Does predation mediate the effects of temperature and nutrient additions in food webs?

Background/Question/Methods

Rising global temperatures and increases in nutrient deposition through severe air pollution may have catastrophic effects on natural ecosystems. Warmer temperatures increase metabolic rates, leading to an increase in energetic demands among endotherms, affecting how they might respond to available nutrient content. The burning of fossil fuels, emission of ammonia by industrial regions, and the indiscriminate use of fertilizer have contributed to ever-increasing nutrient deposition rates. Worse, anthropogenic nutrient additions are known to interact with warming temperatures but in unpredictable ways. While community temperature responses have shown to be mediated by ecological interactions, how microbial communities may respond to temperature and nutrient additions in the presence of predators is poorly understood. To test how predation mediates the joint effects of nutrient additions and temperature on microbial communities, we set up microbial microcosms where we manipulated temperature, nutrients, and the presence and absence of protists. Microbial microcosms were assembled using moss-associated bacteria from northern Minnesota. We manipulated temperature (22℃ and 25℃), nutrient conditions (standard and half concentration Carolina protist media), and the presence of protist predators in 200mL glass jars over three weeks. To assess the response of the bacterial communities, we quantified total microbial biomass and total respiration rate.

Results/Conclusions

Our preliminary results show that temperature, nutrient concentration, and the presence of protists have a significant impact on bacterial growth. Increased temperature and low nutrient conditions negatively affect bacteria, while the presence of protists has a positive effect on bacteria independently of temperature and nutrient conditions. These results suggest that resource partitioning between predators and the constant availability of resources for bacterial growth due to protist predation can stabilize the system and keep bacterial biomass high despite warming temperatures and low nutrient availability. Alternatively, protists may have been acting as keystone predators, thus maintaining a high level of bacterial diversity and biomass by keeping bacterial competitors at bay. Overall, these results can help improve predictions on how the combined effects of nutrient additions and warming will affect ecological communities and food web structure.