Experimental Evolution Of The Coral Algal Endosymbiont, Cladocopium Goreaui: Lessons Learnt Across A Decade Of Stress Experiments To Enhance Coral Heat Tolerance

Projected increases in sea surface temperatures will exceed corals' ability to withstand heat stress within this century. Experimental evolution of cultured symbionts (Symbiodiniaceae) at high temperatures followed by reintroduction into corals can enhance coral heat tolerance. Several studies have selected for enhanced tolerance in Cladocopium goreaui (C1) over multiple time scales and then compared the performance of coral juveniles infected with the heat-tolerant C1 selected strain (SS) to the performance of juveniles infected with the C1 wild type (WT). To derive lessons about host benefits when symbionts are experimentally selected, here we compare the performance of SS- and WT-juveniles after 21 cell generations of heat selection versus longer periods (73-131) in recently published experiments. After 21 generations, we found rapid improvement in heat tolerance of SS through an overall shift in the mean tolerance to temperature. This did not translate to improved growth and survivorship of the coral. Specifically, survival did not differ significantly between juveniles of Acropora tenuis hosting WT versus SS at any temperature. Juveniles infected with WT exhibited greater skeletal growth than those infected with SS at 27 and 31 degrees C but not at 32.5 degrees C. SS-juvenile symbiont cell densities increased significantly at 27 degrees C relative to SS-juveniles in the 31 and 32.5 degrees C. Photosynthetic efficiencies in SS-juveniles were higher compared to WT-juveniles at 31 degrees C, equal at 27 degrees C, and lower at 32.5 degrees C. These results suggest that selection over longer generation (>130) times will be needed to confer host benefits and will be dependent on the stability of this association being maintained in nature.