Stress in the City: Disentangling multi-stressor effects on an urbanized coral in a changing ocean

Reducing the negative impacts of global change on organismal physiology is a critical area of scientific investigation in the Anthropocene. Marine coastal ecosystems that exist downstream from urban centers are often subjected to excess nutrients, pathogens, and chemicals via runoff, which can harm organismal function, and may interact with climate change stress. To simultaneously investigate the individual and combined effects of locally-mediated (nutrient, bacterial) and globally-mediated (temperature) stressors on coral physiology, we conducted a multi-factor experiment utilizing the temperate, urban coral Astrangia poculata. Corals were incubated for 12 days with two levels of field-relevant nitrate concentrations at ambient (20 °C) and elevated (29 °C) temperatures, under fed or starved conditions. After 12 days, corals were challenged with an acute 4-h exposure to Escherichia coli, a known urban pathogen for A. poculata. Results show that the physiological impacts of E. coli exposure, nitrate, elevated temperatures, and starvation were interactive and nuanced. Elevated temperatures had the largest single factor impact, resulting in metabolic lethargy regardless of pathogen exposure, nitrate enrichment, or food level. However, corals under the combination of E. coli exposure, elevated nitrate, starvation, and high temperatures demonstrated metabolic hyperactivity, indicating energetic investment by hyper stressed corals in response to the pathogen. With this work, we move beyond pairwise interactions to demonstrate that the interactive effects of combined stressors may offer the key (and more realistic) answer to the fundamental question "How resilient will marine organisms be to locally-mediated stressors in an era of global change?"