Episodic temperature tradeoffs: secondary metabolites vs enzymes in lab and field studies with model integration for Microcystis

Cyanobacteria are photosynthetic microbes and keystone members of aquatic systems, yet in some cases they produce compounds that poison water supplies. The metabolic landscape for freshwater cyanobacteria is complicated. Cyanobacteria persist in environments where short-term changes in temperature, light, and nutrient availability vary during the summer as well as during episodic events (i.e., large storms). This variability leads to many issues for biology, including oxidative stress in photosynthetic organisms. We describe a multidisciplinary research program to quantify the mechanistic role of oxidative stress responses in a model cyanobacterium (Microcystis) to episodic events. Living cells cope with oxidative stress using multiple mechanisms: e.g., all use enzymes to degrade reactive oxygen, while some also employ biologically expensive secondary metabolites (for example, microcystins, which are dangerous liver toxins) for protection. Our goal is to quantitatively demonstrate how more biologically expensive but longer-term coping mechanisms (in this case microcystin) vs. short-term and cheaper strategies (peroxiredoxin enzymes) influence cyanobacteria strain succession and composition in lake communities. Using this data, we will update an existing mathematical model that predicts microcystin production from cell conditions and add a module that predicts growth response, microcystin production, and strain competition due to episodic events. The resulting model has implications for the bioeconomy through the improvement of in-season predictions of toxin production that will help resource managers better protect the health of lake-dependent residents. We will broadly communicate results to both the public and scientific community. We will train students and stakeholders, all with the goal of better informing the public while advancing science. We hypothesize that increased episodic weather events will select for microcystin producers. To test this, we will directly compete strains with and without the ability to make microcystin (M. aeruginosa PCC7806 and a microcystin-free mutant, ΔmcyB) in continuous cultures under conditions that mimic current and projected seasonal conditions for Lake Erie (June, when microcystin producers dominate, and August, when non-microcystin producers dominate). We will evaluate this in the context of increased episodic events (storms) that may alter Microcystis community composition through selection of one coping strategy over another. Strain composition will be monitored by established qPCR methods and changes in physiology quantified with flow cytometry, fluid imaging (FlowCam), pigment content, and photosynthetic analyses. We will track cellular microcystin concentrations and glutathione-dependent peroxiredoxin activity to determine how this enzyme performs in the presence or absence of microcystin across conditions of episodic change (i.e., temperature drops consistent with large storms). We will conduct mesocosm studies with natural communities to validate in-lab observations on short-term temperature effects, collecting metatranscriptomes to monitor both the target cyanobacteria as well as the entire community. Lab studies will also help us reinterpret existing metatranscriptomic datasets from Lake Erie (300+ samples collected from 2012-2024) for indications of similar responses under natural field conditions. All this information will be used to redesign an existing agent-based, mechanistic model that predicts toxin production on a per cell basis in response to environmental conditions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.