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.