Oligomesotrophic Lake Pavin Research Articles

Parasitic chytrids sustain zooplankton growth during inedible algal bloom.

This study assesses the quantitative impact of parasitic chytrids on the planktonic food web of two contrasting freshwater lakes during different algal bloom situations. Carbon-based food web models were used to investigate the effects of chytrids during the spring diatom bloom in Lake Pavin (oligo-mesotrophic) and the autumn cyanobacteria bloom in Lake Aydat (eutrophic). Linear inverse modeling was employed to estimate undetermined flows in both lakes. The Monte Carlo Markov chain linear inverse modeling procedure provided estimates of the ranges of model-derived fluxes. Model results confirm recent theories on the impact of parasites on food web function through grazers and recyclers. During blooms of “inedible” algae (unexploited by planktonic herbivores), the epidemic growth of chytrids channeled 19–20% of the primary production in both lakes through the production of grazer exploitable zoospores. The parasitic throughput represented 50% and 57% of the zooplankton diet, respectively, in the oligo-mesotrophic and in the eutrophic lakes. Parasites also affected ecological network properties such as longer carbon path lengths and loop strength, and contributed to increase the stability of the aquatic food web, notably in the oligo-mesotrophic Lake Pavin.

Phytoplankton chytridiomycosis: community structure and infectivity of fungal parasites in aquatic ecosystems

Fungal parasitism is recurrent in plankton communities, especially in the form of parasitic chytrids. However, few attempts have been made to study the community structure and activity of parasites at the natural community level. To analyse the dynamics of zoosporic fungal parasites (i.e. chytrids) of phytoplankton, samples were collected from February to December 2007 in two freshwater lakes. Infective chytrids were omnipresent in lakes, with higher diversity of parasites and infected phytoplankton than in previous studies. The abundance and biomass of parasites were significantly higher in the productive Lake Aydat than in the oligomesotrophic Lake Pavin, while the infection prevalence in both lakes were similar and averaged about 20%. The host species composition and their size appeared as critical for chytrid infectivity, the larger hosts being more vulnerable, including pennate diatoms and desmids in both lakes. The highest prevalence (98%) was noted for the autumn bloom of the cyanobacterium Anabaena flosaquae facing the parasite Rhizosiphon crassum in Lake Aydat. Because parasites killed their hosts, this implies that cyanobacterial blooms, and other large size inedible phytoplankton blooms as well, may not totally represent trophic bottlenecks because their zoosporic parasites can release dissolved substrates for microbial processes through host destruction, and provide energetic particles as zoospores for grazers. Overall, we conclude that the parasitism by zoosporic fungi represents an important ecological driving force in the food web dynamics of aquatic ecosystems, and infer general empirical models on chytrid seasonality and trophodynamics in lakes.

High Lytic Infection Rates but Low Abundances of Prokaryote Viruses in a Humic Lake (Vassivière, Massif Central, France)

We explored the abundance and infection rates of viruses on a time series scale in the euphotic zone of the humic mesotrophic Lake Vassivière (Massif Central, France) and compared them to nonhumic lakes of contrasting trophy (i.e., the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat) located in the same geographical region and sampled during the same period. In Lake Vassivière, the abundances of virus-like particles (range, 1.7 × 10(10) to 2.6 × 10(10) liter(-1)) were significantly (P < 0.001) lower than in Lakes Pavin and Aydat. The percentage of virus-infected prokaryotic cells (mean, 18.0%) was significantly higher (P < 0.001) in Vassivière than in Pavin (mean, 11.5%) and Aydat (mean, 9.7%). In Vassivière, the abundance of prokaryotes was a good predictor (r = 0.78, P < 0.001) of the number of virus-like particles, while the potential grazing rate from heterotrophic nanoflagellates was positively correlated to the viral infection rate (r = 0.75, P < 0.001; n = 20), indicating the prevalence of cycling interactions among viruses, prokaryotes, and grazers, which is in agreement with past experiments. The absence of correlation between chlorophyll a concentrations (Chl) and viral parameters suggested that the resources for the lytic activity of viruses in Vassivière were mainly under allochthonous control, through host activity. Indeed, compilation of data obtained from several nonhumic lakes in the French Massif Central revealed that Chl was positively correlated to the abundance of virus-like particles at concentrations above 0.5 μg Chl liter(-1) and negatively at concentrations below 0.5 μg Chl liter(-1), suggesting that phytoplankton-derived resources could force prokaryotic growth to attain a certain threshold level when the host availability is sufficient to boost the proliferation of viruses. Therefore, based on the high level of lytic infection rates in Lake Vassivière, we conclude that viruses are key agents for prokaryotic mortality and could influence the food web dynamics in humic lakes, which may ultimately depend on the internal cycling of resources and, perhaps, mainly on the allochthonous inputs and the associated humic substances.

A Preliminary Study of Heterotrophic Picoflagellates Using Oligonucleotidic Probes in Lake Pavin

Free-living heterotrophic flagellates have been widely studied as key intermediates in aquatic food webs. However, very few attempts have been made (all in marine systems) to study the smallest cells belonging to these communities. In this study, we report on the quantitative importance and the vertical distribution of heterotrophic picoflagellates during spring in the oligomesotrophic Lake Pavin, using the FISH approach. The rRNA-based probes used, initially designed for two novel lineages of heterotrophic picoflagellates (i.e., the Stramenopiles NS3 and NS4) in marine systems, revealed high abundances (in the order of 104 cells ml−1 and up to about 6 × 104 cells ml−1) of the targeted picoflagellates in Lake Pavin, i.e. one order of magnitude higher than in seawater samples. 8NS39 cells were more often detected than 8NS49 cells, the average contributions of the two populations to the total abundance of heterotrophic flagellates being at 25 and 9%, respectively. However, the probes used were apparently non-specific to the targeted cells, implying that the occurrence of the two novel lineages NS3 and NS4 of Stramenopiles in freshwaters needs to be confirmed. Although, this study clearly confirms the occurrence, in high abundance, of small heterotrophic eukaryotes in Lake Pavin. In a near future, we believe that the availability of microbial clone libraries will allow us to improve our knowledge of the composition and the functional role of small heterotrophic eukaryotes in freshwaters.

Effects of Viruses and Predators on Prokaryotic Community Composition

Dialysis bags were used to examine the impact of predation and viral lysis on prokaryotic community composition (PCC) over a 5-day experiment in the oligomesotrophic Lake Pavin (France). The impact of the different predator communities (protists and metazoans) of prokaryotes was estimated by water fractionation (<5 microm: treatment filtered on 5 microm, without ciliates and metazoans; UNF: unfiltered treatment with all planktonic communities). Enrichments of natural viruses (<1.2 microm: with a natural virus concentration; <1.2 mum V and VV: with enrichment leading to a double or triple concentration of viruses, respectively) were used to indirectly assess the control of virioplankton. Viral activity was estimated from the frequency of visibly infected cells (FVIC). PCC was determined by fluorescence in situ hybridization (FISH) and terminal restriction fragment length polymorphism (T-RFLP). In this study, PCC was affected by the eukaryote communities (especially flagellates), and viruses to a lesser extent. Cyanobacteria declined significantly during the experiment and were highly correlated with the FVIC. In addition, the 503-bp terminal restriction fragment (T-RF) disappeared in treatments with virus enrichments, suggesting possible viral-associated mortality processes, whereas the 506-bp T-RF was not affected in these treatments. On one hand, these results suggest a control of the PCC: first, by viral lysis of some dominant phylotypes and second, by interspecific competition between resistant strains for the uptake of substrates released by this lysis. The increase of Archaea may suggest that these cells benefit such resources. On the other hand, the disappearance and the stable proportion of some dominant phylotypes suggested a selection pressure due to the predatory activity on prokaryotes. In conclusion, prokaryotic abundance appears to be mainly controlled by flagellate protists, which also affected PCC, whereas viruses seemed to be essentially responsible for profound changes in PCC via direct and indirect actions.

Genetic Diversity of Small Eukaryotes in Lakes Differing by Their Trophic Status

Small eukaryotes, cells with a diameter of less than 5 mum, are fundamental components of lacustrine planktonic systems. In this study, small-eukaryote diversity was determined by sequencing cloned 18S rRNA genes in three libraries from lakes of differing trophic status in the Massif Central, France: the oligotrophic Lake Godivelle, the oligomesotrophic Lake Pavin, and the eutrophic Lake Aydat. This analysis shows that the least diversified library was in the eutrophic lake (12 operational taxonomic units [OTUs]) and the most diversified was in the oligomesotrophic lake (26 OTUs). Certain groups were present in at least two ecosystems, while the others were specific to one lake on the sampling date. Cryptophyta, Chrysophyceae, and the strictly heterotrophic eukaryotes, Ciliophora and fungi, were identified in the three libraries. Among the small eukaryotes found only in two lakes, Choanoflagellida and environmental sequences (LKM11) were not detected in the eutrophic system whereas Cercozoa were confined to the oligomesotrophic and eutrophic lakes. Three OTUs, linked to the Perkinsozoa, were detected only in the Aydat library, where they represented 60% of the clones of the library. Chlorophyta and Haptophyta lineages were represented by a single clone and were present only in Godivelle and Pavin, respectively. Of the 127 clones studied, classical pigmented organisms (autotrophs and mixotrophs) represented only a low proportion regardless of the library's origin. This study shows that the small-eukaryote community composition may differ as a function of trophic status; certain lineages could be detected only in a single ecosystem.

Low consumption of virus-sized particles by heterotrophic nanoflagellates in two lakes of the French Massif Central

Seasonal and depth-related variability in the grazing activity of heterotrophic nano- flagellates (HNF) on viruses was examined in the oligo-mesotrophic Lake Pavin and in the eutrophic Lake Aydat, French Massif Central, between May and November 2000. Ingestion rates (IR) were determined using 50 nm diameter fluorescent microspheres, as virus analogues. In both lakes, high- est grazing activities on virus-sized particles were recorded in the metalimnion, at the beginning and the end of the thermal stratification period. Estimated IRs in Lake Pavin (mean = 0.4 viruses HNF -1 h -1 , CV = 38.0%) and in Lake Aydat (mean = 0.3 viruses HNF -1 h -1 , CV = 35.6%) were not signifi- cantly different, in contrast to clearance rates (CR), which were significantly higher in the oligome- sotrophic (2.3 × 10 -2 nl HNF -1 h -1 ) than in the eutrophic lake (0.7 × 10 -2 nl HNF -1 h -1 ). CRs for viruses were correlated with CRs for bacteria in Lake Aydat but not in Lake Pavin, suggesting a greater abundance within the HNF assemblages of virus-sized particle feeders in the less productive lake. We estimated that 4.1 and 0.8% of viral production were grazed by HNF in Lake Pavin and Lake Aydat, respectively. Finally, although viruses seem to represent a minor food source for HNF (i.e. compared to bacteria), they may not be inconsequential in their diet, especially in oligotrophic lakes.

Viral activity in two contrasting lake ecosystems.

For aquatic systems, especially freshwaters, there is little data on the long-term (i.e., >6-month period) and depth-related variability of viruses. In this study, we examined virus-induced mortality of heterotrophic bacteria over a 10-month period and throughout the water column in two lakes of the French Massif Central, the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Concurrently, we estimated nonviral mortality through heterotrophic nanoflagellate and ciliate bacterivory. Overall, viral infection parameters were much less variable than bacterial production. We found that the frequency of visibly infected cells (FVIC), estimated using transmission electron microscopy, peaked in both lakes at the end of spring (May to June) and in early autumn (September to October). FVIC values were significantly higher in Lake Pavin (mean [M] = 1.6%) than in Lake Aydat (M = 1.1%), whereas the opposite trend was observed for burst sizes, which averaged 25.7 and 30.2 virus particles bacterium(-1), respectively. We detected no significant depth-related differences in FVIC or burst size. We found that in both lakes the removal of bacterial production by flagellate grazing (M(Pavin) = 37.7%, M(Aydat) = 18.5%) was nearly always more than the production removed by viral lysis (M(Pavin) = 16.2%, M(Aydat) = 19%) or ciliate grazing (M(Pavin) = 2.7%, M(Aydat) = 8.8%). However, at specific times and locations, viral lysis prevailed over protistan grazing, for example, in the anoxic hypolimnion of Lake Aydat. In addition, viral mortality represented a relatively constant mortality source in a bacterial community showing large variations in growth rate and subject to large variations in loss rates from grazers. Finally, although viruses did not represent the main agent of bacterial mortality, our data seem to show that their relative importance was higher in the less productive system.

8 Grazing by heterotrophic nanoflagellates on virus‐ and bacteria‐sized particles in two lakes of different trophy

Although viruses are now widely recognized to infect aquatic bacteria as well as prokaryotic and eukaryotic phytoplankton, our understanding of how they fit into aquatic foodwebs remains still deficient. The present study examined the potential nutritional food source that viral particles represent for natural assemblages of heterotrophic nanoflagellates (HNF). We determined the seasonal and depth‐related variability in the grazing activity of HNF on virus‐ and bacteria‐sized particles in the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat. Ingestion rates were determined using 50 and 500 nm diameter fluorescent microspheres. Estimated ingestion rates ranged from 0.01 to 1.7 viruses cell−1·h−1 (Aydat) and from 0.01 to 2 viruses cell−1·h−1 (Pavin). Derived clearance rates for viruses represented 3–5% (Aydat) and 10–12% (Pavin) of those for bacteria. In general, HNF grazing on both viruses and bacteria showed a similar pattern characterized by an apparent decrease in summer followed by autumnal peaks, with lowest values always being recorded in the hypolimnion. Viral production, estimated from the frequency of visibly infected cells (using transmission electronic microscopy), was consumed by HNF at rates averaging 2% (Aydat) and 10% (Pavin). This study suggests that, viruses, which are assumed to correspond to 1×10−7 ng C/particle, are likely to be of low nutritive value for HNF in both lakes. However, these results imply that HNF grazing may contribute significantly to the removal of viruses in freshwaters, especially in low productive systems.

Distribution, size, and bacterial colonization of pico‐ and nano‐detrital organic particles (DOP) in two lakes of different trophic status

Abundance, size spectra, and bacterial colonization of three types of freshwater pico‐ and nano‐detrital organic particles (DOP), namely transparent exopolymeric particles (TEP), Coomassie‐stained proteinaceous particles (CSP), and 4',6‐diamidino‐2‐phenylindole (DAPI) yellow particles (DYP), were examined by epifluorescence microscope equipped with a color camera and a video recorder. The study was conducted during spring in the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat, France. Pico‐ and nano‐DOP were abundant (105–108 particles L−1) and exhibited significant variations in morphometric characteristics and numerical density within and among lakes, which is consistent with comparable studies known mostly from marine systems. Surface area and abundance of pico‐ and nano‐DYP and nano‐TEP were significantly higher in Lake Aydat than in Lake Pavin. Both these variables significantly increased with chlorophyll a concentration and the abundance of total heterotrophic bacteria. Bacterial colonization of DOP was highly related to the nature of the particle. Attached bacterial counts were on the order of 107–108 bacteria L−1, representing 1–17% of the total bacterial counts. Overall, we conclude that nonliving particulate organic matter are ubiquitous and abundantly occurring components in lakes where, at least, they serve as support for the attachment and as substrate for the metabolism of bacteria, the latter role being likely dependent on the composition of the particles and the trophic status of the milieu.

Diel Pattern of Photosynthate Partitioning in Phytoplankton Population in an Oligomesotrophic Lake

AbstractThe aim of this study was to measure the short‐term changes in inorganic carbon allocation into various macromolecular compounds (proteins, polysaccharides and lipids) throughout a diurnal cycle in the oligomesotrophic Lake Pavin (Massif Central of France) at the depths of 5, 15 and 30 m. Biochemical fractionation was done by consecutive differential extractions in order to separate proteins, polysaccharides, lipids and low molecular weight compounds (LMW) by virtue of their relative solubilities in different extraction solvents. Over the entire diurnal cycle inorganic carbon was preferentially incorporated into proteins (M = 30%), then into polysaccharides (M = 28%), LMWs (M = 27%) and lipids (M = 15%). However, at 5 m, diurnal variations were reflected by the high percentage of the inorganic carbon incorporated into polysaccharides during periods of high light intensity and decreased at dawn and dusk. The reverse pattern was observed for the allocation of inorganic carbon to proteins.

Seasonal dynamics and vertical distribution of planktonic ciliates and their relationship to microbial food resources in the oligomesotrophic Lake Pavin

The planktonic ciliated protozoa and their potential prey (bacteria, autotrophic picoplankton, flagellates, algae) in Lake Pavin (Massif central of France) were investigated during the period of April to December, 1992. Ciliate abundance varied from 300 to 24,200 cells/l. Peak densities were observed during mixis for all sampling depths and in May-June in the cold aerobic hypolimnion. During the rest of the year, the species composition varied greatly with depth and season. Small algivorous prostomatids and large mixotrophic oligotrichs were most abundant in the euphotic zone during summer. Small filter-feeders were present at each depth throughout the study whereas haptorids and large size taxa were restricted to 15 m depth during spring and autumn. The highest density of ciliates occurred in the deep (40 m) cold aerobic hypolimnion due to the major spring development of the scuticociliate Uronema nigricans. This species seems to be adapted to cold and dilute freshwater habitats. Evidence for a major impact of availability and diversity of appropriate food in determining ciliate composition at different seasons and depths are given.

Potential glutamine synthetase and glutamate dehydrogenase activities in the oligo-mesotrophic Lake Pavin: annual comparison with an indirect estimation of inorganic nitrogen utilization

The annual distribution of potential glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities was studied in the oligo-mesotrophic Lake Pavin (France, Massif Central). The validity of these enzymatic activies as an index of N-assimilation in field studies was tested by a comparison with the coupling of distributions of inorganic nitrogen uptake rate based on 14 C incorporation into proteins together with several abiotic and biotic parameters. The potential GS and GDH activities recorded during the annual phytoplankton succession were in the same order of magnitude as those reported from the very scanty literature on this subject. Moreover, specific potential GS and GDH activities were significantly correlated with the external NO 3 - -N concentration suggesting that these enzymatic activities reflected nitrate utilization by algae. Specific activities of GDH were about 2.5 fold higher than GS specific activities suggesting that the GDH pathway may play a leading part in inorganic nitrogen assimilation in the oligo-mesotrophic Lake Pavin. However, N-assimilation rates based on the enzymatic approach were lower than those obtained through indirect isotopic determination of N-uptake rates. The reasons for such a discrepancy are discussed.