Coast-reef scale physiological responses of Acropora muricata harboring Symbiodinium clade A

Coral bleaching events are increasing worldwide as a result of climate change. Coral show differential bleaching susceptibilities depending on host and/or symbiont responses to changing environmental factors. Coral responses have been shown to vary over broad geographical scales with thermally-disparate environments. Of these, physiological characteristics such as Symbiodinium density and size, chlorophyll a content and estimated productivity along a coast-reef scale (<1 km) remain to be thoroughly understood. In this study, we examined these characteristics seasonally in Acropora muricata colonies that harbor Symbiodinium clade A along a coast-reef scale in Belle Mare lagoon, Mauritius. The studied reef and coast habitats are characterized by contrasting thermal and light conditions as well as bleaching histories, with bleaching occurring only at the reef site in 2009. We observed higher symbiont density in winter for coast colonies but similar symbiont cell size compared to reef colonies. These two parameters showed seasonal variation. Chlorophyll a content was 30% lower in reef than coast colonies, irrespective of sampling season and was significantly influenced by site. Estimated productivity was influenced by both site and season, with summer samples of coast colonies displaying 56% higher values than reef colony samples. This significant difference was maintained but less pronounced (18%) in winter samples. Our data suggest that Symbiodinium clade A may show physiological acclimatization along a coast-reef scale possibly as a consequence of difference in temperature and light conditions and nutrient concentrations at these two sites