Increasing comparability among coral bleaching experiments.

Coral bleaching is the single largest global threat to coral reefs worldwide. Integrating the diverse body of work on coral bleaching is critical to understanding and combating this global problem. Yet investigating the drivers, patterns, and processes of coral bleaching poses a major challenge. A recent review of published experiments revealed a wide range of experimental variables used across studies. Such a wide range of approaches enhances discovery, but without full transparency in the experimental and analytical methods used, can also make comparisons among studies challenging. To increase comparability but not stifle innovation, we propose a common framework for coral bleaching experiments that includes consideration of coral provenance, experimental conditions, and husbandry. For example, reporting the number of genets used, collection site conditions, the experimental temperature offset(s) from the maximum monthly mean (MMM) of the collection site, experimental light conditions, flow, and the feeding regime will greatly facilitate comparability across studies. Similarly, quantifying common response variables of endosymbiont (Symbiodiniaceae) and holobiont phenotypes (i.e., color, chlorophyll, endosymbiont cell density, mortality, and skeletal growth) could further facilitate cross-study comparisons. While no single bleaching experiment can provide the data necessary to determine global coral responses of all corals to current and future ocean warming, linking studies through a common framework as outlined here, would help increase comparability among experiments, facilitate synthetic insights into the causes and underlying mechanisms of coral bleaching, and reveal unique bleaching responses among genets, species, and regions. Such a collaborative framework that fosters transparency in methods used would strengthen comparisons among studies that can help inform coral reef management and facilitate conservation strategies to mitigate coral bleaching worldwide.

Survival and settlement success of coral planulae: independent and synergistic effects of macroalgae and microbes.

Restoration of degraded coral reef communities is dependent on successful recruitment and survival of new coral planulae. Degraded reefs are often characterized by high cover of fleshy algae and high microbial densities, complemented by low abundance of coral and coral recruits. Here, we investigated how the presence and abundance of macroalgae and microbes affected recruitment success of a common Hawaiian coral. We found that the presence of algae reduced survivorship and settlement success of planulae. With the addition of the broad-spectrum antibiotic, ampicillin, these negative effects were reversed, suggesting that algae indirectly cause planular mortality by enhancing microbial concentrations or by weakening the coral's resistance to microbial infections. Algae further reduced recruitment success of corals as planulae preferentially settled on algal surfaces, but later suffered 100% mortality. In contrast to survival, settlement was unsuccessful in treatments containing antibiotics, suggesting that benthic microbes may be necessary to induce settlement. These experiments highlight potential complex interactions that govern the relationships between microbes, algae and corals and emphasize the importance of microbial dynamics in coral reef ecology and restoration.