Biotic stress contributes to seawater temperature induced stress in a site-specific manner for Porites astreoides

Abstract

Increasing seawater temperatures (SWT) resulting from climate change have greatly reduced coral cover world-wide; however, disease has also greatly reduced coral cover in many regions and is especially apparent for offshore bank reefs of the Florida Reef Tract (FRT). The deleterious capacity of future stressors is often dependent upon previous stress. Chronic microbial-related stress has received less attention despite documented links between chronic activation of the innate immune system and the diminished capacity to respond to simultaneously acting stressors. Therefore, differential response of corals to bacterial stress may contribute to decreased coral cover indirectly. To identify differences in bacterial stress response between coral populations, Porites astreoides colonies (n = 6) were collected from an adjacent inshore patch reef and offshore bank-reef of the FRT. Coral were affixed to a concrete substrate at the collection site, and remained there for 2 years to diminish reef environment interspecific effects. Coral were collected after 2 years and subjected to one of three conditions for 8 h: a control condition (28 °C), a thermal stress treatment (32 °C), and thermal and bacterial stress treatment (5 µg mL−1 Serratia marcescens lipopolysaccharide). Offshore reef colonies displayed two- to fourfold increase in innate immune system-related gene expression following temperature stress and LPS, but not with temperature stress alone. Inshore P. astreoides maintained lower levels of expression independent of treatment. These results provide evidence for site-specific sensitivity to bacterial stress and indicate that bacterial stress may indirectly contribute to decreased resistance to climate-related stressors such as SWT.