How does warmer sea water change the sensitivity of a Mediterranean thermophilic coral after immune-stimulation?

Abstract

Anthropogenic climate change is warming sea water worldwide, pushing the limits of tolerance for marine organisms and driving a decline in biodiversity. The risk of thermal anomalies has increased particularly in the Mediterranean region over the last 30 years, where intense warming has been identified as one of the main stressors in coastal regions. To determine the influence of warmer conditions on the immunity of an endemic Mediterranean coral species, different immune activity parameters were compared in response to elevated temperature (~ 28°C) and the presence of a pathogen-associated molecular pattern (PAMP) - Escherichia coli lipopolysaccharide (LPS) - as an elicitor of the innate immune response of Astroides calycularis. Immune parameters, which included phenoloxidase-like, glutathione peroxidase, lysozyme-like, alkaline phosphatase, and esterase enzyme activity, were measured over time after LPS balneation (0-, 12-, 48-, and 120-hour time point). All five enzymes demonstrated constant values under environmental conditions (~ 23°C), indicating a constituent activity. LPS at environmental temperature induced significant upregulation immediately after exposure (0h-time point), demonstrating an immune response to the pathogen elicitor. Under warmer conditions (~ 28°C), constituent values increased over time, indicating a shift in the immune strategy to maintain homeostasis. However, warmer sea water, within the summer range experienced by this coral species, impaired the immune response to LPS, delaying it over time. These changes in immune strategy indicate that temperature affects coral immunity and, in thermophilic A. calycularis, results in an energy trade-off that could maintain its health-state through suboptimal conditions during multiple perturbations, such as summertime diseases.