Warmer seawater affects the immune activities of thermophilic coral Astroides calycularis under LPS challenge

A worldwide increase in coral diseases and mortality has been linked to anthropogenic ocean warming due to changes in pathogen virulence and coral immune functions. The anomalous temperature pattern has particularly worried the Mediterranean region over the last 30 years, where intense warming has caused recurring mass-mortality events. To evaluate how warmer seawater conditions influence the immune responses of an endemic coral species, colonies of Astroides calycularis were exposed to environmental (23°C) or elevated (28°C) temperatures, and subsequently challenged with Escherichia coli lipopolysaccharide (LPS). Several enzyme activities, which included phenoloxidase-like, glutathione peroxidase, lysozyme-like, alkaline phosphatase, and esterase, were measured after 6 hours of LPS balneation and over time (0-, 12-, 48-, and 120-h). The five enzyme trends showed upregulation immediately after the LPS balneation under environmental conditions, demonstrating an immune response, while warmer seawater impaired the enzyme activities, delaying it over time. Furthermore, through immunolabeling with specific antibodies, was also detected the regulation of Toll-like receptor 4 (TLR4), nuclear factor kappa B (NF-kB), and heat shock protein 70 (HSP70) activity. The activity of this markers after the LPS stimulation revealed a modulation at environmental temperature. Elevated temperature and LPS-challenge almost suppressed TLR4-NF-kB activity, while HSP70 up-regulation appeared in both treatments under warmer conditions. Such an approach is useful for understanding the pathogen-defence mechanisms in corals in order to disentangle the complex interactive effects related to global climate change.