Bouncing Back in a Warming World

We have not given corals enough credit for their resilience and ability to bounce back. Panama’s Tropical Eastern Pacific provides a glimpse into the diverse strategies corals are implementing to survive in an increasingly warmer, high CO2 world.

The smell that permeated across the deck was a diaphanous veil of sea salt, with notes of seaweed and sand. Then there were crackling sounds, as we placed corals into various tanks at different temperatures. This was the first experiment of my PhD, in collaboration with the Smithsonian Tropical Research Institute (STRI) in Panama. At the end of 2020, we collected corals from across Panama’s Tropical Eastern Pacific (TEP) and placed them in tanks representing various temperatures to understand corals’ responses to thermal stress. When you think about coral reefs, the spectacular structures and incredible biodiversity of the Great Barrier Reef are likely the first images that come to mind. You have also likely heard about the equally astounding threats these rainforests of the sea are facing due to anthropogenic climate change, with coral bleaching being one of the most devastating. By exceeding the region’s average thermal maxima by a single degree, corals experience extreme stress which can eventually result in death. In the process, in the midst of notes of sea salt and crackling, corals become white, hence the term “coral bleaching.” This all stems from the breakdown of what I think is one of the most successful, and yet paradoxically delicate relationships in marine ecosystems.  
Photograph of the thermal stress assay, known as the Coral Bleaching Automated Stress System (CBASS), we ran in November-December 2020 across Panama’s TEP. The data, which is currently being analyzed, will present what is driving differences in bleaching responses for corals in the region. Daviana Berkowitz-Sklar
Over 200 million years ago when modern reef-building corals had just emerged (forming an order called Scleractinia), corals allowed algae to live inside their tissues, instead of digesting them. Corals, like other members of the animal kingdom, cannot make their own food, but algae, being plants, can. This was the beginning of a remarkable partnership that continues to this day. The coral allows the algae to live safely within its tissues, and even forms a special home for them called the “symbiosome.” In exchange, the algae provides sugars to the coral, by converting the sun’s energy into a usable end-product in a process known as photosynthesis. The coral can then use this sugar to produce complex carbohydrates and other organic substances, which it also shares with its algal guest. This nutrient bartering, where both partners are benefiting, is known as a symbiotic relationship. Although this relationship is ancient and central to the establishment of coral reefs, it is easily disrupted by environmental stressors, temperature being one of the most well documented. Bleaching is the breakdown of this symbiosis, where temperature stress causes the coral to kick out its algal partner, and in many cases, starves itself to death. With heat waves becoming more frequent and reaching higher temperatures each year, and the ocean’s temperature steadily increasing, bleaching is becoming a yearly occurrence on some reefs. And yet, not all reefs are bleaching at the same rate – why?  
Comic breaking down what is bleaching. Victoria Glynn
The question of why certain corals bleach while others do not, has captured the scientific communities’ interest for decades. I am certainly not the first nor last researcher focusing on this topic. Yet very few studies focus on coral reefs in Central and South America, which represent some of the world’s most resilient reefs. Panama provides a unique natural laboratory to test what are the factors driving coral bleaching. Panama’s Tropical Eastern Pacific (TEP) as a whole presents a rough environment for coral reefs to exist, and yet they do. This is because this region experiences drastic seasonal fluctuations in temperature on both annual and interannual cycles. In Panama’s TEP, upwelling results in extreme environmental heterogeneity, driven by the trade winds in the neotropical dry season, spanning from January to March. This occurs when cold, nutrient rich water replaces warmer, nutrient poor water at the surface. Diving under these conditions was strenuous, with visibility being under 1 meter, and the temperatures close to 16°C on some days – not precisely what comes to mind when you think of a tropical coral reef. Just 3 months later after the winds had subsided, the same reef site was 30°C and the visibility was remarkably better. Besides upwelling, El Niño Southern Oscillation (ENSO) events are also a driving force in reef dynamics across the TEP. Two of the strongest ENSO events in written history left scars in the TEP, causing close to complete collapse of these reefs due to record-breaking heat waves. Recent papers have suggested that these stark abiotic conditions reflect what all reefs will experience in the future, providing a glimpse into the fate of corals at the turn of the century.  
Illustration depicting how coral’s microbial community shifts under different bleaching conditions, and in turn is in conversation with the surrounding reef community. Victoria Glynn
The 1982-83 ENSO event was particularly devastating, and we located a plot during my time in Panama that once looked like a debris-filled war zone. But now, it was a healthy reef, resembling pre-ENSO conditions, according to Dr. Robert Richmond and Dr. Peter Glynn who initially marked the plot. The fact that this plot was able to recover likely spans past the microbiome. This resilience also likely centers itself on the coral host's own adaptive potential. Evolution suggests that over time, the population will undergo trials and tribulations until only the most well-adapted individuals remain – this is often referred to as “survival of the fittest.” The genetic landscape of the TEP is proving to be as complex as the coral microbiome. Our data is beginning to reveal that there are relatively few genotypes (genetically-distinct individuals) across our reef sites. This perhaps reflects that these genotypes are the best adapted to local conditions, and yet the same genotypes can be found over 100 km apart, underscoring the power of currents in moving adaptive traits throughout the region and sustaining these reefs’ overall existence.  
Photographs of the same reef site in Panama’s TEP, in Saboga Island, taken approximately 40 years apart. The image on the left was taken by Dr. Robert Richmond after the devastating 1982-83 ENSO event, while the picture on the right was taken by Victoria Marie Glynn in 2022 on the same reef, and said to resemble the site prior to the ENSO event.
We have not given corals enough credit for their resilience and ability to bounce back. Yes, climate change is placing these organisms and the diverse communities they uphold in cataclysmic risk, but this is not their first close encounter with extinction. In fact, the fossil record presents that with changing oceanic conditions, there have been boom and bust cycles in marine communities. Some periods are dominated by hard corals, which are those that form modern coral reef communities, but these were sometimes followed by periods where softer bodied corals took their place. A key factor to coral’s resilience is their diverse microbial community, where corals seem to associate with different partners to increase their chances of survival when faced with stressful conditions. This goes past their algal community. Corals in and of themselves are one of the most biodiverse ecosystems on the planet – a single coral polyp is in association with all six kingdoms and three domains of life. Upwelling is such a strong environmental stressor, that within the span of a single year, corals seem to be able to tweak their microbiome to match the different temperature, nutrients, and pH regimes that the trade winds give way to. Diversity is their insurance policy, where depending on what the coral experiences, it can quickly swap and/or give preference to different microorganisms. How flexible these microbial dynamics are, is perhaps also constrained by the host’s own genetic background, underscoring the synergy between the coral host and the microbial community it harbors.  My 11-month fellowship at STRI was a transformative experience. The scientific literature rightfully presents the TEP’s corals as experiencing extreme seasonal environmental stress, however, these coral reefs have persisted and are teeming with life. The data I collected in the TEP over the past year, across upwelling seasons, have provided me with rich insights as to the mechanisms that corals could be implementing to bounce back and thrive in our currently warming world. I am feeling quite hopeful for the future of the region's coral reefs. 
Panama’s TEP reefs are filled with life, where various fish are found cohabiting alongside extraordinarily resilient corals. Victoria Glynn