Influence of tides on the dissemination and related health risks of intestinal helminths along the Kribi beaches (Atlantic Coast, Southern Cameroon).

Kribi is a seaside town that welcomes thousands of tourists each year. However, the poor sanitation condition of its beaches along the Atlantic coast is not without risk for visitors. In this study, we used the formol-ether concentration technique to identify and quantify larvae or eggs of intestinal helminths in waters of three regularly visited Kribi beaches (Mpalla, Ngoyè, and Mboamanga). Results revealed that all identified larvae and eggs were cestodes (Hymenolepis nana) and nematodes (Strongyloides sp., Ascaris sp., Ancylostoma duodenale and Trichuris trichiura). All the helminth eggs and larvae showed high abundance at low tide during rainy seasons. Ancylostoma duodenale eggs, totally absent at Mpalla, were densely present at low tide at Ngoyè (301 ± 15 eggs/L). Trichuris trichiura eggs showed the lowest abundance (0 to 62 eggs/L) at all sites. Abiotic variables indicated that waters at the various beaches were basic (pH: 8.75-9.77), generally warmer (32.44°C at Mpalla in the Short Rainy Season), more oxygenated at low tide, and moderately mineralized at high tide. Positive and significant correlations were observed at Ngoyè at low tide between Strongyloides sp. larvae and dissolved oxygen (P ˂ 0.05); and between Ancylostoma duodenale eggs and temperature (P ˂ 0.05). The overall results indicated that the beaches studied are subjected to fecal pollution. This pollution is more accentuated during low tides than during high tides. Depending on tidal movements, swimmers risk exposure to helminth eggs and larvae known to be responsible for gastroenteritis.

Distinctive Patterns in the Taxonomical Resolution of Bacterioplankton in the Sediment and Pore Waters of Contrasted Freshwater Lakes.

Bacteria assemblages in lake sediments play a key role in various biogeochemical processes, yet their association with interstitial pore waters has been scarcely investigated. In this study, we utilized Illumina next-generation amplicon sequencing of the 16S rRNA gene to characterize the seasonal bacterial communities in the sediments and pore waters of three contrasted temperate freshwater lakes, namely Pavin, Aydat, and Grangent (French Massif Central). Despite occupying seemingly similar habitats, bacterial communities differed substantially between sediments and pore waters at all seasons with low sharing of operational taxonomic units (OTUs, 6.7 to 20.3%) between them. Sediment-associated bacteria were more rich and diverse than pore water bacteria, indicating a high heterogeneity in the sediment microhabitat. The changes in both sediment and pore water bacterial communities were lake and season specific. The bacterial community showed distinct differences between the lakes, with larger presence of strict anaerobes such as Syntrophus, Syntrophorhabdus, and Sulfuricurvum in the pore water and sediments of Pavin responsible for carbon and sulfur cycling. In both Aydat and Grangent, the hgcI_clade dominated throughout the study period in the pore waters. The higher representation of lesser-known transient members of lake communities such as Methylotenera in the pore waters of Aydat, and Clostridium and Sulfuricurvum in the pore and sediments of Grangent, respectively, were observed during the period of temporary anoxia in summer caused by lake stratification. Our study revealed that in the investigated lakes, the prevailing environmental factors across time and space structured and influenced the adaptation of bacterial communities to specific ecological niches.

Taxonomical Resolution and Distribution of Bacterioplankton Along the Vertical Gradient Reveals Pronounced Spatiotemporal Patterns in Contrasted Temperate Freshwater Lakes.

We examined the relationship between viruses and co-occurring bacterial communities across spatiotemporal scale in two contrasting freshwater lakes, namely meromictic Lake Pavin and dimictic Lake Aydat (Central France). Next-generation sequencing of 16S rRNA genes suggested distinct patterns in bacterioplankton community composition (BCC) between the lakes over depths and seasons. BCC were generally dominated by members of Actinobacteria, Proteobacteria, and Bacteroidetes covering about 95% of all sequences. Oxygen depletion at the bottom waters in Aydat and existence of permanent anoxia in the monimolimnion of Pavin resulted in the occurrence and dominance of lesser known members of lake communities such as Methylotenera, Methylobacter, Gallionella, Sulfurimonas, and Syntrophus in Pavin and Methylotenera and Sulfuritalea in Aydat. Differences in BCC appeared strongly related to dissolved oxygen concentration, temperature, viral infection, and virus-to-bacteria ratio. UniFrac analysis indicated a clear distinction in BCC when the percentage of viral infected bacterial cells and virus-to-bacteria ratio exceeded a threshold level of 10% and 5, respectively, suggesting a link between viruses and their potential bacterial host communities. Our study revealed that in both the lakes, the prevailing environmental factors across time and space structured and influenced the adaptation of bacterial communities to specific ecological niches.

Microbial diversity in support of anaerobic biomass valorization.

Microbial diversity provides an immense reservoir of functions and supports key steps in maintaining ecosystem balance through matter decomposition processes and nutrient recycling. The use of microorganisms for biomolecule production is now common, but often involves single-strain cultures. In this review, we highlight the significance of using ecosystem-derived microbial diversity for biotechnological researches. In the context of organic matter mineralization, diversity of microorganisms is essential and enhances the degradation processes. We focus on anaerobic production of biomolecules of interest from discarded biomass, which is an important issue in the context of organic waste valorization and processing. Organic waste represents an important and renewable raw material but remains underused. It is commonly accepted that anaerobic mineralization of organic waste allows the production of diverse interesting molecules within several fields of application. We provide evidence that complex and diversified microbial communities isolated from ecosystems, i.e. microbial consortia, offer considerable advantages in degrading complex organic waste, to yield biomolecules of interest. We defend our opinion that this approach is more efficient and offers enhanced potential compared to the approaches that use single strain cultures.

Functional Responses of Bacterioplankton Diversity and Metabolism to Experimental Bottom-Up and Top-Down Forcings.

We conducted an experimental approach using microcosms to simultaneously examine the functional response of natural freshwater bacterial assemblages to the impact of resources (nutrients) and top-down factors (viruses and grazers) on bacterial physiological state and their community structure. Addition of organic and inorganic nutrients led to the proliferation of high nucleic acid content bacterial cells accompanied by high bacterial growth efficiency (considered as proxy of bacterial carbon metabolism) estimates, suggesting that this subgroup represented the most active fraction of bacterial community and had a high capacity to incorporate carbon into its biomass. However, their rapid growth induced the pressure of viral lytic infection which led to their lysis toward the end of the experiment. In microcosms with flagellates plus viruses, and with viruses alone, the selective removal of metabolically active high nucleic acid cells through viral lysis benefitted the less active low nucleic acid content cells, perhaps via the use of lysis products for its growth and survival. Changes in bacterial physiological state in microcosms were reflected in their community structure which was examined using 16S ribosomal RNA (rRNA) gene sequencing by Illumina's Miseq platform. Chao estimator and Shannon diversity index values suggested that bacterial species richness was highest in the presence of both the top-down factors, indicating a tighter control of bacterioplankton dominants within a relatively stable bacterial community. The increase in bacterial metabolism with nutrient addition followed by subsequent lysis of bacterial dominants indicate that both resources and top-down factors work in concert for the sustenance of stable bacterial communities.

Linking host prokaryotic physiology to viral lifestyle dynamics in a temperate freshwater lake (Lake Pavin, France).

In aquatic ecosystems, fluctuations in environmental conditions and prokaryotic host physiological states can strongly affect the dynamics of viral life strategies. The influence of prokaryote physiology and environmental factors on viral replication cycles (lytic and lysogeny) was investigated from April to September 2011 at three different strata (epi, meta, and hypolimnion) in the mixolimnion of deep volcanic temperate freshwater Lake Pavin (France). Overall, the euphotic region (epi and metalimnion) was more dynamic and showed significant variation in microbial standing stocks, prokaryotic physiological state, and viral life strategies compared to the aphotic hypolimnion which was stable within sampled months. The prokaryotic host physiology as inferred from the nucleic acid content of prokaryotic cells (high or low nucleic acid) was strongly regulated by the chlorophyll concentration. The predominance of the high nucleic acid (HNA) prokaryotes (cells) over low nucleic acid (LNA) prokaryotes (cells) in the spring (HNA/LNA = 1.2) and vice versa in the summer period (HNA/LNA = 0.4) suggest that the natural prokaryotic communities underwent major shifts in their physiological states during investigated time period. The increase in the percentage of inducible lysogenic prokaryotes in the summer period was associated with the switch in the dominance of LNA over HNA cells, which coincided with the periods of strong resource (nutrient) limitation. This supports the idea that lysogeny represents a maintenance strategy for viruses in unproductive or harsh nutrient/host conditions. A negative correlation of percentage of lysogenic prokaryotes with HNA cell abundance and chlorophyll suggest that lysogenic cycle is closely related to prokaryotic cells which are stressed or starved due to unavailability of resources for its growth and activity. Our results provide support to previous findings that changes in prokaryote physiology are critical for the promotion and establishment of lysogeny in aquatic ecosystems, which are prone to constant environmental fluctuations.

Variable viral and grazer control of prokaryotic growth efficiency in temperate freshwater lakes (French Massif Central).

The effects of viral lysis and heterotrophic nanoflagellate grazing (top down forces) on prokaryotic mortality and their subsequent impact on their metabolism were estimated in the upper euphotic and deeper aphotic depth of 11 freshwater lakes located in the French Massif Central. The standing stocks of viruses (VA) and heterotrophic nanoflagellate (HNF) varied significantly (p < 0.05) with sampled depth. VA was substantially (twofold on an average) and significantly higher (p < 0.03) at the aphotic compared to euphotic depth, whereas the reverse was true (p < 0.02) for HNF. Among the prokaryote subgroup, high nucleic acid content prokaryotes explained for significant variability in the total VA and served as principle host target for viral proliferation. Like standing stocks, flagellate grazing and viral infection rates also followed similar patterns. In the investigated lakes, the mechanism for regulating prokaryotic production varied with sampled depth from grazing control in the euphotic to control due to viral lysis in the aphotic. We also tested the hypothesis of top down control on prokaryotic growth efficiency (PGE, which we used as an index of prokaryotic physiological and energetic status at the community level) at both depths. Overall, among the studied lakes, PGE varied widely (4-51 %) with significantly (p < 0.05) lower values in the aphotic (mean = 18 ± 4 %) than euphotic depth (mean = 32 ± 9 %). Contrasting observations on the top down control of PGE between sampled depths were observed. The presence of grazers was found to stimulate PGE at the euphotic, whereas viruses through their lytic infection had a strong negative impact on PGE at the aphotic depth. Such observed differences in PGE and the mechanism controlling prokaryotic production with depth could eventually have strong implication on carbon and nutrient flux patterns in the studied lakes.

[Revision of of the subfamily of Metaracoelophryinae de Puytorac 1972 (Oligohymenophora: Hoplytophryida: Hoplytophryidae), astome ciliates of the digestive tract of Oligochaeta worms of Africa: description of five new species]. / Révision de la sous-famille des Metaracoelophryinae de Puytorac 1972 (Oligohymenophora : Hoplytophryida : Hoplytophryidae), ciliés astomes du tube digestif d'oligochètes terricoles d'Afrique : description de cinq espèces nouvelles.

Five new species belonging to the astome ciliates, living in the digestive tract of Oligochaeta worms belonging to the genus Alma from Cameroon, have been described. The techniques used are: vital staining, staining of the nucleus with Diamidino Phenyl Indol (DAPI), scanning electron microscopy and silver staining method (Fernandez Galiano, 1976, 1994). This work confirms the presence of the genus Paracoelophrya and Dicoelophrya in the digestive track of the oligochaete Alma from Gabon and Cameroon; it helps to understand the general taxonomy of this Metaracoelophryinae subfamily. Moreover, the homogeneity of this group is confirmed and the phylogenetic relationship inside the Hoplitophryida order need more studies to be solved.

Trophic interactions between viruses, bacteria and nanoflagellates under various nutrient conditions and simulated climate change.

Population dynamics in the microbial food web are influenced by resource availability and predator/parasitism activities. Climatic changes, such as an increase in temperature and/or UV radiation, can also modify ecological systems in many ways. A series of enclosure experiments was conducted using natural microbial communities from a Mediterranean lagoon to assess the response of microbial communities to top-down control [grazing by heterotrophic nanoflagellates (HNF), viral lysis] and bottom-up control (nutrients) under various simulated climatic conditions (temperature and UV-B radiations). Different biological assemblages were obtained by separating bacteria and viruses from HNF by size fractionation which were then incubated in whirl-Pak bags exposed to an increase of 3°C and 20% UV-B above the control conditions for 96 h. The assemblages were also provided with an inorganic and organic nutrient supply. The data show (i) a clear nutrient limitation of bacterial growth under all simulated climatic conditions in the absence of HNF, (ii) a great impact of HNF grazing on bacteria irrespective of the nutrient conditions and the simulated climatic conditions, (iii) a significant decrease in burst size (BS) (number of intracellular lytic viruses per bacterium) and a significant increase of VBR (virus to bacterium ratio) in the presence of HNF, and (iv) a much larger temperature effect than UV-B radiation effect on the bacterial dynamics. These results show that top-down factors, essentially HNF grazing, control the dynamics of the lagoon bacterioplankton assemblage and that short-term simulated climate changes are only a secondary effect controlling microbial processes.

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.

Resources drive trade-off between viral lifestyles in the plankton: evidence from freshwater microbial microcosms.

Viruses have evolved different life strategies for coping with environmental challenges and this is a key explanation for their omnipresence in aquatic systems. However, factors that determine the balance between lytic versus lysogenic decision within natural virioplankton are poorly documented, primarily in freshwaters. This study was designed to investigate the experimental short-term (24 h incubation) effects of added organic and inorganic nutrients on the two viral lifestyles in nutrient-depleted freshwater microbial (i.e. <0.8 microm fraction) microcosms, using mitomycin C as prophage inductor agent. In the absence of mitomycin, viral lytic production increased as a functional response to the strong stimulation of bacterial growth rates (0.7-0.8 day(-1)) by the added nutrients, primarily the organic nutrients which appeared scarcer than inorganic nutrients and was related to the sampling period and the geomorphological peculiarities of Lake Pavin. In the presence of mitomycin, temperate phage production (frequency of lysogenically infected bacterial cells, FLC=17-19% of total cells) significantly exceeded lytic production (frequency of lytically infected bacterial cells, FIC=9-11%) in natural samples (i.e. without nutrient additions) as a result of enhanced prophage induction, which relatively increased with the decreasing contact probability between viruses and their potential hosts. In contrast, addition of nutrients drastically reduced FLC (<4%) and increased FIC (>22%). Both variables were antagonistically correlated as was the correlation between FLC and bacterial growth rates, supporting the idea that lysogeny may represent a maintenance strategy for viruses in harsh nutrient/host conditions which appeared as major instigators of the trade-off between the two viral lifestyles. Overall, at the community level, we reject the hypothesis that nutrients but mitomicyn C stimulate temperate phage induction, and retained the hypotheses that nutrients rather (i) stimulate lytic viruses via enhanced host growth and (ii) when limiting, promote lysogenic conversion in natural waters.

Differential impact of top-down and bottom-up forces in structuring freshwater bacterial communities.

Limited data exist on the simultaneous impact of bottom-up (nutrients) and top-down (viruses and heterotrophic nanoflagellates) forces in shaping freshwater bacterial communities. In our laboratory microcosms, nutrient additions (organic and inorganic) and viral reduction approach led to the proliferation of high nucleic acid (HNA) bacterial subpopulation without an increase in phage abundance. High viral-mediated bacterial lysis in the presence of nanoflagellates yielded high proportion of low nucleic acid bacterial subpopulation. 16S rRNA gene sequence analysis indicated that members of classes Proteobacteria and Bacteroidetes evoked differential responses to nutrients and mortality forces, thereby resulting in differences (P < 0.001) in bacterial community composition and diversity, as observed from analysis of similarities and UniFrac analysis. Bacterial species richness (Chao) and diversity (Shannon) index was significantly higher (P < 0.001) in the presence of both the top-down factors and viruses alone, whereas lower host diversity was observed under nutrient relaxation of growth-limiting substrates due to the explosive growth of opportunistic HNA bacterial subpopulation. Our results are in agreement with the theoretical model of 'killing the winner', where the availability of growth-limiting substrates can act as a stimulating factor for host community composition while top-down forces can operate in the control of host diversity.

Seasonal depth-related gradients in virioplankton: standing stock and relationships with microbial communities in Lake Pavin (France).

This study presents a depth-related survey of virioplankton abundance in Lake Pavin (Massif Central, France), in relation to the abundances of heterotrophic prokaryotes, picocyanobacteria (Pcy), autotrophic picoeukaryotes (Peu), and of autotrophic (ANF) and heterotrophic (HNF) nanoflagellates. The sampling strategy was designed to be representative of the physico-chemical gradients of the whole water column of the lake, and the seasonal variability as well. In mixolimnic surface waters, all communities were present and viral abundance peaked in summer and autumn. Viral abundance was significantly correlated (p < 0.001) with Pcy, Peu, and ANF, indicating that cyanophages and perhaps other phytoplankton viruses represent a significant pool of viral standing stocks in the mixolimnion of Lake Pavin. Microautotrophs were absent in the deep monimolimnic water masses, where viruses and heterotrophic prokaryotes exhibited highest seasonal abundances in summer and/or autumn and were significantly correlated (p < 0.001) to each other. This indicates that the anoxic monimolimnion of Lake Pavin is an exclusive habitat for viruses and heterotrophic prokaryotes. We conclude that in this habitat, host availability is prevalent over other factors (temperature, oxygen, nutrients, grazers) in favoring viral proliferation.

Virioplankton 'pegylation': use of PEG (polyethylene glycol) to concentrate and purify viruses in pelagic ecosystems.

We have described the use of Polyethylene glycol (PEG) for the precipitation of natural communities of aquatic viruses, and its comparison with the usual concentration method based on ultracentrifugation. Experimental samples were obtained from different freshwater ecosystems whose trophic status varied. Based on transmission electron microscope observations and counting of phage-shaped particles, our results showed that the greatest recovery efficiency for all ecosystems was obtained when we used the PEG protocol. On average, this protocol allowed the recovery of >2-fold more viruses, compared to ultracentrifugation. In addition, the diversity of virioplankton, based on genomic size profiling using pulsed field gel electrophoresis, was higher and better discriminated when we used the PEG method. We conclude that pegylation offers a valid, simple and cheaper alternative method to ultracentrifugation, for the concentration and the purification of pelagic viruses.

The Microbial Food Web in the Recently Flooded Sep Reservoir: Diel Fluctuations in Bacterial Biomass and Metabolic Activity in Relation to Phytoplankton and Flagellate Grazers.

The spatial distribution of the bacterial biomass and production and of potential heterotrophic activity (PHA) were measured every 4 h between 23 July (10:00 h) and 25 July (10:00 h) 1997 in a recently flooded oligo-mesotrophic reservoir (the Sep Reservoir, Puy-de-Dôme, France), in relation to temperature, the phytoplankton biomass and production, and the abundance of heterotrophic flagellates. The temperature varied slightly with time during the study, but the well-established thermal stratification agreed well with vertical distribution of the biological variables that were measured. Only the bacterial production and the PHA showed significant diel changes (t-test, p <0.05), with maxima at 18:00 h and minima at 02:00 h. A significant positive relation was found between bacterial abundance and that of heterotrophic flagellates, which, rather than being an association related to the thermal stratification of the water column, was considered to reflect a trophic relation between these two communities. A carbon balance analysis suggested that at least 30% of the C from primary production measured during the sampling period was used by bacteria, and that 42% of this secondary production, or 6% of the primary production, would be used for the development of the heterotrophic flagellates present. We conclude that the bacterioplankton forms, at least occasionally, an important source of carbon for higher trophic levels, and reject the hypothesis that bacterial production in the Sep Reservoir depends exclusively on organic matter of allochthonous origin.

Protozoan Bacterivory in the Ice and the Water Column of a Cold Temperate Lagoon.

> Abstract Bacterial abundance and bacterivorous protist abundance and activity were examined in ice-brine and water column communities of a cold temperate Japanese lagoon (Saroma-Ko Lagoon, Hokkaido, 44 degreesN, 144 degreesE), during the late winter phase of ice community development (February-March 1992). Bacterial abundance averaged 6 and 1 x 10(5) cells ml-1 in the ice-brine and plankton samples, respectively, and generally decreased during the sampling period. Bacterivorous protists, identified based on direct observation of short-term (<1 h) ingested fluorescently labeled bacteria (FLB) in their food vacuoles, were largely dominated by flagellates, mainly cryothecomonad-type and chrysomonad-like cells and small dinoflagellates of the genus Gymnodinium. Bacterivorous ciliates included mainly the prostomatid Urotricha sp., the scuticociliates Uronema and Cyclidium, the choreotrichs Lohmaniella oviformis and Strobilidium, and the hypotrich Euplotes sp. Protist abundance averaged 4 x 10(3) and 8.1 cells ml-1 in the ice-brine and 0.3 x 10(3) and 1.2 cells ml-1 in the plankton, for flagellates and ciliates, respectively. In contrast to bacteria, the abundance of protists generally increased throughout the sampling period, indicating predator-prey interactions. Protistan bacterivory, measured from the rate of FLB disappearance over 24 h, averaged 36% (ice) and 24% (plankton) of bacterial standing stock and exhibited the same seasonal pattern as for protist abundance. The calculated specific clearance (range, 2-67 nl protozoa-1 h-1) and ingestion (<1-26 particles protozoa-1 h-1) rates were likely to be minimal estimates and grazing impact may have been higher on occasion. Indications for the dependence of "bacterivorous protists" on nonbacterial food items were also provided. Although alternative sources of bacterial loss are likely to be of importance, this study provides evidence for the potential of protozoan assemblages as bacterial grazers in both sea ice-brine biota and water column at the southern limit of sea ice in the northern hemisphere.

Short-term variations of the abundance and biomass of planktonic ciliates in a eutrophic lake.

Short-time spatio-temporal variations of planktonic ciliates in a eutrophic lake were examined for evidence of diel vertical migration in relation to food supply (bacteria, nanoplankton, and detritus) and physico-chemical parameters. Two campaigns were conducted during successive summers in Lake Aydat, France. Ciliates were less abundant during the first campaign (July 1988; global means 1500 cells/1 and 61.0 µgC/l) than during the second (July 1989; 5000 cells/l; 110.8 µgC/l). On both dates, ciliate abundance decreased from surface to depth, while biomass increased. Each layer (epilimnion, metalimnion, hypolimnion) was inhabited by a separate ciliate community, generally dominated by bacterivores/detritivores. There was no clear evidence of vertical migrations across major physical-chemical boundaries. Circadian variations in each layer occurred independently of light-dark rhythms, partly due to micropatchiness. Temporal variability increased from the vertically mixed epilimnion (C.V. = 32%-41%) to the well stratified deeper zones (C.V. = 41%-100%). Ciliate biomass was negatively and significantly correlated to temperature and dissolved oxygen, and to most principal food parameters. The vertical differences in ciliate biomass were linked to the long-term (seasonal) evolution of the lake system (correlations with temperature, oxygen) and its larger-sized food resources (correlations with chlorophyll, nanoplankton, detritus). By contrast, circadian variations of ciliates were linked to availability of bacteria, an important food resource for many ciliate species of this study.

Depth-related gradients of viral activity in Lake Pavin.

High-resolution vertical sampling and determination of viral and prokaryotic parameters in a deep volcanic lake shows that in the absence of thermal stratification but within light, oxygen, and chlorophyll gradients, host availability empirically is prevalent over the physical and chemical environments and favors lytic over lysogenic "viral life cycles."