Bacteria represent the majority of biodiversity and play key roles in virtually every ecosystem on Earth. In doing so, bacterial species act as part of complex communities shaped by interactions across all domains of life. Although viruses and protists account for the majority of bacterial mortality, metazoan grazers affect the growth and survival of bacteria directly via consumption and indirectly through trait and density mediated effects. Thus, understanding how metazoan grazers impact bacterial communities is critical to understanding how top-down forces shape bacterial biodiversity.
Here, we report on the impacts of dreissenids, invasive metazoans grazers threatening aquatic systems globally, on freshwater bacteria. Filtration stress added by dreissenids has caused redirection of energy and nutrients, induction of harmful algal blooms, and outbreaks of avian botulism among other impacts. Previous studies have reported that dreissenids can also directly impact bacterial community structure phenotypically and taxonomically by changing trait distributions and enriching/removing specific taxa respectively. However, these studies were conducted using water collected from Lake Michigan (TP ~5 μg/L), and it is unknown whether similar patterns are observed in systems with differing nutrient regimes.
We conducted 9 separate short-term dreissenid grazing experiments over a 6 month period in 2019 (May to Oct) using water collected from a low-nutrient offshore site (TP ~17.7 μg/L)and a high-nutrient nearshore site (TP ~44.3μg/L) in Lake Erie. In addition, we set up two 20L mesocosms (- and + dreissenids) for 8 days using water from the Maple River in Michigan, which drains Douglas Lake.
Results/Conclusions:
Based on 16S rRNA gene sequencing, we observed significant differences in bacterial community structure between - and + dreissenid treatments for all 10 experiments. In Lake Erie, despite clear seasonal differences and differences between sites, we observed consistent shifts in bacterial community structure induced via direct grazing by dreissenids. Across all experiments, we observed strong removal of Verrucomicrobia and enrichment of Actinobacteria and Armatimonadetes phyla, which is consistent with previously reported findings. However, certain phyla such as Cyanobacteria, demonstrated variation in terms of susceptibility to grazing, depending on the taxon and time of the experiment. Taken together, these results suggest that bacterial traits that confer grazing resistance might vary in terms of their phylogenetic conservation. Identifying how these functional traits are distributed will help us better understand how trophic interactions shape bacterial diversity and exert control on ecosystem-level processes.