Experimental evolution of the coral algal endosymbiont, Cladocopium goreaui: lessons learnt across a decade of stress experiments to enhance coral heat tolerance

Abstract

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 C but not at 32.5 C. SS-juvenile symbiont cell densities increased significantly at 27 C relative to SS-juveniles in the 31 and 32.5 C. Photosynthetic efficiencies in SS-juveniles were higher compared to WT-juveniles at 31 C, equal at 27 C, and lower at 32.5 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.

The role of gene expression and symbiosis in reef-building coral acquired heat tolerance

“…To address this, larvae from the aforementioned crosses were induced to metamorphose and then subsequently exposed to a range of cultured or wild Symbiodiniaceae treatments. Juveniles were infected with cultured Cladocopium goreaui (“C1”), Durusdinium trenchii (“D1a”), a heat-evolved species of Cladocopium goreaui (“SS1”) 21 , or a diverse community of putatively heat-tolerant symbionts from the sediments collected from Curd (“SED”), with the dominant infection of each symbiont type confirmed with transcripts mapping to each genus of Symbiodiniaceae (Supplementary Fig. 6 ).…”

Physiological Characterization of the Coral Holobiont Using a New Micro-Respirometry Tool

“…Coral juveniles affix to the substrate they settle on, so it is recommended to settle them on similar plastic material, using an artificial peptide 16 to facilitate their removal for the gluing process. To further minimize handling stress and impact on the respiration response, it is recommended to allow the corals mounted to zip-ties to acclimate for 1-2 weeks, as is common in many adult coral stress experiments.…”

“…When placing the lids, make sure to do it when the chambers are fully submerged and there is absolutely no air inside. 16. Place the chambers firmly in the stirring plate and connect the fiber optics.…”

Drivers of adaptive capacity in wild populations: implications for genetic interventions

“…The extent of phenotypic variation has long been implicated as a driving force in adaptation 29 and there is evidence that particular forms of assisted evolution approaches on organism like dinoflagellates, insects, and corals can influence phenotypic variation by increasing the width of the fitness function instead of the mean (Fig. 5) 24,30,31 . These approaches are promising because a wider environmental envelope buffers against environmental change and oscillation; but they also carry potential long-term risks because environmental tolerance may also slow down adaptation by decreasing the selective pressure 32 .…”