M. Richardot

Proteolytic and glycolytic activities in size-fractionated surface water samples from an oligotrophic reservoir in relation to plankton communities

Abstract: From February to August 1996, the following parameters were determined in surface water samples from an oligotrophic reservoir: concentrations of dissolved combined amino acids (DCAA), monosaccharides (DFCHO) and polysaccharides (DCCHO); activities of glycolytic and proteolytic enzymes associated with three size fractions (0-0.2 μm, 0.2-2 μm, and 2-100 μm); densities and biomass of free-living and attached bacteria, nanoflagellates, and metazoan zooplankton. The mean concentrations of DCAA, DFCHO and DCCHO were 2.7, 0.5 and 1.4 mg C l-1 respectively. Both βGlcA and αGlcA showed a high substrate affinity. The enzyme activities associated with all size fractions fluctuated throughout the year. Highest activities were generally associated with the 2-100 μm fraction; the mean percentages of leucine-aminopeptidase (LeuA), β-glucosidase (βGlcA), α-glucosidase (αGlcA) and β-N-acetylglucosaminidase (βGlcNAcA) activities were 58%, 62%, 76%, and 59%, respectively. Activities of αGlcA and βGlcNAcA in the 0.2-2 μm fraction were correlated with the abundance of free-living picobacteria (bacteria <2 μm). The biomass-specific activity of attached bacteria was 18 to 64 fold higher for attached compared to free-living bacteria. The abundances of autotrophic and mixotrophic nanoflagellates (ANF) were correlated with LeuA (r = 0.59, P < 0.05) and the βGlcA (r = 0.64, P < 0.05) activities in the 2-100 μm fraction.

Incorporation of 3H-Thymidine by Different Prokaryotic Groups in Relation to Temperature and Nutrients in a Lacustrine Ecosystem

The incorporation of [3H-methyl] thymidine (3H-TdR) by Eubacteria, bacterial groups (α- and β-Proteobacteria, Cytophaga–Flavobacter), and Archaea was measured according to temperature (7 and 17°C) and nutrient levels (nitrogen, phosphorus, and carbon) in a lacustrine system (Sep, France). Short-term incubation was performed using a combination of microautoradiography and fluorescent in situ hybridization. Irrespective of the temperatures and nutrients studied, all the major phylogenetic bacterial groups assimilated 3H-TdR, and in most of the treatments studied, the proportion of β-Proteobacteria taking up 3H-TdR was higher than those in the other bacterial groups. The proportion of Bacteria and different bacterial groups studied incorporating 3H-TdR were significantly increased, approximately 1.5-fold, by temperature except for α-Proteobacteria (7.6-fold). The nutrient effect was not the same for the different bacterial groups according to the temperatures studied. The proportions of α-Proteobacteria (at both temperatures) and Cytophaga–Flavobacter (at 7°C) taking up 3H-TdR were significantly decreased and increased by adding N and P, respectively. Also, adding N, P, and C increased and decreased the percentage of β-Proteobacteria incorporating 3H-TdR at 7 and 17°C, respectively. The archaeal community showed a similar proportion of active cells (i.e., 3H-TdR) to the bacterial community, and uptake of 3H-TdR by Archaea was significantly increased (P < 0.05) by both temperature and nutrients. Thus, the assimilation of 3H-TdR by bacterial groups and Archaea in lacustrine system is significantly controlled by both temperature and nutrients.

Regulation of the abundance and growth of ciliates in a newly-flooded reservoir

The specific growth rates of ciliates were measured in diffusion chambers after the first flooding of a reservoir. The abundance of nanoplankton and the high concentrations of dissolved organic matter could explain why the growth rate, whose average value was 0.07 ± 0.06 h -1 , was higher than that recorded in other lakes of similar trophic status. The abundance of ciliates was strongly controlled by Daphnia longispina and Asplanchna priodonta.