Gilles Bourdier

The Microbial Loop at the Surface of a Peatland:Structure, Function, and Impact of Nutrient Input

A bstractThe aim of this study was to assess the structure and function of the microbial loop in a peatland of the French Massif central, and the impact of fertilization on the different microbial communities.In terms of biomass, testate Amoeba (48% of the total microbial biomass), heterotrophic bacteria (15%), cyanobacteria (14%) and Bacillariophyceae (13%) were the dominant groups of microorganisms. Other microalgae (7%), ciliates (2%) and heterotrophic flagellates (1%) accounted for only a low proportion of total microbial biomass. The relative importance of heterotrophic microorganisms was higher than in marine or lacustrine environments. In addition, ciliates and heterotrophic flagellates only constituted a small proportion of the total protozoan biomass, which was heavily dominated by testate amoeba. Thus, the structure of the protist community in the subaquatic peatland was completely different from that reported for lakes or marine environments.In other aspects, the supply of nutrients (PKCa and NPKCa) resulted in increases of the relative biomasses of heterotrophic bacteria, Bacillariophyceae, and ciliates and by a decrease in the relative proportion of testate amoeba and of other microalgae.

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.

Short-term effect of nitrogen enrichment on the microbial communities of a peatland

The aim of this study was to assess the structure of the microbial loop in a Sphagnum fallax —Carex rostrata fen of the French Massif central,and the impact of nitrogen supplies on the different microbial communities. Microalgae (46% of the total microbial biomass),Protozoa (26%) and heterotrophic Bacteria (17%) were the dominant microorganisms.Rotifera (5%),Cyanobacteria (3%),Fungi (2%) and Nematoda (< 1%) were also present.Testate Amoebae were well represented in Sphagnum peatland (14% of total microbial biomass).Thus,the structure of the protist community in the surface of fen peatlands is notably different from that recorded in other environments.The input of nitrogen led to a steady increase in microbial biomasses,but only changed the structure of microbial communities significantly when the input was 50 kg ha-1.At high inputs,nitrogen supply increased the relative importance of Cyanobacteria, Euglenophyceae, Bacillariophyceae and Ciliates, and decreased the relative importance of heterotrophic Bacteria, other microalgae and testate Amoebae. The increase in the values of photosynthetic assimilation and heterotrophic activities could also reflect changes in community functioning.

Short-term variations in specific biovolumes of different bacterial forms in aquatic ecosystems.

Short-term and spatial fluctuations in specific biovolumes (volume x cell−1) of different morphological categories of planktonic bacteria were estimated microscopically. Samples were taken from two lakes occurring in two different climatic systems: Lake Aydat (France) and Lake Cromwell (Canada). The study was done in summer, using 24-hour cycles of sampling.Due to their large size, the specific volume of filamentous bacteria constituted, on average, the major part (>70%) of the total specific volume of all bacterial forms considered. Greatest variations in specific biovolumes were recorded for filamentous bacteria (coefficients of variation ranged from 16 to 109%). These variations were more pronounced in the oxygenated and microaerophilic strata (DOC ≈1.5 mg liter−1). Fluctuations in cell volume were high (coefficients of variation =12–80%) for coccal bacteria, whereas no marked fluctuations were found for the rod and vibrio bacteria (coefficients of variation =4–10%).Evidence of diel patterns of cell volume of filamentous bacteria is provided. These cells displayed their maximum size during the day until early night, indicating cell division was occurring at night. Homogeneous circadian patterns were not provided by specific volume variations of coccal, rod, and vibrio bacteria.Statistical relationships between bacterial specific biovolumes and the biotic and abiotic parameters considered are discussed.

The microbial loop in a humic lake: seasonal and vertical variations in the structure of the different communities

Seasonal and vertical variations of the main microbial communities (heterotrophic bacteria, autotrophic picoplank- ton, auto- and heterotrophic nanoflagellates, ciliated protozoa and microalgae) and auto- and heterotrophic activities were estimated in a brown-colored humic and moderately acid lake in central France, the lake of Vassiviere. The results demonstrated the dominant role of light in the vertical distribution of autotrophic and mixotrophic microor- ganisms which are confined to the 0–5 m layer during thermal stratification. The bacterial biomass was high throughout the water column probably because of the great availability of dissolved organic matter. Consequently, the predatory microzooplankton and particularly the various trophic groups of ciliated protozoa, were distributed in the water column according to the vertical distribution of the particular food resources (detritus, bacteria, algae). However, despite the great abundance of algae and bacteria, biomass of flagellated and ciliated protozoa was relatively weak. Most of the phytoplanktonic biomass was filamentous (Diatoms) or colonial (Cyanobacteria) and therefore almost probably difficult to ingest for algivorous microzooplankton. Regarding the low abundance of bacterivorous protozoa, the relation with the special physicochemical properties of this lake is discussed.

Neutral carbohydrate geochemistry of particulate material (trap and core sediments) in an eutrophic lake (Aydat, France)

Carbohydrate compositions were determined on sinking particles and core samples from eutrophic lake Aydat. Carbohydrate fluxes indicate that phytoplanktonic production is the main organic matter (OM) source whereas the monosaccharides concentrations normalised to OC contents (TImage) mostly reflect degradation. The abundance of rhamnose in all samples, its increasing fluxes (together with fucose) with increasing water depth, and its strong decrease in the top 5 cm of the bottom sediments mark the intense activity of the microbiota. The almost complete removal of sugars in the water and top sediment indicates very little contribution to the preserved OM stock.

Changes in the structure and metabolic activities of periphytic communities in a stream receiving treated sewage from a waste stabilization pond

Abstract In order to determine the impact of the treated water discharge of a waste stabilization pond on the periphyton in the stream receptor, the structure and the metabolic activities of the periphytic communities were studied above and below the outflow. Only slight changes were observed in the bacterial community. The Diatom abundance and biomass and the relative abundance of the different species differed between upstream and downstream stations. The heterotrophic index was generally lower in the upstream control station than in the downstream stations, whereas periphytic chlorophyll a concentrations decreased downstream of the stabilization pond. This implied an increase in heterotrophic microorganisms and/or an adaptation by autochthonous periphytic microorganisms. Photosynthetic carbon fixation decreased beyond the pond outflow maybe because of the disappearance of sensitive species, while the appearance of other resistant species led to an increase in productivity (autotrophic assimilation/chl a concentrations). Finally, potential chemoheterotrophic activity increased in the downstream stations, while the potential photoheterotrophic activity decreased. On the whole, the pond effluent preferentially favoured the development of chemoheterotrophic microorganisms to the detriment of photosynthetic microorganisms.

Effects of a pulp and paper mill effluent on the structure and metabolism of periphytic algae in experimental streams

Abstract The impact of a pulp and paper mill effluent on periphytic algae growing under controlled conditions in experimental streams was manifested as a drop in species richness and diversity, and changes in the taxonomic structure of the algal community. The decrease in diatom density was compensated by the relative contribution of Chlorophyceae, which increased with increasing effluent concentration (0.5%, 2.5% and 10% v/v). The presence of effluent also led to a rise in heterotrophic biomass and activity. However, the stream with the highest effluent concentration (10% v/v) showed a greater autotrophic biomass, and had the lowest photosynthetic efficiency, suggesting that algal species which develop under such conditions exhibit important heterotrophic activity. This hypothesis was supported by micro-autoradiographic studies, which showed that Chlamydomonas sp. cells were very strongly marked after incubation in the presence of amino acids, while diatoms showed little or no radioactivity. This suggested that the dissolved organic carbon in the effluent acted as a eutrophication factor for periphytic algal communities.

From Aquatic to Terrestrial Food Webs: Decrease of the Docosahexaenoic Acid/Linoleic Acid Ratio

Fatty acid composition of the adipose tissue of six carnivorous mammalian species (European otter Lutra lutra, American mink Mustela vison, European Mink Mustela lutreola, European polecat Mustela putorius, stone marten Martes foina and European wild cat Felis silvestris) was studied. These species forage to differing degrees in aquatic and terrestrial food webs. Fatty acid analysis revealed significant differences in polyunsaturated fatty acid composition between species. More specifically, our results underline a gradual significant decrease in the docosahexaenoic acid (DHA)/linoleic acid (LNA) ratio of carnivore species as their dependence on aquatic food webs decreases. In conclusion, the use of the DHA/LNA ratio in long-term studies is proposed as a potential proxy of changes in foraging behaviour of semi-aquatic mammals.

Importance of ciliated protozoa in relation to the bacterial and phytoplanktonic biomass in an oligo-mesotrophic lake, during the spring diatom bloom

The abundance and the biomass of bacterial, phytoplanktonic, and ciliate communities were estimated at different depths during the spring planktonic development in an oligo-mesotrophic lake (the Pavin lake).The bacterial population, which consists mainly of free bacteria (94% of the total bacterial abundance), displays only low cell densities (0.6 to 7 × 105 cells · ml−1) and represents low biomass values (0.9 to 11.5 µgC·l−1) The bacteria represent from 0.9 to 23.8% (M = 9.7%) of the “microplanktonic biomass” (with the exclusion of heterotrophic nanoflagellates, i.e. bacteria + phytoplankton + ciliates, size range 0.2–160 µm). The abundance of the phytoplankton varies between 0.5 and 1.8 × 106 cells·l−1, and the biomass values between 12 and 118 µC·l−1. The phytoplankton population constitutes the largest part of the microplanktonic biomass (51.9 to 96.6%, M = 80.6%), and the diatomMelosira italica subsp.subarctica is the largely dominant species of this community. The population of ciliates, essentiallyOligotrichida andScuticociliatida, displays densities between 1.3 and 38.3 × 103 cells·l−1 (M = 6.7 × 103 cells·l−1), and biomass values vary from 0.10 to 16.30 µgC·l−1 (M = 6.01 µgC·l−1). The ciliates constitute thus from 0.1 to 26.4% (M = 9.8%) of the microplanktonic biomass. Whereas the oligotrichs are best represented in the euphotic zone, the small-sized scuticociliates dominate in the hypolimnion. Besides, species having symbionts and considered to be mixotrophic (Strobilidium gyrans, Strombidium viride, Stokesia vernalis) develop preferentially in the epilimnion and constitute more than 50% of the total ciliate biomass.