Know more

Abstract:

A pluri-annual study was conducted in the three lakes: Annecy, Bourget and Geneva. It allowed us to observe spatial and temporal variations for the abundances of viruses, heterotrophic and autotrophic bacteria, flagellates, ciliates and small-pigmented eukaryotes between 0 and 50 meter depth between July 2002 and December 2006. The results and statistical analysis showed that:

• (i) the dynamics of these micro organisms was reproducible from one year to the other
• (ii) abiotic parameters (temperature) and depth are significantly involved in the variability of micro organism abundance
• (iii) some differences were observed according to the trophic status of the system mesotrophic lakes vs oligotrophic lake,
• (iv) at a pluri-annual scale, the variation of abundance of grazers and viruses explains a low fraction of bacterial abundance variability.

A second approach (dilution method) was used to quantify the impact of virus and predators (<10µm) in the mortality of bacteria. Lyses and predation (predators <10µm) pressure exerted on heterotrophic bacteria was undetectable in winter, whereas in summer they can explain 71% of the heterotrophic bacteria mortality in Lake Geneva.Also, in autumn, lysis and predation reached 42 and 18% in Lake Geneva and Bourget respectively. At the same period, viruses have explained 9.8% and 7.6% of the picocyanobacteria mortality in the two mesotrophic lakes, suggesting that they are widely responsible for their annual termination bloom. In contrast, for the oligotrophic Lake of Annecy, viruses and predators may not have played a key role for controlling bacteria community abundance.

These results allowed us to verify and specify the variability and the punctually strong role of viruses and predators to regulate bacteria abundance. This work also highlights the necessity to progress in the knowledge of the different viral life strategies and the factors that control these strategies.