell size ≤5 μm) in the East China Sea (ECS) was revealed in this study. In a summer without flooding (i.e., July 2009), photosynthetic picoeukaryotes (PPEs) were more abundant in the area covered by the Changjiang River diluted water (CDW, salinity ≤31) than in the non-CDW affected area. According to the 18S ribosomal RNA phylogeny, Alveolata (all from the superclass Dinoflagellata) was the main community component accounting for 72 to 99% of the community at each sampling station during the nonflooded summer. In addition to Dinoflagellata, diatoms or Chlorophyta also contributed a considerable proportion to the PE assemblage at the stations close to the edge of CDW coverage. In July 2010, an extreme flooding event occurred in the Changjiang River basin and led to the CDW covering nearly half of the ECS. In the flooded summer, the abundance of PPEs in the CDW-covered area decreased significantly to less than 1 × 104 cells ml-1. Compared to that during the nonflooded summer, the diversity of the PE composition was increased. While Dinophyceae still dominated the surface waters, Syndiniophyceae, which were represented by the uncultured Marine Alveolata Group (MALV)-I and MALV-II, accounted for a substantial amount in the Dinoflagellata superclass relative to this community composition in the nonflooded summer. Furthermore, a variety of plankton, including Cryptophyta, Haptophyta, Picobiliphyta, the uncultured Marine Stramenopiles (MASTs) and heterotrophic nanoflagellates, were observed. The nutrition modes of these PEs have been reported to be mixotrophic or heterotrophic. Therefore, it was inferred that the potentially mixotrophic and heterotrophic PE compositions might be favored in the marginal sea in the flooded summer.

Mechanisms and fluid dynamics of foraging in heterotrophic nanoflagellates

AbstractHeterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean. In their microscale world, viscosity impedes predator–prey contact, and the mechanisms that allow flagellates to daily clear a volume of water for prey corresponding to 106 times their own volume is unclear. It is also unclear what limits observed maximum ingestion rates of about 104 bacterial preys per day. We used high‐speed video microscopy to describe feeding flows, flagellum kinematics, and prey searching, capture, and handling in four species with different foraging strategies. In three species, prey handling times limit ingestion rates and account well for their reported maximum values. Similarly, observed feeding flows match reported clearance rates. Simple point force models allowed us to estimate the forces required to generate the feeding flows, between 4 and 13 pN, and consistent with the force produced by the hairy (hispid) flagellum, as estimated using resistive force theory. Hispid flagella can produce a force that is much higher than the force produced by a naked (smooth) flagellum with similar kinematics, and the hairy flagellum is therefore key to foraging in most nanoflagellates. Our findings provide a mechanistic underpinning of observed functional responses of prey ingestion rates in nanoflagellates.