Environmental Stress Shapes Bacterial Community Structure and Function Through Interactive Abiotic Effects.

Microbial communities play critical roles in ecosystem functioning across a wide range of environmental conditions. The physiological stress imposed by temperature, pH and resource levels can shape the structure and function of microbial communities; however, while often tested independently, factors influencing physiological stress on a community rarely occur in isolation from each other. Controlled experiments simultaneously testing multiple interactive stressors allow researchers to better assess the dynamical responses of microbial communities to rapidly changing environments. Using a full factorial, controlled experiment, we tested three hypotheses for how independent and interactive effects of abiotic stresses impact bacterial community composition, structure and function in a model system. We utilised an aquatic, pitcher plant-associated bacterial community in which microbial nutrient cycling is essential to the host plant and ecosystem. Temperature, pH and resource (food) concentration had strong independent and interactive effects on bacterial community composition, structure and function. Community functions did not respond to interactive stressors in the same way. Chitinase and protease enzymatic activities had opposite responses to temperature and pH changes, indicating that diverse functional measures are necessary for understanding the varied effects of interacting stressors. The most extreme abiotic stress combination (high temperature, lowest pH and excess food) resulted in the lowest enzyme activity and reduced species richness as compared to the other treatments. Stressful conditions, especially high temperature, strengthened correlations between community structure and function. Higher phylogenetic dispersion under abiotic extremes suggested selection for diverse taxa adapted to similar conditions through convergent evolution. These interactive effects highlight the often greater-than-additive impact of multiple stressors and demonstrate that environmental filtering and trait convergence shape microbial responses to stress.