Contrasting the thermal performance of cultured coral endosymbiont photo-physiology

Abstract

Cnidarian-microalgae (Family: Symbiodiniaceae) endosymbiotic relationships are central to the success of reef building corals, particularly shaping stress tolerance and capacity to adapt to ever-changing environmental conditions. Studies have particularly examined Symbiodiniaceae susceptibility (and adaptation) to thermal stress to aid reef management as reef temperatures continue to rise. Thermal Performance Curves (TPCs) have previously been well studied to resolve how changing temperatures influence organism competitive fitness but have only recently been applied to corals and never before to Symbiodiniaceae. We therefore developed and applied rapid assay TPCs to 7 Symbiodiniaceae isolates (spanning 5 genera) from different reef origins. Isolates were exposed to temperature gradients, ramping up (26-35 °C) and down (26-17 °C) by 1 °C every 3 days. Photophysiological performance (maximum and effective photochemical efficiency, Fv/Fm and Fq´/Fm´) measured at each temperature was used to fit TPC algorithms describing temperature for optimal performance (Topt), critical thermal minimum or maximum (Ctmin or Ctmax), and thermal performance breadth (Tbr). TPC characteristics varied across the isolates revealing diverse performance ranges. Fv/Fm-derived Topt across all 7 isolates was 25.84 °C, with isolate WT3.2 (Symbiodinium spp.) returning the highest overall Topt (26.89 °C) and Ctmax (36.63 °C) values. In contrast, isolate RT141 (Breviolum psygmophilum) had the greatest Tbr (16.15 °C). As such, Fv/Fm-derived TPCs revealed thermal “generalism” of the temperate originating RT141 in comparison to more thermal “specialism” of WT3.2 originating from a hot tropical mangrove lagoon. Thermal characteristics were more varied across isolates when fitting TPCs to the effective photochemical efficiencies (Fq´/Fm´). We discuss how TPC-based characteristics are ultimately operationally defined in terms of thermal perturbation, choice of TPC algorithm and physiological descriptor, and in turn how more standardised approaches could aid future efforts attempting to resolve differences in thermal adaptation in coral endosymbionts.