Rémy D. Tadonléké

Microbial Food Webs in Boreal Humic Lakes and Reservoirs: Ciliates as a Major Factor Related to the Dynamics of the Most Active Bacteria

In order to assess the factors that determine the dynamics of bacteria with high nucleic acid content in aquatic systems, we (i) conducted 24-h in situ dialysis experiments, involving different fractions of plankton and unfiltered water and (ii) examined empirical relationships between bacteria and both abiotic factors and protists, in boreal humic freshwaters (reservoir and lakes) in the James Bay region (Québec, Canada). Bacteria were subdivided into two subgroups on the basis of their nucleic acid content assessed by flow cytometry. The abundance of bacteria with the highest nucleic acid content and high light scatter (HNA-hs) was significantly correlated, across sites, to bacterial production, whereas bacteria with lower nucleic acid content (LNA) and total bacteria were not. In addition, HNA-hs growth was higher and more variable than LNA growth, indicating that HNA-hs were the most dynamic bacteria. Heterotrophic nanoflagellate and ciliate biomass represented, on average, 5 and 13% of bacterial biomass, respectively. Both in ambient waters and in experiments, ciliates were significantly and negatively correlated with bacteria, whereas heterotrophic nanoflagellates, likely under the grazing pressure from ciliates and metazooplankton, were not. Among ciliates, Cyclidium glaucoma appeared to play an important role. Its growth was significantly and negatively correlated to that of HNA-hs but not to that of LNA. In ambient waters, the abundance of this species explained 56% of the variations in HNA-hs abundance and only 27% of those for LNA. The abundances of total bacteria and LNA significantly increased with chlorophyll a, whereas those of HNA-hs did not. In addition, during the experiments, the estimated potential losses of HNA-hs significantly increased with the initial abundance of C. glaucoma. These results suggest selective removal of the most dynamic bacteria by C. glaucoma and indicate that ciliates may play an important role in the dynamics of active bacteria in natural waters. These findings suggest the existence, within the aquatic microbial food webs, of keystone species that are very important in regulating the activity structure of bacteria.

Short-term variations in the abundance and cell volume of bacterioplankton in an artificial tropical lake

Circadian and spatial fluctuations in bacterioplankton abundance and cell volume were examined, for the first time, in the Municipal Lake located in the down town area of Yaoundé (Capital of Cameroon, Central Africa, ca 3° 52′ N, 11° 31′ E). Bacterial cell volumes (range, 0.05 to 0.2 μm3) were consistent with those reported for other aquatic systems while bacterial densities (0.8 to 2 × 108 cells ml-1) were among the highest values reported in pelagic systems. These variables and chlorophyll a and dissolved oxygen concentrations within a single depth-zone varied from 13 to 61%, while water temperature fluctuated only from 2 to 6%. Spatial fluctuations of physical-chemical and biological variables were generally higher during the day-time than during the night-time. A significant diel variation was provided for bacterial cell volume in the surface waters where synchronized cell division was occurring during the night. The measured bacterial abundances in this study were 4 to 17 fold higher than values known from other lakes of similar trophic status, and both cell abundance and volume were not correlated with chlorophyll. We conclude that this was due to the dependence of bacterial populations to different sources of allochthonous substrates, including untreated sewage from the major influents of the lake, resuspension of benthic material, and substrate releasing from macrophytes which are prevalent in the littoral zone of the lake.

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) weree 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.

Assessing factors underlying variation of CO2 emissions in boreal lakes vs. reservoirs

Reservoirs and lakes were compared to test the hypothesis that they are similar with respect to factors driving the variation in CO(2) emissions to the atmosphere. Understanding this variation is necessary for the assessment of the contribution of these freshwater ecosystems to the global carbon cycle. This study, in contrast to previous ones, included analyses of the relationships between CO(2) emissions and microbial communities. Pooled data (lakes and reservoirs) showed that variations in CO(2) emissions were strongly related to variations in temperature, dissolved organic matter (DOM) quality, and bacterial production (BP). Results also showed that lakes were characterized by higher water temperature, lower DOM quality, larger size of Daphnia, and enriched δ(13) C zooplankton compared to reservoirs. Moreover, interactions within plankton communities and relationships between CO(2) emissions and zooplankton δ(13) C signatures differed in lakes vs. reservoirs, indicating among-system type differences in food web structure and carbon cycling. As a result of these ecosystem-type characteristics, CO(2) emission variation was mainly explained by temperature and BP in lakes, and by DOM quality and the ratio of phytoplankton biomass to microheterotroph biomass in reservoirs. These results showed that differences in temperature and DOM quality between lakes and reservoirs translate into differences in microbial interactions and ultimately in the importance of factors driving CO(2) emissions to the atmosphere. They indicated that considering microbial communities and environmental variables such as temperature and DOM quality can help improve our understanding of the variation in CO(2) emissions from freshwater ecosystems.

Planctomycetes in Lakes: Poor or Strong Competitors for Phosphorus?

ABSTRACT Experiments were conducted with water samples from two perialpine lakes with differing eutrophication status in order to examine the effects of inorganic-nutrient amendments (nitrogen as NO3 − or NH4 + and phosphorus as PO4 3−) on the dynamics, structure, and composition of Planctomycetes and to test the hypothesis that the community structure of Planctomycetes members and that of the other bacteria (without Planctomycetes, here referred to as bacteria-wP, the most represented groups within the community) would be similarly impacted by nutrient additions. Initial samples were characterized by high total nitrogen-to-total phosphorus ratios (range, 39 to 55), suggesting P rather than N was the limiting nutrient for microbial communities. Consistent with this, P additions stimulated phytoplankton growth and affected the community structure of bacteria-wP but, surprisingly, not that of Planctomycetes. N additions did not significantly affect the community structures of bacteria-wP and Planctomycetes or the Planctomycetes phylotype composition. The estimated generation time of Planctomycetes was 123 h. These findings could suggest that the generally well-accepted statement that bacteria (as a whole) are superior to phytoplankton in the ability to obtain phosphorus under P limitation might actually not hold for Planctomycetes. Planctomycetes might be poor competitors for P that do not respond quickly to the nutrient supply, which may help explain why their abundance is low in aquatic systems. The alternative view that Planctomycetes could be strong competitors for P (storing it) is also discussed. Our findings highlight the need for further studies examining Planctomycetes-phosphorus relationships in aquatic ecosystems.

Bacterial Activity in the Water Column and its Impact on the CO 2 Efflux

As part of a comprehensive study intended to elucidate mechanisms that drive carbon dioxide (CO2) emissions from hydroelectric reservoirs, we examined bacterial abundance and production in the water column of three hydroelectric reservoirs of different ages and their nearby lakes, in relation to temperature, dissolved organic carbon (DOC), Chlorophyll a, phytoplankton production and CO2 fluxes from these ecosystems to the atmosphere. The summer values of bacterial production and bacterial specific production in each reservoir were similar to those in the nearby lakes. There was no clear evidence that the age of the reservoir per se had a strong effect on the measured bacterial activities, even though the highest values of these activities were found in the youngest reservoir. DOC and nutrient availability were among the major factors driving bacterial activities. DOC was indeed positively related to bacterial production, bacterial specific production and the proportion of bacteria with high nucleic acid content (i.e. bacteria with higher activity = % HNA) in these sites, where nutrients were, most of the time, found to be limiting for bacterial growth. Among the bacterial variables tested, the % HNA appeared to be important in determining changes in CO2 emissions at least in reservoirs, where it explained 38% of the variance of CO2 fluxes to the atmosphere. Such a relationship was not found in lakes. These results indicate that examining different aspects of the functioning of bacterial communities may help to understand the mechanisms underlying CO2 emissions from aquatic ecosystems, and suggest that the relative importance of factors driving bacterial activities and CO2 efflux may be quite different in lakes versus reservoirs.

Bacteria—phytoplankton relationships in a recently formed reservoir (Sep, France)

The role ofbacterioplankton in the cycling of carbon and nutrients in pelagic systems is increasingly recognized as important. lndeed, heterotrophic planktonic bacteria mediate a significant conversion of poor-quality dissolved organic carbon imo highquality particulate organic carbon available to higher trophic levels, and so, occupy a pivotal position in aquatic ecosystems (AzAM et al. 1983). Bacteria are generally considered as the principal decomposers of organic carbon and generators of minerals in aquatic systems. A number of studies from natural or artificial stabilized lakes have shown dose coupling between heterotrophic bacterial activity and growth and phytoplankton primary production, presumably related to the production of dissolved organic carbon by autotrophs (e.g. COLE et al. 1988, WHITE et al. 1991). However, the absence of or weak (i.e. nonsignificant) coupling between bacteria and autotrophs is sometimes reported in plankton, especially in coastal and inland waters where carbon fixed within the systems can play a minor role in plankton dynamics, compared to the impact of allochthonous substrate inputs from the catchment (CoFFIN & SHARP 1987, LAVANDIER 1990, FINDLAY et al. 1991, JuGNIA et al. 1998).