What’s a Strong Coral Anyway?

[I would be remiss if I didn’t mention coronavirus. I’m sitting in my newly converted home office, which is actually a small and surprisingly functional corner of my bedroom. I haven’t been to my lab in over two weeks now and counting. I also canceled a trip to check on my field experiment in Palau a mere nine hours prior to my flight taking off. In truth, the quest for a PhD takes place on ever constant battlegrounds of twists and turns, ups and downs, accomplishments and adversities… Well Coronavirus is a particularly scary ongoing challenge. Graduate students all over are reevaluating carefully predetermined research plans and shifting focus to remote work, all while trying to maintain our health and keep track of loved ones. To any graduate student reading this, you are doing great.]

Coral reefs are spectacular ecosystem creations. All the reefs in the world inhabit less than 1% of the entire ocean floor, and yet they’re estimated to support 25% of all marine biodiversity. Reefs also bring in as much as $375 billion per year in revenue for humans through channels that include tourism and large-scale fishing. Sadly, up to 50% of coral reef coverage has been lost since the 1980s, and today a whopping 75% of existing reefs are threatened by anthropogenic causes like pollution, overfishing, habitat destruction, and increasing ocean temperatures.

I grew up in New York but had the opportunity at a young age to snorkel on some reefs in the Caribbean. One of the earliest observations I made as an aspiring scientist was that there were patches of colorful corals and completely white corals on the same reefs. What I’m referencing is a difference between a healthy coral animal that has lots of colorful tiny algae living inside its tissue, and an unhealthy coral animal that has lost its algae and is left with a white skeleton you can see through transparent tissue. Under normal conditions, plant-like algae give the coral oxygen and a metabolism energy boost while corals provide the algae with protection and nutrients. So what separates a healthy colored coral from an unhealthy “bleached” white coral? It comes down to how much stress the coral animal can withstand from a stressfully hot environment, and even from similarly stressed out algae that can go from helpful to harmful.

Here I am free-diving on a degraded section of a fore reef in Palau’s southern lagoon.

Fast forward nearly fifteen years to today, and now a major part of my PhD research is trying to identify what makes some corals better able to handle environmental stress as our oceans continue to warm due to anthropogenic climate change effects.

Coral bleaching can be a devastating process; coral animals must deal with extreme stress from an uncomfortably hot environment while fending for themselves without any help from algae. A traditional view of the “strongest” corals is those that can hold onto their algae during intense ocean warming events. It makes sense that the most bleaching resistant corals might be good candidates as it gets hotter. But while coral bleaching is detrimental, it is not an automatic death sentence. Corals can recover from bleaching—that is regain their colorful algae. This begs the question whether the predominant view of the “strongest” corals should be expanded.

Why should we care about corals that can effectively recover from bleaching? For one, even the most bleaching resistant corals will eventually fall victim to bleaching as the oceans keep warming. Another reason is that just giving attention to a small subset of the highest bleaching resistant corals could result in an extreme loss of genetic diversity amongst future generations. For example, cheetahs are so inbred that all individuals are nearly identical at the DNA level. If a sudden disease were to kill one cheetah, likely all cheetahs would die due to low diversity. As a final compelling reason, if it were so overwhelmingly beneficial for corals to be bleaching resistant you would expect all corals to possess the trait. The fact that not all corals are high bleaching resistant suggests there might be some bad tradeoffs that reduce fitness (e.g. growth or reproduction) in certain environments.

Left: Coral pieces, called “nubbins”, sitting in the Palau International Coral Reef Center (PICRC) following experimental heat stress. Right: Here I am returning bleached corals to their coral reef to begin monitoring their bleaching recovery.

My research seeks to start challenging the notion that the strongest corals are high bleaching resistant. To do this, I am conducting experiments on how individual coral colonies are able to resist and then recover from bleaching. I get to travel about 20 hours from California to the beautiful archipelago nation of Palau for my research and have visited the country six times at this point in my PhD. First, I snorkeled on reefs to collect pieces of coral colonies then brought live corals back to the laboratory environment. Next, I exposed these collected corals to artificially heated ocean water until they bleached and returned them to their reef environment. Now I am eight months into a year-long study of how these bleached corals recover from their artificial extreme bleaching event. I’m studying how quickly corals recover their algae and metabolic energy, and how much they can grow. I am also investigating coral health from the DNA level to see if there are differences in genetics between colonies that allow us to better understand the impacts of heat stress AND to better predict how corals will react to hotter ocean temperatures.

Can the highest bleaching resisters also be the best at recovering from bleaching when it just gets too hot to handle? Are there serious undesirable tradeoffs associated with being high resistant? Is bleaching recovery actually a significant indicator of “strength” that has been largely undervalued? These are some of the questions I hope my PhD research will begin to address. Ultimately, an important goal of this research is to preserve coral reefs for future generations. In order to manage reefs, we need to develop appropriate criteria for determining the “strongest” corals to protect and even breed. As a young scientist, I am excited to apply my own interests and expertise to the sorts of time sensitive questions being explored by the coral world.

Nia Walker is a 3rd year Biology Ph.D. candidate and Editor of High Tidings. She is generally interested in using genomics and physiology techniques to better understand how organisms function under normal and high stress conditions. Funding for this fieldwork comes from the National Science Foundation and a Hopkins Marine Station Lederberg Research Grant.