Demonstrating the effects of environmental variables on community composition in an undergraduate laboratory can be difficult. Measuring shifts in species composition of many aquatic communities can be challenging due to time and/or taxonomic constraints. Changes in the fish, plant, and macroinvertebrate communities could require sustained ecosystem manipulations as few of these communities will change in less than a growing season. Phytoplankton and zooplankton communities could be altered by environmental conditions more rapidly but require a high level of taxonomic proficiency. Naturally occurring bacterial communities from lakes, ponds or streams are easy to manipulate over a short time period, and can be used to explore community composition without the need for complex enrichment media or molecular techniques using commercially available Biolog EcoPlates to assess functional community diversity. Bioassays have been used to demonstrate nutrient limitation and toxicological responses in aquatic communities and we have adapted this approach to bacterial communities to demonstrate how environmental variables generate shifts in bacterial community composition. A single EcoPlate contains 31 carbon sources in triplicate, and visually detectable colorimetric changes indicate that a member or members of the community are capable of using that carbon source. By evaluating the combinations of carbon sources utilized, non-taxonomic descriptions of the community can be generated. We exposed naturally occurring planktonic bacterial communities to readily available reagents to cause shifts in the functional community: nutrient additions (nitrate and phosphate), organic supplementation (starch and gluten), and increased temperature (25°C vs. 40°C).  After 48 hours, aliquots of enriched lake water were used to inoculate EcoPlates which were then monitored visually and using a spectrophotometric plate reader for 24 hours.

Results/Conclusions

We observed shifts in bacterial communities in response to all of these manipulations. The control community utilized 4 of the 31 carbon sources, but when temperature was increased 6 carbon sources were used (4 in common with the control plus 2 additional). In the nutrient addition assays, the nitrate treatment only used 2 carbon sources, while the phosphorus treatment used those 2 plus 11 additional. The communities supplemented with organic matter had the highest overall richness of substrate utilization; the starch treatment utilized 15 substrates and the gluten treatment utilized 29 substrates. These qualitative results can be supplemented with quantitative results if a plate reader is available. This exercise can be adapted using many combinations of substrates and encourages active learning as students test hypotheses to match their interests.