A finely tuned symphony of factors modulates the microbial food web of a freshwater reservoir in spring

Abstract

We employed high frequency sampling to determine the major factors modulating microbial food‐web composition and dynamics through the spring phytoplankton bloom and clear‐water phases in a freshwater reservoir. We examined effects of a changing trophic structure of the planktonic community cascading from the level of zooplankton, through phytoplankton composition and exudation rates to the level of growth responses and losses to grazers of phylogenetically narrow bacterial lineages. Specific probes for fluorescence in situ hybridization (FISH) revealed important short‐lived peaks of genus‐like (Fluviiciola sp. and Limnohabitans spp.) or even taxonomically narrower populations of Betaproteobacteria and Flavobacteria (such as FlavB and Flav2 lineages). Protozoan grazing on bacterioplankton was studied by using fluorescently labeled bacteria and by direct analyses of FISH‐probe—targeted bacterial phylotypes in flagellate food vacuoles. Evaluations of selective bacterivory, growth responses, and cell biovolumes of various bacterial groups during the spring bloom indicated that certain bacterial groups such as Limnohabitans can contribute to carbon flow to the grazer food chain up to 10‐fold more than similarly abundant small cells from the Ac1 lineage of Actinobacteria. During the clear‐water phase, filter‐feeding cladocera had dominant effects on bacterioplankton abundance and community dynamics, likely through direct grazing on larger bacteria along with altering major substrate pools (via e.g., the herbivores' sloppy feeding on algae). Fine‐temporal resolution data revealed several environmental scenarios, in which the interplay of distinct top‐down and bottom‐up factors resulted in a competitive advantage of particular bacterial lineages.