Heterotrophic Nanoflagellates Community Research Articles

Cryptophyta as major bacterivores in freshwater summer plankton

Small bacterivorous eukaryotes play a cardinal role in aquatic food webs and their taxonomic classification is currently a hot topic in aquatic microbial ecology. Despite increasing interest in their diversity, core questions regarding predator–prey specificity remain largely unanswered, e.g., which heterotrophic nanoflagellates (HNFs) are the main bacterivores in freshwaters and which prokaryotes support the growth of small HNFs. To answer these questions, we fed natural communities of HNFs from Římov reservoir (Czech Republic) with five different bacterial strains of the ubiquitous betaproteobacterial genera Polynucleobacter and Limnohabitans. We combined amplicon sequencing and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) targeting eukaryotic 18 S rRNA genes to track specific responses of the natural HNF community to prey amendments. While amplicon sequencing provided valuable qualitative data and a basis for designing specific probes, the number of reads was insufficient to accurately quantify certain eukaryotic groups. We also applied a double-hybridization technique that allows simultaneous phylogenetic identification of both predator and prey. Our results show that community composition of HNFs is strongly dependent upon prey type. Surprisingly, Cryptophyta were the most abundant bacterivores, although this phylum has been so far assumed to be mainly autotrophic. Moreover, the growth of a small lineage of Cryptophyta (CRY1 clade) was strongly stimulated by one Limnohabitans strain in our experiment. Thus, our study is the first report that colorless Cryptophyta are major bacterivores in summer plankton samples and can play a key role in the carbon transfer from prokaryotes to higher trophic levels.

Bacterial prey food characteristics modulate community growth response of freshwater bacterivorous flagellates

AbstractDifferent bacterioplankton species represent different food quality resources for heterotrophic nanoflagellate (HNF) communities, potentially affecting HNF growth, community dynamics and carbon flow to higher trophic levels. However, our knowledge of such dynamics is still very limited. Here, we describe the results of 11 experiments with natural HNF communities from distinct seasonal phases in two freshwater habitats. The HNF communities were released from predation pressure of zooplankton and incubated with 16 distinct ecologically relevant prey bacterial strains from important Betaproteobacteria genera (Limnohabitans, Polynucleobacter, and Methylopumilus) and one Actinobacteria strain from the Luna 2 cluster. We observed remarkable prey‐ and season‐specific variability in community HNF growth parameters, i.e., doubling time, volumetric gross growth efficiency (GGE), and length of lag phase. All strains, except for the actinobacterium, supported rapid HNF population growth with an average doubling time of 10 h and GGE of 29%. Our analysis revealed that 59% of the variability in flagellate GGE data was explained by the length of lag phase after prey amendments. This indicates a considerable “adaptation time,” during which the predator communities undergo compositional shifts toward flagellate bacterivores best adapted to grow on the offered prey. Importantly, the rapid HNF growth detected on various bacteria tightly corresponds to doubling times reported for fast growing bacterioplankton groups. We propose a conceptual model explaining the tight linkages between rapid bacterial community shifts and succeeding HNF community shifts, which optimize prey utilization rates and carbon flow from various bacteria to the microbial food chain.

Trophic relationships between planktonic bacteria, heterotrophic nanoflagellates and viruses in a mesoeutrophic reservoir

The abundance, biomass, and production (РВ) of bacrerioplankton; the taxonomic composition, abundance, biomass of heterotrophic nanoflagellates (HNF) and the rate of consumption of bacteria by HNFs; and the abundance of virioplankton, frequency of visibly infected bacterial cells, virus-induced mortality of bacterioplankton, and viral production were estimated in the mesoeutrophic Rybinsk Reservoir. The rate of bacterial mortality due to viral lysis (7.8–34.1%, on average 17.2 ± 2.0% of daily РВ) was lower than the consumption of bacteria by the HNF community (15.4–61.3%, on average 32.0 ± 4.2% of daily РВ). While consuming bacteria, HNFs simultaneously absorbed a significant number of viruses residing on the surface and inside the bacterial cells.

Differential freshwater flagellate community response to bacterial food quality with a focus on Limnohabitans bacteria

Different bacterial strains can have different value as food for heterotrophic nanoflagellates (HNF), thus modulating HNF growth and community composition. We examined the influence of prey food quality using four Limnohabitans strains, one Polynucleobacter strain and one freshwater actinobacterial strain on growth (growth rate, length of lag phase and growth efficiency) and community composition of a natural HNF community from a freshwater reservoir. Pyrosequencing of eukaryotic small subunit rRNA amplicons was used to assess time-course changes in HNF community composition. All four Limnohabitans strains and the Polynucleobacter strain yielded significant HNF community growth while the actinobacterial strain did not although it was detected in HNF food vacuoles. Notably, even within the Limnohabitans strains we found significant prey-related differences in HNF growth parameters, which could not be related only to size of the bacterial prey. Sequence data characterizing the HNF communities showed also that different bacterial prey items induced highly significant differences in community composition of flagellates. Generally, Stramenopiles dominated the communities and phylotypes closely related to Pedospumella (Chrysophyceae) were most abundant bacterivorous flagellates rapidly reacting to addition of the bacterial prey of high food quality.

Microbial dynamics and flagellate grazing during transition to winter in Lakes Hoare and Bonney, Antarctica

The planktonic microbial communities of Lakes Hoare and Bonney were investigated during transition into winter. We hypothesized that the onset of darkness induces changes in the functional role of autotrophic and heterotrophic microplankton. Bacteria decreased in Lake Hoare during March-April, while in Lake Bonney bacterial abundances varied. Heterotrophic nanoflagellates (HNAN), phototrophic nanoflagellates (PNAN) and ciliates showed no marked decline with the onset of winter. PNAN outnumbered HNAN in both lakes. Grazing rates of HNAN in Lake Hoare ranged up to 30.8 bacteria per cell day(-1). The HNAN community grazed between 3.74 and 36.6 ng of bacterial carbon day(-1). Mixotrophic PNAN had grazing rates up to 15.2 bacteria per cell day(-1), and their daily community grazing exceeded bacterial production. In Lake Bonney East, PNAN grazing rates ranged up to 12.48 bacteria per cell day(-1) and in Lake Bonney West up to 8.16 bacteria per cell day(-1). As in Lake Hoare, the mixotrophic PNAN grazing rates (up to 950 ng C day(-1)) usually exceeded bacterial production. HNAN grazing rates were generally similar to those in Lake Hoare. As winter encroaches, these lakes move progressively towards heterotrophy and probably function during the winter, enabling populations to enter the short austral summer with actively growing populations.

Flagellate nutritional versatility as a key to survival in two contrasting Antarctic saline lakes

Summary1. Seasonal patterns of grazing and photosynthesis were investigated in two saline Antarctic lakes (Highway and Ace) in the Vestfold Hills (68°S). The phototrophic nanoflagellate (PNAN) community was dominated by Pyramimonas gelidicola and two morphological forms of a cryptophyte species that occurred throughout the year. Both species were mixotrophic on bacteria, and in Highway Lake they also exploited dissolved organic carbon as determined by the uptake of fluorescently labelled dextrans.2. Clearance rates ranged between 0.02 and 0.21 nL h−1 cell−1 in Ace Lake and 0.004–1.05 nL h−1 cell−1 in Highway Lake. On occasion cryptophyte grazing equalled that of the heterotrophic nanoflagellates (HNAN).3. Photosynthetic rates showed similar trends in both lakes, but there were differences in chlorophyll a specific rates and photosynthetic efficiency, probably related to the meromictic characteristic of Ace Lake. Primary production was measurable in winter and peaked in summer following the maxima of mixotroph grazing.4. The HNAN community of Highway Lake achieved clearance rates of 0.02–1.80 nL h−1 cell−1 and removing between 50 and 693 ng bacterial carbon L−1 day−1, with highest impact in winter when HNAN were most abundant. The HNAN also ingested fluorescently labelled dextrans showing a preference for 4 and 500 kDa molecules. The more diverse HNAN community of Ace Lake had lower clearance rates (0.04–0.37 nL h−1 cell−1) and exerted a lower grazing pressure on bacterioplankton. In Highway Lake, where the HNAN community was dominated by the choanoflagellate Diaphanoeca grandis, there was a significant correlation between mean cell volume and clearance rate.5. The major feature was that the microbial plankton functioned throughout the year by employing nutritional versatility.

Abundância de nanoflagelados heterotróficos no plâncton de reservatórios com distintos graus de trofia

The aim of this study was to investigate the influence of the trophic state on the planktonic heterotrophic nanoflagellates abundance from three reservoirs of the State of Parana: Irai (eutrophic), Rosana (mesotrophic) and Chavantes (oligotrophic). The samples were collected during dry (July) and rainy (November) periods in 2001 and were taken at the sub-surface and mixing layers of the lacustrine zone of the reservoirs. Heterotrophic nanoflagellates abundance was determined by epifluorescence microscopy, after staining with Dapi, and cells volumes were calculated aiming to estimate biomass, using a cell volume-to-carbon conversion factor. Higher values of density and biomass were observed in Irai Reservoir, the most eutrophic environment, whereas the lower ones were registred in Chavantes Reservoir, in both studied periods. In all reservoirs, a clear vertical pattern was observed, with higher values in the sub-surface layer. The results suggested that the trophic state influenced strongly the abundance patterns of planktonic heterotrophic nanoflagellates, indicating that the resources available determined the patterns. In other words, although there certainly is a predation pressure on the heterotrophic nanoflagellates community, it is not enough to prevent that the increase of resources determine a significant enlargement in these organisms abundance

Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa): a mesocosm study

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 40:13-24 (2005) - doi:10.3354/ame040013 Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa):a mesocosm study Marc Troussellier1, Patrice Got1, Maimouna Mboup2, Daniel Corbin2,Laura Giuliano3, Simone Cappello3, Marc Bouvy1,2,* 1Laboratoire Ecosystèmes Lagunaires UMR-CNRS 5119, Equipe EMMA, Université Montpellier II, 34095 Montpellier, France2Centre IRD Bel Air, BP 1386, UR 098 Dakar, Senegal3Istituto Sperimentale Talassografico—Consiglio Nazionale delle Ricerche, Spianata San Raineri 84, 98122 Messina, Italy *Corresponding author. Email: bouvy@dakar.ird.sn ABSTRACT: Temporal variability in bacterial communities was studied on a daily scale in the estuarine part of the largest river on the West African coast, the Senegal River. Duplicate mesocosms (3 m3 volume) were placed in the upper part of the estuary at the end of the dry season (May 2002) and treated with low and high inorganic nutrient (N and P) enrichment. High nutrient additions were followed by a 7-fold increase in phytoplankton biomass and a 6-fold increase in bacterial abundance after 4 and 9 d, respectively. Heterotrophic nanoflagellates (HNF) showed their maximal abundance (1 × 106 ml–1) 2 d after the bacterial peak. The low bacteria to flagellate ratios recorded on Day 10 may suggest enhanced bacterivory from HNF. Simultaneous measurements of growth and grazing rates on bacteria during the bacterial growth phase were performed with the dilution method and seemed to indicate that the HNF community was capable of quickly controlling bacterial development. However, estimates of the carbon demand of HNF during their growth phase (915 µg C l–1 d–1) appeared to be more elevated than the bacterial carbon production (63% of the HNF carbon demand). To cover HNF carbon requirements, an alternative/complementary prey might be the picophytoplanktonic cells, which were very numerous during the study (i.e. 85% of the total phytoplankton count). During the period of high grazing pressure, bacterial populations were characterized by higher specific activity (from tritiated thymidine incorporation) and culturability (from plate counts). The presence of very large bacteria, as detected by epifluorescence microscopy and flow cytometry measurements, may be an escape response from flagellate grazing. 16S rDNA sequences from bacterial isolates showed the presence of 2 types of taxonomic units (Vibrio natriegens– and Flexibacter maritimus–like bacteria), which can be considered by their forms and growth rates to be strains that have developed strategies to protect against grazing. Thus, as demonstrated in various temperate systems, predation by HNF in estuarine tropical ecosystems may also be of importance in shaping the structure and functions of the bacterial community. KEY WORDS: Bacteria · Heterotrophic nanoflagellates · Mesocosms · Growth and grazing rates · Senegal estuary · West Africa Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 40, No. 1. Online publication date: July 18, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.

Seasonal succession and taxon-specific bacterial grazing rates of heterotrophic nanoflagellates in Lake Constance

We investigated the taxonomic composition of the heterotrophic nanoflagellate (HNF) assemblage and its taxon-specific bacterial grazing rates in Lake Constance (Germany) over the course of 1 yr. Bacterial grazing rates were measured using natural fluorescently labelled bacteria (FLB) and compared to bacterial production estimated by the uptake of 14 C-leucine incorporation. Glutaraldehyde-fixed, DAPI-stained flagellates were counted using epifluorescence microscopy. Based on annual averages, small species such as Spumella sp. (2 to 6 µm) were the most numerous HNF and the dominant bacterivores. Larger flagellates such as Kathablepharis sp. contributed sig- nificantly to total HNF biomass, in particular during spring, but were relatively unimportant as bac- terial grazers. The HNF community structure changed during the transition from the phytoplankton spring bloom to the clearwater phase, with small flagellates such as heterokonts, kinetoplastids and choanoflagellates becoming increasingly abundant. The flagellate community composition was more diverse during summer and autumn than in spring. Per capita ingestion rates ranged from 0 to 31 bac- teria HNF -1 h -1 and changed seasonally up to 10-fold within a given taxon. Mixotrophic species con- tributed little to total bacterivory. We provide evidence that the relative significance of bacterial ingestion by a given flagellate taxon may change seasonally. Based upon our experimental results, we discuss potential shortcomings inherent in the FLB technique.

Viral lysis and grazing loss of bacteria in nutrient- and carbon-manipulated brackish water enclosures

A 3 week enclosure experiment was carried out at the Gulf of Finland, the Baltic Sea. After additions of inorganic nutrients [nitrogen (N) + phosphorus (P)] and a carbon source (sucrose), we followed bacterial, viral and heterotrophic nanoflagellate (HNF) abundances, as well as bacterial production and the frequency of bacteria visibly infected with viruses. Furthermore, the decay rate of virus particles was measured three times during the enclosure experiment from the KCN-treated water samples. Bacterial mortality caused by viral lysis was estimated using the decay rates and the fraction of bacteria infected. Nutrient (N + P) additions stimulated phytoplankton growth [the chlorophyll (Chl)<it>a</it> concentration increased from &lt;5 <IMG SRC="/math/mu.gif">g l^-1 up to 19 &mgr;g l^-1], while sucrose additions increased bacterial production (from 4-6 x 10⁷ l^-1 h^-1). The phytoplankton blooms affected bacterial production only slightly. Bacterial mortality that was explained by viruses ranged from &lt;2% to 13% when estimated from the visibly infected cells, and from 8% to 808% when the decay rates (range 0.052-0.765 h^-1) were used. Assuming a clearance rate of 5 nl flagellate^-1 h^-1, the HNF community could graze 16-135% of total bacterial loss.

Microbial dynamics in coastal waters of East Antarctica:bacterial production and nanoflagellate bacterivory

Bacterial production and heterotrophic nanoflagellate (HNAN) bacterivory were determined concurrently with measurements of abundance and biomass at weekly intervals between 30 December 1993 and 11 February 1994 at a shallow, coastal location in Prydz Bay, eastern Antarctica. Bacterial production was measured by [3H]thymidine incorporation and HNAN bacterivory by the uptake of fluorescently labelled bacteria. Bacterial abundance, biomass and production ranged from 2 to 8 x 108 l-1, 13 to 64 µg C l-1 and 8 to 14 µg C l-1 d-1, respectively, with maximum values recorded in mid January. The HNAN community comprised choanoflagellate, non-collared and colonial taxa, with non-collared forms dominating abundance and biomass in late January and early February. Total HNAN abundance and biomass ranged from 1.6 to 4.2 x 106 l-1 and 8 to 16 µg C l-1, respectively. HNAN cellular ingestion and clearance rates differed between taxa with maximum rates of 8.28 particles cell-1 h-1 and 9.32 nl cell-1 h-1 recorded for large non-collared forms. During the study period the HNAN community grazed 0.9 to 4.7 µg bacterial C l-1 d-1, equivalent to 3 and 12% of bacterial biomass, and 10 and 36% of daily bacterial production; however, these values are likely to be minimal estimates and grazing impact may have been higher on occasion. Choanoflagellates were responsible for much of the grazing impact at the beginning of the study period, while non-collared HNAN were the dominant grazers in late January and early February. The HNAN community therefore appears to graze substantial bacterial production in Antarctic coastal waters during the austral summer, although alternative sources of bacterial mortality are likely to be of importance.

Top-down effects on the size-biomass distribution of a freshwater bacterioplankton community

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 10:255-263 (1996) - doi:10.3354/ame010255 Top-down effects on the size-biomass distribution of a freshwater bacterioplankton community Pernthaler J, Sattler B, Simek K, Schwarzenbacher A, Psenner R Protistan size-selective grazing on aquatic bacteria has frequently been demonstrated in the laboratory, but its postulated effects on the size distribution of natural bacterioplankton communities remain uninvestigated. For a period of 5 wk we followed the spring development of the epilimnetic microbial community in an oligo-mesotrophic lake. Changes in bacterial size distributions and growth parameters were quantified and compared with the abundance, biomass and bacterivory of the heterotrophic nanoflagellate (HNF) community. We observed pronounced shifts within 2 distinct size ranges of the bacterioplankton. Short, rod-shaped cells between 0.4 and 1.2 um cell length formed between 30 to 70% of total bacterial biomass; cells &GT2.4 um were responsible for another 20 to 50%. High HNF community grazing rates (&GT100% of newly produced cells) were reflected in a biomass drop of bacteria between 0.4 and 1.2 um cell length, whereas large cells (&GT2.4 um) were almost half of total bacterial biomass during this period. When HNF abundance and bacterivory decreased, the opposite pattern was observed. [3H]Thymidine and [14C]leucine uptake per respiring bacterial cell were maximal during the period of rapid increase of bacterial cells between 0.4 and 1.2 um. Both flagellate total grazing and individual clearance rates were negatively correlated with growth per active bacterial cell. The biomass of 'grazing-resistant' bacteria was correlated with flagellate bacterivory and biomass, and size-selective protistan grazing influenced the size structure of bacterioplankton as well as the mean per-cell growth of the active bacterial fraction. We suggest a model that divides freshwater bacterioplankton into 4 size ranges of different vulnerability to size-selective protistan grazing. Bacterioplankton size structure . Bacterial biomass . Image analysis . Grazing-resistant bacteria . Protistan size selectivity . Cell-specific activity Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 10, No. 3. Publication date: June 27, 1996 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1996 Inter-Research.

Bacterivory by heterotrophic nanoflagellates and bacterial production in sediments of a freshwater littoral system

We made simultaneous measurements of benthic bacterial production, using [3H]thymidine assays, and bacterivory by benthic heterotrophic nanoflagellates (HNAN), using fluorescently labeled bacteria. Sediment samples were collected at two stations in a eutrophic freshwater littoral in different seasons. Bacterial production ranged from 19.7 to 263.0 × 109 bacteria dm‐3 h‐1, with the highest values measured in summer. HNAN grazing rates (3.8–64.2 bacteria flagellate‐1 h‐1) varied significantly with season and station, and rates were correlated with flagellate biovolume. At both stations, there were large discrepancies between bacterial production and HNAN grazing. The estimated bacterivory ranged from 0.4 to 5.2% of bacterial production. The minor impact of HNAN as grazers on bacteria during summer months was caused by high bacterial production and low HNAN abundances. During winter, bacterial production decreased, whereas HNAN abundance increased. However, due to the low grazing rate of these relatively small flagellates, the total HNAN community still consumed only an insignificant amount of the bacterial production. The results of our measurements show that bacterivory by HNAN cannot account for the fate of bacterial production in the littoral studied.

Short-term changes of protozoan control on autotrophic picoplankton in an oligo-mesotrophic lake

In May 1994, we investigated the short-term development of the planktonic community in the epi- and metalimnion of an oligo-mesotrophic lake (Piburger See, Tyrol), focusing on trophic links between protists and picoplankton, but also including phyto- and zooplankton. Uptake experiments with fluorescently labelled bacteria (FLB) and picocyanobacteria (FIX) were performed in order to compare the importance of both prey types as carbon sources for bacterivorous protists. Heterotrophic nanoflagellates (HNF) were responsible for —90% of total protozoan picoplanktivory (FLB + FIX); ciliates accounted for —10%. A selectivity index related to prey density showed that both HNF and ciliates clearly preferred FIX over FLB. The mean cell size of autotrophic (prokaryotic) picoplankton (APP) was nearly three times larger (0.323 jim3) and much less variable than mean bacterial cell volume (0.122 (inV). Although APP biomass was on average only 8.6% of total picoplankton biomass, pico- cyanobactena accounted for a mean 15.9% of total HNF carbon uptake. We calculated that total HNF grazing could match potential APP maximum growth rates at the beginning of the study period. A strong decrease in HNF individual clearance rate (CR) on APP was clearly related to a fall in the percentage of choanofiagellates (from 75 to —10% of the HNF community). A simultaneous decrease in HNF biomass and CR was followed by a steep increase in APP abundance; APP abundance and HNF biomass were highly negatively correlated both in the epi- and the metalimnion (/-, = -0.879, r, META = -0.907; P < 0.001). Total rotifer abundance increased by a factor of 50 within 2 weeks and was also negatively correlated with HNF biomass (r, EPI = -0.852, P < 0.001; r, MlrTA = -0.659, P < 0.05). HNF grazing was found to exert a strong short-term control on picocyanobacteria and this link was probably broken by an increase in metazooplankton, especially due to rotifer predation on HNF.