Ecological stoichiometry and prokaryotic diversity

Background/Question/Methods
Ecological stoichiometry examines the interactions among organisms, their elemental composition, and the elemental composition of their environment.  Recent research from our laboratory indicated that individual heterotrophic bacteria isolated from freshwater lakes are relatively homeostatic – that is, the composition of their cellular constituents varies only slightly when the composition of their growth medium varies by orders of magnitude.  In contrast, the bacterial communities from freshwater lakes are not homeostatic – that is, their nutrient composition closely mimics the nutrient composition of their growth medium.  This suggests that freshwater bacterial communities may be comprised of organisms that are both genetically and stoichiometrically diverse.  We hypothesize that the bacterial communities in freshwater lakes with more dynamic nutrient concentrations should be more diverse than the bacterial communities from lakes with more static nutrient concentrations.  To test this hypothesis, we collected samples from eight lakes (ranging from oligotrophic to eutrophic) within the Twin Cities Metropolitan Region in southeastern Minnesota, eleven lakes in and near Itasca State Park in northwestern Minnesota, and eleven lakes in and near the Boundary Waters Canoe Area Wilderness in Northeastern Minnesota.  Automated ribosomal intergenic spacer analysis (ARISA) was used to fingerprint bacterial communities, which were then correlated to the particulate and dissolved concentrations of carbon, nitrogen, and phosphorus in these lakes. 

Results/Conclusions

The bacterial communities in all of these lakes were complex (typically 20-40 organisms were detected) and temporally dynamic.  Nutrient levels were also dynamic – the carbon-to-phosphorus ratio oscillated by as much as three orders of magnitude in some of the lakes.  Correspondence analysis demonstrated that the bacterial communities were more diverse in lakes with greater fluctuations of dissolved and particulate nutrient concentrations, supporting our hypothesis that bacterial communities in freshwater lakes with more dynamic nutrient concentrations are more diverse.