The role of alternative microbial resources in competition between Daphnia species

We studied competitive interactions between Daphnia longispina and D. magna competing for a single algal resource of varying quality in treatments with low and enhanced amounts of bacteria. We proposed that the outcome of competition is influenced by the concentration of bacteria. We used three types of food including high-quality green alga Chlamydomonas klinobasis, phosphorus-poor C. klinobasis and the cyanobacterium Synechococcus elongatus, which is deficient in essential lipids. The medium in the low-bacteria experiment was replaced daily, while the medium in the enhanced-bacteria experiment was changed every other day, thus allowing for prolonged bacterial growth as determined by an additional short-term experiment. Furthermore, we found significantly higher abundances of both Daphnia species in monocultures with enhanced bacteria compared to those in the low-bacteria experiment, indicating that they were feeding on bacteria. In the experiment involving low bacterial densities, the dominance of both Daphnia species depended on the quality of the algae. However, D. magna was the superior competitor across all food quality treatments in the enhanced bacteria experiment. We conclude that enhanced bacteria are a significant factor that shapes competitive interactions in cladoceran communities.