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) a concentration increased from <5 g l^-1 up to 19 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 <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.      

Protistan bacterivory in an oligomesotrophic lake: Importance of attached ciliates and flagellates

Seasonal and depth variations of the abundance, biomass, and bacterivory of protozoa (heterotrophic and mixotrophic flagellates and ciliates) were determined during thermal stratification in an oligomesotrophic lake (Lake Pavin, France). Maximal densities of heterotrophic flagellates (1.9 × 10³ cells ml^–1) and ciliates (6.1 cells ml^–1) were found in the metalimnion. Pigmented flagellates dominated the flagellate biomass in the euphotic zone. Community composition of ciliated protists varied greatly with depth, and both the abundance and biomass of ciliates was dominated by oligotrichs. Heterotrophic flagellates dominated grazing, accounting for 84% of total protistan bacterivory. Maximal grazing impact of heterotrophic flagellates was 18.9 × 10⁶ bacteria 1^–1h^–1. On average, 62% of nonpigmented flagellates were found to ingest particles. Ciliates and mixotrophic flagellates averaged 13% and 3% of protistan bacterivory, respectively. Attached protozoa (ciliates and flagellates) were found to colonize the diatom Asterionella formosa. Attached bacterivores had higher ingestion rates than free bacterivorous protozoa and may account for 66% of total protozoa bacterivory. Our results indicated that even in low numbers, epibiotic protozoa may have a major grazing impact on free bacteria. Correspondence: C. Amblard.     

Bacterivory of metazooplankton, ciliates and flagellates in a newly flooded reservoir

Bacterial consumption by metazoan zooplankton and phagotrophic protists was measured in situ during the period of thermal stratification in the epilimnion (1 m) and metalimnion (7 m) of a newly flooded reservoir (Sep reservoir, France). The mean bacterial consumption was 2.53 x 10⁶ bacterial l^-1 h^-1 at 1m and 0.97 x 10⁶ bacteria l^-1 h^-1 at 7m. The main consumers over the whole study period were the cladocerans Daphnia longispina and Ceriodaphnia quadrangula, accounting on average for 72% of the potential total predation of bacteria at 1 m and 56% at m, especially during the months of May-June and August. Heterotrophic nanoflagellates (HNF), which accounted for 12% estimated total predation of bacteria at 1 m and 13% at 7m, only exerted a limited predation, mainly by a Monas-type cell. Ciliates, dominated in terms of abundance by Pelagohalteria viridis, accounted for 4% of total predation in the epilimnion (0.00-0.42 x 10⁶ bacteria l^-1 h^-1). In a newly flooded reservoir, metazoan zooplankton appear to be the main consumers of bacteria. Predation of ciliates and HNF by zooplanktonic crustaceans could account for the low contribution of components of the microbial loop to bacterial consumption.      

Short communication. Biomass and production characteristics of the first red tide noticed in Kuwait Bay, Arabian Gulf

Observations on the first red tide off Kuwait, Arabian Gulf, yielded high biomass [55.4-262.7 g chlorophyll (Chl) a l^-1] and primary production (507.9-571.2 g C h^-1l^-1), comparable with some of the highest values reported. There were contrasting changes in the carbon assimilation ratios, composition of the phytoplankton and the contribution of nanoplankton (<20 m) between 11 and 12 May. On 11 May, carbon assimilation by nanoplankton was 2.2 g C (g Chl a)^-1 h^-1, but increased to 9.2 g C (g C (g Chl a)^-1 h^-1 the following day. Nanoplankton contributed 56 and 9% of Chl a and production, respectively, on 11 May, while net plankton (>20 m) accounted for >65% of the biomass and primary production on 12 May. Picoplankton (<3 m) contributed <8% of Chl a and <3% of production. Frequent sampling will be necessary to capture such dynamic changes and ephemeral events in these waters.      

The Soil Flagellate Heteromita globosa Accelerates Bacterial Degradation of Alkylbenzenes through Grazing and Acetate Excretion in Batch Culture

The impact of grazing by soil flagellates Heteromita globosa on aerobic biodegradation of benzene by Pseudomonas strain PS+ was examined in batch culture. Growth of H. globosa on these bacteria obeyed Monod kinetics (_max, 0.17 ± 0.03 h^–1; K_s, 1.1 ± 0.2 × 10⁷ bacteria mL^–1) and was optimal at a bacteria/ flagellate ratio of 2000. Carbon mass balance showed that 5.2% of total [ring-U-¹⁴C]benzene fed to bacteria was subsequently incorporated into flagellate biomass. Growth-inhibiting concentrations (IC₅₀) of alkylbenzenes (benzene, toluene, ethylbenzene) were inversely related with their octanol/ water partitioning coefficients, and benzene was least toxic for bacteria and flagellates with IC₅₀ values of 4392 (± 167) M and 2770 (± 653) M, respectively. The first-order rate constant for benzene degradation (k₁, 0.48 ± 0.12 day^–1) was unaffected by the presence or absence of flagellates in cultures. However, the rate of benzene degradation by individual bacteria averaged three times higher in the presence of flagellates (0.73 ± 0.13 fmol cell^–1 h^–1) than in their absence (0.26 ± 0.03 fmol cell^–1 h^–1). Benzene degradation also coincided with higher levels of dissolved oxygen and a higher rate of nitrate reduction in the presence of flagellates (p < 0.02). Grazing by flagellates may have increased the availability of dissolved oxygen to a smaller surviving population of bacteria engaged in the aerobic reactions initiating benzene degradation. In addition, flagellates may also have increased the rate of nitrate reduction through the excretion of acetate as an additional electron donor for these bacteria. Indeed, acetate was shown to progressively accumulate in cultures where flagellates grazed on heat-killed bacteria. This study provided evidence that grazing flagellates stimulate bacterial degradation of alkylbenzenes and provide a link for carbon cycling to consumers at higher trophic levels. This may have important implications for bioremediation processes.     

Benthic Bacterial Production and Protozoan Predation in a Silty Freshwater Environment

The interrelation of heterotrophic bacteria with bacterivorous protists has been widely studied in pelagic environments, but data on benthic habitats, especially in freshwater systems, are still scarce. We present a seasonal study focusing on bacterivory by heterotrophic nanoflagellates (HNF) and ciliates in the silty sediment of a temperate macrophyte-dominated oxbow lake. From January 2001 to February 2002 we monitored the standing stock of bacteria and protozoa, bacterial secondary production (BSP, ³H-thymidine, and ¹⁴C-leucine incorporation), and grazing rates of HNF and ciliates on bacteria (FLB uptake) in the oxic sediment of the investigated system. BSP ranged from 470 to 4050 µg C L^–1 wet sediment h^–1. The bacterial compartment turned out to be highly dynamic, indicated by population doubling times (0.6–10.0 d), which were comparable to those in the water column of the investigated system. Yet, the control mechanisms acting upon the bacterial population led to a relative constancy of bacterial standing stock during a year. Ingestion rates of protozoan grazers were 0–20.0 bacteria HNF^–1 h^–1 and 0–97.6 bacteria ciliate^–1 h^–1. HNF and ciliates together cropped 0–14 (mean 4)% of BSP, indicating that they did not significantly contribute to benthic bacterial mortality during any period of the year. The low impact of protozoan grazing was due to the low numbers of HNF and ciliates in relation to bacteria (1.8–3.5 × 10⁴ bacteria HNF^–1, 0.9–3.1 × 10⁶ bacteria ciliate^–1). Thus, grazing by HNF and ciliates could be ruled out as a parameter regulating bacterial standing stock or production in the sediment of the investigated system, but the factors responsible for the limitation of benthic protistan densities and the fate of benthic BSP remained unclear.     

Trophic interactions among heterotrophic microplankton,nanoplankton, and bacteria in Lake Constance

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 µm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics.Clearance rates varied between 0.2 and 122.8 nl HNF^–1 h^–1 and between 0.2 and 53.6 µl ciliate^–1 h^–1, respectively.Ingestion and clearance rates measured for HNF and ciliates are in good agreement with results obtained by other investigators from different aquatic environments and from laboratory cultures. Both the abundance of all three major microheterotrophic categories — bacteria, HNF, and ciliates — and the grazing pressure within the microbial loop show pronounced seasonal variations.     

Spatial variability of the photosynthetic parameters and biomass of the Gulf of California phytoplankton

Spatial variability of the central Gulf of California (CGC) phytoplankton biomass and photosynthetic parameters in relation to physical forcing was studied. Sampling was carried out in November, and the surface TC range was 20-27.5°C. Strong tidal mixing in the midrift islands regions injects relatively cool, nutrient-rich waters to the euphotic zone. Some of this water is transported via jets and cool filaments throughout the Gulf. In general, chlorophyll a (Chl) of small phytoplankton (<8 m) (up to >2.5 mg m^-3) was higher than that of large phytoplankton. Highest values of phytoplankton assimilation numbers (P^Bm) [3.17 mg C (mg Chla)^-1 h^-1], and photosynthetic efficiency ^B) [0.23 mg C (mg Chl a)^-1 h^-1 (W m^-2)^-1] were determined for the large phytoplankton cells (>8 m). Our hypothesis that P^Bm values increase from cooler to warmer waters is not supported by the data. We found a 27-fold spatial difference of Chl, compared with a 10-fold difference of P^Bm and a 6-fold difference of ^B. Thus, in our study area, the major source of variability for primary productivity (PP) comes from Chl, and not from P^Bm and ^B. Therefore, we propose that it is possible to estimate late-fall PP for the CGC using average photosynthetic parameters. Average values for P^Bm and ^B of total phytoplankton were 0.72 mg c (mg Chl a)^-1 h^-1 and 0.12 mg C (mg Chl a^-1 h^-1, (W m^-2)^-1, with standard errors of 0.07 and 0.03, respectively.      

Indirectly fluorescently labelled flagellates (IFLF): a tool to estimate the predation on free-living heterotrophic flagellates

A procedure was developed to estimate the direct grazing impacton free-living heterotrophic nanoflagellates (HNF). Culturedflagellates were labelled by feeding on brightly fluorescingbacteria (FLB) and then offered as indirectly fluorescentlylabelled flagellates (IFLF) to potential predators of HNF. Thenumber of FLB in the predators' food vacuoles could be convertedinto IFLF uptake and consumption of HNF. This new techniquewas used to study the HNF-ciliate relationship in the pelagiczone of Lake Constance. Three groups of ciliates were detectedas HNF grazers: small representatives of the genus Strobilidium.a small Haltena-like ciliate (probably Halteria grandinella)and a Codonella sp. Tintinnidium sp. group The ingestion ofHNF by these groups of ciliates ranged between 3 and 15, 3 and39, and 3 and 7 HNF ciliate^–1 h^–1; respectively.The IFLF method allows the direct determination of ingestedflagellate prey in the food vacuoles of their predators. Becauseindigenous living prey organisms were used, tracer discriminationcan be reduced.     

Seasonal variation of particulate organic carbon, dissolved organic carbon and the contribution of microbial communities to the live particulate organic carbon in a shallow near-bottom ecosystem at the Northwestern Mediterranean Sea

Microbial planktonic communities (i.e. bacteria and protozoa), phytoplankton, dissolved organic carbon (DOC) and particulate organic carbon (POC) were seasonally examined at Medes Islands (Northwestern Mediterranean) to assess their variation in abundance and composition throughout the year in a near-bottom littoral ecosystem. From October 1995 to November 1996, samples were collected between two and six times per month at 0.5 m above the bottom. Mean DOC and POC values throughout the year were 2560 180 (SE) and 387 ± 35 g C l^-1, respectively. All year, detrital organic carbon (detrital=total POC - live carbon) represented the main POC fraction, and mean live carbon was 24 ± 9 g C l^-1. Winter and spring had maximum values of POC, and spring and summer had maximum values of DOC. Heterotrophic bacteria, with a mean abundance of 5.16 ± 0.08 x 10⁵ cells ml^-1, were the main contributor to live carbon (26 ± 7%). During winter, heterotrophic bacterial biomass decreased 40% due to a decrease in mean biovolume per cell. Synechococcus sp. and Prochlorococcus sp. abundance were 2.24 ± 0.09 x 10⁴ and 1.05 ± 0.07 x 10⁴ cells ml^-1, respectively. However, while Synechococcus sp. were present all year, Prochlorococcus sp. were not observed from April to July. Mean phytoplankton (i.e. diatoms and dinoflagellates) abundance was 2.06 ± 0.40 x 10⁴ cells l^-1 with biomass at a maximum during the winter months, the period with the lowest temperature and the highest nutrient concentration. The size composition of live carbon showed two clearly distinct periods: from December to March, live carbon was dominated in biomass by microplankton, while from April to November, pico- and nanoplankton cells were dominant. Overall, the dynamics of the near-bottom planktonic communities was characterized by a low biomass of heterotrophic and autotrophic bacteria, phytoplankton and ciliates in contrast to previous water column studies. This pattern and the high temporal heterogeneity of the different planktonic communities are discussed in relation to the physical and chemical characteristics of the environment, as well as to the potential role that benthic communities may be exerting in the control of the near-bottom planktonic communities.      

Seasonal cycle of the microbial plankton in Crooked Lake,Antarctica

Summary Changes in the abundance of the components of the microbial plankton between July 1990 and March 1991 in Crooked Lake, one of the largest and deepest freshwater lakes in Antarctica, are described. Chlorophyll a concentration is low (0.2–0.4g·1^–1) and there is no discernable spring increase. The phytoplankton is largely dominated by flagellates. Bacterioplankton exhibits a seasonal pattern of abundance ranging from 1.0 × 10⁸·1^–1 in July to 3.25 × 10⁸·1^–1 in September. Changes in bacterial abundance probably relate to temperature and grazing by heterotrophic and mixotrophic flagellates. Total flagellated protozoan concentrations ranged between 25–136 × 10²·l^–1. Autotrophic and heterotrophic flagellate abundances were coupled and peaks in their abundance oscillated with peaks in bacterioplankton concentration. Four species of ciliated protozoa, dominated by oligotrichs, particularly the plastidic Strombidium, inhabit the lake. The plankton is characterised by the presence of floes which act as loci for bacteria, flagellates and amoebae and feeding sites for the ciliates and the two sparce metazoan components of the plankton. Crooked Lake is extremely oligotrophic but nonetheless supports a plankton community with a low species diversity and simple trophodynamics.     

Dynamics of Microbial Communities on Marine Snow Aggregates: Colonization, Growth, Detachment, and Grazing Mortality of Attached Bacteria

We studied the dynamics of microbial communities attached to model aggregates (4-mm-diameter agar spheres) and the component processes of colonization, detachment, growth, and grazing mortality. Agar spheres incubated in raw seawater were rapidly colonized by bacteria, followed by flagellates and ciliates. Colonization can be described as a diffusion process, and encounter volume rates were estimated at about 0.01 and 0.1 cm³ h⁻¹ for bacteria and flagellates, respectively. After initial colonization, the abundances of flagellates and ciliates remained approximately constant at 10³ to 10⁴ and ~10² cells sphere⁻¹, respectively, whereas bacterial populations increased at a declining rate to >10⁷ cells sphere⁻¹. Attached microorganisms initially detached at high specific rates of ~10⁻² min⁻¹, but the bacteria gradually became irreversibly attached to the spheres. Bacterial growth (0 to 2 day⁻¹) was density dependent and declined hyperbolically when cell density exceeded a threshold. Bacterivorous flagellates grazed on the sphere surface at an average saturated rate of 15 bacteria flagellate⁻¹ h⁻¹. At low bacterial densities, the flagellate surface clearance rate was ~5 × 10⁻⁷ cm² min⁻¹, but it declined hyperbolically with increasing bacterial density. Using the experimentally estimated process rates and integrating the component processes in a simple model reproduces the main features of the observed microbial population dynamics. Differences between observed and predicted population dynamics suggest, however, that other factors, e.g., antagonistic interactions between bacteria, are of importance in shaping marine snow microbial communities.     

Selection for fertility in mice — the selection plateau and how to overcome it

Summary A long-term experiment for increasing the traits first day litter size (LS1) and litter weight (LW1) was conducted with two populations for 33 generations. The selection plateau was reached in population DU-C (selection and estrus synchronization (h² = 0.02±0.01); in population DU-K (selection) the plateau (h² = 0.05±0.2) was nearly reached. Selection progress per generation was in LS1 b = 0.11±0.02; b = 0.12±0.04 (1st to 18th generation DU-K, DU-C) and b = 0.10±0.03; b = 0.07±0.05 (19th to 33rd generation, DU-K, DU-C) in LW1 b = 0.16±0.04 g; 0.19±0.07 g (DU-K, DU-C) b = 0.20±0.09 g; 0.001±0.09 g (DU-K, DU-C). Reverse and relaxe selection as well as systematic inbreeding was applied for 10 generations. Reverse selection yielded h² = 0.28±0.11 (R-DU-K) and h² = 0.17±0.05 (R-DU-C) and showed that there was still additive genetic variance. Relaxe selection did not cause alterations in the selection parameters, whereas inbreeding lead to inbred depressions (b = LS1 = –0.42±0.15; –0.45±0.12; b = LW1 = –1.13±0.20; –0.82±0.18 I-DU-K, I-DU-C). The plateau was based upon the heterozygote advantage. Several methods for overcoming the plateau were applied. A new selective useful variance could be created by crossing the plateau populations (h² = 0.14±0.04). A short-term progress in overcoming the plateau (1st to 3rd generation) could be obtained by litter size standardization (LS = 388). Tandem selection (selection for body weight — BW42) as well as crossing of inbred strains were not suitable for overcoming the selection plateau. Altering the environmental conditions as a possibility for overcoming the plateau has been discussed.     

Effects of salinity, light and inorganic nitrogen on growth and toxigenicity of the marine dinoflagellate Alexandrium tamarense from northeastern Canada

Growth and toxin production of a highly toxic clone of the marine dinoflagellate Alexandrium tamarense, isolated from the lower St Lawrence estuary (Quebec) in eastern Canada, were studied in unialgal batch cultures under different conditions. Controlled experiments were conducted on the production of paralytic shellfish poisoning (PSP) toxins under conditions of varying light (40, 60, 150, 230 and 470 mol m^-21 s^-1), salinity (10, 15, 20, 25 and 30) and nitrate concentrations (0, 88, 364, 528 and 880 mol l^-1). The effects of variable environmental factors on both toxin composition (% molar) and cell toxicity [pg STXeq (saxitoxin equivalents) cell^-1] were determined through the culture cycle. The toxin profile (% molar; mean SD), determined by high-performance liquid chromatography with fluorescence detection (HPLC-FD), remained stable and was consistently dominated by the low-potency N-sulfocarbamoyl toxins C1/C2 (64.0 ± 3%). There were also substantial relative amounts of the high-toxicity carbamate derivatives gonyautoxin 1-4 (GTX1-4) (1.7 ± 0.5%), neosaxitoxin (NEO) (16.2 ± 2%) ans saxitoxin (STX) (17.8 ± 2%). The cellular toxicity (mean ± SD: 58.8 ± 7 pg STXeq cell^-1) was essentially independent of light, salinity and nitrate concentration throughout the exponential growth phase, but varied over the growth stages in cultures. A positive correlation was observed between cellular toxicity and salinity-dependent growth rate, indicating that cell toxin quota may be affected by extrinsic factors, but it is not always a direct functional response to specific environmental stress.      

An approach to measure ciliate grazing on living heterotrophic nanoflagellates

The complicated routes by which organic material is channelled up to higher trophic levels via bacteria and protozoans is a major issue in aquatic microbial ecology. Because of the fragile nature of protists it is not straightforward to perform experimental studies of prey–predator interactions. Here we present an approach for the assessment of ciliate grazing on living heterotrophic nanoflagellates. Stationary phase cultures of a heterotrophic nanoflagellate (Cafeteria sp.) were live-stained by allowing them to take up fluorescently labelled macromolecules. Controls revealed that this label persisted for several hours. Fluorescently labelled living flagellates (FLLF) were added into enriched natural assemblages of marine oligotrich ciliates and uptake of FLLF was monitored over time. Oligotrich ciliates did not incorporate fluorescent-labelled macromolecules but a linear FLLF uptake over time was observed for 20–30 min at 20°C. Ingestion rates were 21–46 FLLF h^–1 at a concentration of about 2×10⁴ FLLF ml^–1, which corresponded to clearance rates of 0.7–0.8 l ciliate^–1 h^–1. These results are in the same order of reported ciliate grazing on phytoplankton of similar size. This method represents a direct approach to measure ciliate grazing specifically on living heterotrophic nanoflagellates.     

Adaptation of renal function to hypotonic medium in the winter flounder (Pseudopleuronectes americanus)

Summary The kidneys of winter flounders transferred to hypotonic medium were investigated for glomerular and tubular handling of fluid and electrolytes and for the urinary excretion of proteins. Media were sea water (925 mosm·kg^–1) and brackish water (70 mosm·kg^–1).In sea water, the urine was hypertonic to the plasma in 7 fish of this study. Urine flow rate was correlated with the GFR. After adaptation to brackish water a delay of 1 to 3 days was observed until the kidneys switched from fluid retention to the excretion of large amounts of dilute urine. GFR and urine flow rate were increased from 0.61±0.08 to 1.58±0.29 ml·h^–1·kg^–1 and from 0.14±0.02 to 0.68±0.08 ml·h^–1·kg^–1, respectively . With increased filtered load the tubular reabsorption of fluid decreased from 74±2.4% to 45±11.2%. The excretion rates of sodium and potassium were increased due to decreased fractional sodium and potassium reabsorption. The urinary excretion of divalent cations, however, was reduced because the net tubular reabsorption of calcium was increased and the net secretion of magnesium was diminished.Both the urinary total protein concentration and the protein pattern showed no significant change, but the rate of protein excretion was increased from 0.21±0.04 to 0.60±0.05 mg·h^–1·kg^–1. The comparison of protein patterns obtained from urine and serum samples revealed that high molecular weight (HMW) proteins prevail in the serum whereas low molecular weight (LMW) proteins dominate in the urine. The diminished quantity of the HMW-protein fraction in the urine thus may reflect size selectivity of the glomerular filtration barrier for serum proteins also in the winter flounder.Abbreviations BW brackish water - SW sea water - GFR glomerular filtration rate - HMW heigh molecular weight - LMW low molecular weight     

Changes in cell volume of bacteria and heterotrophic nanoflagellates in a hypereutrophic pond

Changes in cell volume of planktonic bacteria and heterotrophic nanoflagellates (HNF) were examined in a hypereutrophic pond from April to October, 1997. There were marked changes in the abundance of bacteria, HNF and ciliates and in protistan bacterivory during this period. The cell volume of free-living bacteria (0.121 ± 0.031 m³, mean ± SD) was large relative to that reported in the literature. The cell volumes of HNF was 71.1 ± 24.8 m³. Both cell volumes did not follow a seasonal trend. The dominant size class of bacteria was seasonally variable, whereas density of filamentous bacteria was relatively high between August and September. Biomass of filamentous bacteria accounted for up to 33.6% of total bacterial biomass. A correlation analysis for cell volume of bacteria and HNF, density of filamentous bacteria and some microbial variates was performed. The positive correlations detected (p<0.05) were between density of bacteria and cell volume of HNF, and between density of filamentous bacteria and cell volume of HNF.     

Abundance and biomass of heterotrophic microorganisms in Lake Tanganyika

1. This study focused on heterotrophic microorganisms in the two main basins (north and south) of Lake Tanganyika during dry and wet seasons in 2002. Bacteria (81% cocci) were abundant (2.28–5.30 × 10⁶ cells mL^?1). During the dry season, in the south basin, bacterial biomass reached a maximum of 2.27 g C m^?2 and phytoplankton biomass was 3.75 g C m^?2 (integrated over a water column of 100 m). 2. Protozoan abundance was constituted of 99% of heterotrophic nanoflagellates (HNF). Communities of flagellates and bacteria consisted of very small but numerous cells. Flagellates were often the main planktonic compartment, with a biomass of 3.42–4.43 g C m^?2. Flagellate biomass was in the same range and often higher than the total autotrophic biomass (1.60–4.72 g C m^?2). 3. Total autotrophic carbon was partly sustained by the endosymbiotic zoochlorellae Strombidium. These ciliates were present only in the euphotic zone and usually contributed most of the biomass of ciliates. 4. Total heterotrophic ciliate biomass ranged between 0.35 and 0.44 g C m^?2. In 2002, heterotrophic microorganisms consisting of bacteria, flagellates and ciliates represented a large fraction of plankton. These results support the hypothesis that the microbial food web contributes to the high productivity of Lake Tanganyika. 5. As the sole source of carbon in the pelagic zone of this large lake is phytoplankton production, planktonic heterotrophs ultimately depend on autochthonous organic carbon, most probably dissolved organic carbon (DOC) from algal excretion.     

Contribution of Ciliated Microprotozoans and Dinoflagellates to the diet of three Copepod species in the Bay of Biscay

Predation of three calanoid copepods (Calanus helgolandicus, Temora longicornis and Centropages chierchiae) on phytoplankton, dinoflagellates and ciliates was estimated in the Gironde estuarine plume (Bay of Biscay) during winter by means of in situ incubations. Both phytoplankton and ciliates were part of the diet of those three species, while only Centropages chierchiae also included a significant portion of dinoflagellates in its diet. The clearance rates of Calanus helgolandicus for ciliates and phytoplankton reached 2.8 and 4.0 ml copepod^–1 h^–1, respectively, those of Temora longicornis were 3.2 and 1.8 ml copepod^–1 h^–1, and those of Centropages chierchiae were 4.3 and 0.8 ml copepod^–1 h^–1.Neither Calanus helgolandicus nor Temora longicornis selected dinoflagellates, given the low clearance rates measured for this prey category (0.05 and 0.03 ml copepod^–1 h^–1, respectively). By contrast, Centropages chierchiae included dinoflagellates in its diet, with a clearance rate of 4.9 ml copepod^–1 h^–1. Within a given prey category (ciliates or dinoflagellates), all three copepods selected larger prey types (>40 m) over smaller ones (40 m). This implies a better detection and capture of larger motile prey compared to small ones. The results are discussed with regard to the omnivorous feeding behavior of these copepods observed here, during a late winter phytoplankton bloom.     

An in situ enclosure experiment to test the solar UVB impact on plankton in a high-altitude mountain lake. II. Effects on the microbial food web

We studied the impact of ambient levels of solar ultraviolet B (UVB) radiation on the planktonic microbial food web (viruses, heterotrophic bacteria, heterotrophic nanoflagellates and ciliates) of a high-mountain lake (2417 m above sea level) under in situ conditions for 16 days. Enclosures of 1 m³ receiving either the full sunlight spectrum or sunlight without UVB radiation were suspended at the lake surface. We found that the abundance of heterotrophic nanoflagellates was always lower in the +UVB treatment than in the -UVB one. In addition, bacterial consumption, measured by the disappearance of fluorescently labelled bacteria, was significantly (P < 0.05) reduced in the +UVB treatment. The abundance of non-filamentous bacteria (<10 m long) was also lower in the +UVB treatment, suggesting a direct effect of UVB on their growth. This was supported by the significantly (P < 0.05) lower cell-specific activity ([³H])thymidine incorporation) found on the fifth day of the experiment. In contrast, UVB radiation had no effect on filamentous bacteria (>1 m long), which represented only a small fraction of the total abundance (<4%), but up to 70% of the total bacterial biovolume. Ciliates, mainly Urotricha pelagica and Urotricha furcata, were less impacted by UVB radiation, although the net growth rate during the first week of the experiment was lower in the +UVB treatment than in the -UVB one (0.22 and 0.39 day^-1, respectively). The abundance of virus-like particles during the first week of the experiment was higher in the -UVB treatment. After reaching the maximum value for the interaction viruses x bacteria, viral abundance decreased dramatically (by 85%) in both treatments with a decay rate of 0.017 h^-1. This study illustrates the complexity in assessing the impact of UVB radiation when more than one trophic level is considered and indicates the existence of different sensitivity to UVB radiation among components of the microbial food web.