Effect of grazers and viruses on bacterial community structure and production in two contrasting trophic lakes

Lyria Berdjeb,Stéphan Jacquet,Thomas Pollet,Isabelle Domaizon

doi:10.1186/1471-2180-11-88

Abstract

BackgroundOver the last 30 years, extensive studies have revealed the crucial roles played by microbes in aquatic ecosystems. It has been shown that bacteria, viruses and protozoan grazers are dominant in terms of abundance and biomass. The frequent interactions between these microbiological compartments are responsible for strong trophic links from dissolved organic matter to higher trophic levels, via heterotrophic bacteria, which form the basis for the important biogeochemical roles of microbial food webs in aquatic ecosystems. To gain a better understanding of the interactions between bacteria, viruses and flagellates in lacustrine ecosystems, we investigated the effect of protistan bacterivory on bacterial abundance, production and structure [determined by 16S rRNA PCR-DGGE], and viral abundance and activity of two lakes of contrasting trophic status. Four experiments were conducted in the oligotrophic Lake Annecy and the mesotrophic Lake Bourget over two seasons (early spring vs. summer) using a fractionation approach. In situ dark vs. light incubations were performed to consider the effects of the different treatments in the presence and absence of phototrophic activity.ResultsThe presence of grazers (i.e. < 5-μm small eukaryotes) affected viral production positively in all experiments, and the stimulation of viral production (compared to the treatment with no eukaryotic predators) was more variable between lakes than between seasons, with the highest value having been recorded in the mesotrophic lake (+30%). Viral lysis and grazing activities acted additively to sustain high bacterial production in all experiments. Nevertheless, the stimulation of bacterial production was more variable between seasons than between lakes, with the highest values obtained in summer (+33.5% and +37.5% in Lakes Bourget and Annecy, respectively). The presence of both predators (nanoflagellates and viruses) did not seem to have a clear influence upon bacterial community structure according to the four experiments.ConclusionsOur results highlight the importance of a synergistic effect, i.e. the positive influence of grazers on viral activities in sustaining (directly and indirectly) bacterial production and affecting composition, in both oligotrophic and mesotrophic lakes.

Highlights

Over the last 30 years, extensive studies have revealed the crucial roles played by microbes in aquatic ecosystems
The heterotrophic bacterial community is the most important biological compartment involved in the transformation and mineralization of the organic matter in aquatic systems
Different biotic and abiotic factors have been identified as players acting on the activity and composition of the bacterial community, and resources are considered one of the main factors controlling this community [2,6]

Summary

Introduction

Over the last 30 years, extensive studies have revealed the crucial roles played by microbes in aquatic ecosystems. The roles of bacterivory and viral lysis are not insignificant, and may strongly affect bacterial abundance, activity and structure Both heterotrophic nanoflagellate (HNF) grazing and viral lysis are known to be variable causes of bacterial mortality, and can be responsible for 10 to 60% of daily bacterial loss in lacustrine systems [e.g. If bacteria are grazed by flagellates, nutrients and energy can reach higher trophic levels via the connection between the microbial loop and the classical food chain [15] These processes can significantly influence the production of dissolved organic carbon and the recycling of nutrients [14,16] and can impact/modify bacterial diversity [9,17] and the relationship between diversity and ecosystem functioning [18]

Methods

Results

Conclusion