COS 54-5 - Predation by a protist community mediates temperature and nutrient effects on a prey microbial community

Abstract: Rising global temperatures and increases in nutrient deposition through air pollution may have catastrophic effects on natural ecosystems. Warmer temperatures increase metabolic rates, leading to increased energetic demands among exotherms. The burning of fossil fuels, the emission of ammonia by industrial regions, and indiscriminate fertilizer use contributed to ever-increasing nutrient deposition rates. Microbial respiration rates change dramatically due to temperature increases via metabolism and community structure. While it is known that ecological interactions mediate the temperature responses of microbial communities, how these communities may respond to both temperature increases and nutrient additions in the presence of predators is unknown. Understanding how species interactions influence the effect of abiotic change on microbial function is thus central. Here, we examine how predation of a prey microbial community by a protist community mediates the combined effects of nutrient additions and temperature on microbial diversity and biomass.

To test how predation mediates the combined effects of nutrient additions and temperature, we set up microbial microcosms where we manipulated temperature, nutrients, and the presence of protists. Specifically, microbial microcosms were assembled using moss-associated bacteria from the DOE-supported SPRUCE site in the Marcell experimental forest in Minnesota. We manipulated microcosm temperatures (22℃ and 25℃), nutrient levels (standard and half concentration Carolina protist media), and the presence of protist predators over three weeks. To assess the response of the microbial communities, we quantified total microbial biomass using OD600, total respiration rate using real-time respirometry, and changes in community structure using amplicon sequencing.

Our results show that temperature, nutrient levels, and the presence of a protist community have a significant independent and interactive impact on microbial biomass and community composition. Increased temperature and low nutrient levels resulted in less biomass, while the presence of protists resulted in more. Shannon diversity increased in the presence of protists under a high nutrient level and low temperature (22℃). Beta diversity was significantly affected by temperature, nutrient levels, and the presence of protists.

These results suggest that while temperature and nutrients may interactively influence microbial communities' structure and function, their response is determined by top-down controls (exerted by bacterivore organisms responding to increased temperature). Moreover, we show that while protist presence may provide top-down control, it can also fertilize microbial growth by recycling nutrients stored in bacterial biomass. Overall, these results can help improve predictions on how the combined effects of nutrient additions and warming affect ecological communities and food web structure.