Ecological stoichiometry and microbial biodiversity effects on water quality in Minnesota lakes

PIs: James Cotner, Department of Ecology, Evolution, and Behavior and Timothy LaPara, Department of Civil Engineering, University of Minnesota

Funding Source: USGS-WRRI 104B/ CAIWQ Competitive Grants Program

Project Duration: 3/1/06 - 2/29/08

Cultural eutrophication is one of the most important issues facing water quality managers sin Minnesota. Most of the nutrient loading into Minnesota waterways are focused on limiting phosphorous (P) availability because of large quantities of this potentially limiting nutrient's contributing to excessive plant growth and low (hypoxic) or anoxic conditions and fish kills. Heterotrophic bacteria are important competitors with plants for P in north temperate lakes. Furthermore, they typically have low growth efficiencies leading to high rates of carbon decomposition. When these high rates of decomposition occur in the surface waters rather than hypolimnion, the high rates of oxygen consumption can be replenished through photosynthesis and mixing with atmospheric oxygen. Heterotrophic bacteria, therefore, are a key component to carbon decomposition and P cycling in surface waters. Yet, very little is known about the efficiencies at which these processes operate. Our work has shown that the biomass stoichiometry of microbial communities can play a key role in these processes and the work proposed here will address the relationships among carbon (C), nitrogen (N), and P content of bacteria and how that affects water quality. Furthermore, this study will examine potential interactions among microbial diversity and stoichiometry, and ecosystem stability. We hypothesize that high microbial diversity may promote stability and increased water quality by increasing decomposition rates in the epi- rather than the hypolimnion. Specific hypotheses tested are that (a) individual strains of bacteria are strongly homeostatic and (b) variable microbial community stoichiometry is achieved through variability in community composition. It is further hypothesized that (c) the efficiency at which nutrients are remineralized by the microbial community is directly dependent on the diversity present in a given lake/ecosystem. Bacterial strains will be isolated from several lakes and characterized with respect to their stoichiometry under conditions of varying nutrients, resources ratios, and growth rates. Bacterial diversity will be ascertained using denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments (detects ~ 99% of bacterial community without cultivation). It is expected that higher diversity increases the potential for a given system to respond to various resource ratios. More diverse communities are likely to demonstrate a low degree of community homeostasis, i.e., little change in biomass stoichiometry with large changes in substrate stoichiometry.

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