What Are Zooxanthellae? And Why Should You Care?
Recently at the Royal BVI Yacht Club a large crowd of concerned residents gathered to hear environmental scientist Dr. Lianna Jarecki present “Coral Reefs in Crisis: Bleaching and Resilience in the BVI.” During her presentation, a result of her visit to the National Oceanic and Atmospheric Administration (NOAA) Reef Resilience Conference, Dr. Jarecki explained what coral bleaching is and how environmental stressors can be decreased in order to ensure recovery when a bleaching event does occur.
Coral bleaching is a misnomer. When we see white, living coral, it’s not actually bleached or dead or diseased. Rather, the whiteness is an indication that the coral no longer hosts its symbiotic, energy- and colour-providing algae. These algae, called zooxanthellae (pronounced zoo-oh-zan-THELL-ee), give healthy corals a golden brown hue. The single-celled, photosynthetic algae also provide corals with 90% of their energy requirement through excess sugar compounds that they don’t use.
According to Dr. Jarecki, when sea surface temperatures increase, the zooxanthellae produce free radicals which are damaging to the structure of the coral animal. The corals then purge the zooxanthellae from their tissue in order to protect themselves from the toxic compound, Dr. Jarecki said. But, the problem is that when the corals expel the algae from their bodies, they are also expelling the provider of 90% of their food. “The bleached corals are not actually dead, but they are basically starving; they are severely weakened,” Dr. Jarecki added.
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Two options exist for corals that have been bleached: recovery or disease and death of the coral head. “Under good conditions, with good water quality and low impact, [the coral] could eventually regrow,” Dr. Jarecki said, but if the local marine environment is already compromised, by stressors such as sediment and sewage pollution or boat anchors, the corals will be more susceptible to disease. “If you have multiple stressors and a huge thing like a bleaching event or a storm,” said Dr. Jarecki, “the reef can’t recover.”
Dr. Jarecki cited the lack of sediment control when building new roads as one of the main reasons that the BVI is not providing a low-stress environment for the corals. Dr. Jarecki showed an image of the sedimentation runoff on the North Shore after a recent rainfall and commented that the roads being built to Little Bay, Trunk Bay and Lava Flow “generally have very little sediment control. Even though the technology is there, it’s not required by law, and it’s cheaper to build roads and house sites without them.” So every time it rains, “we get a huge pollution spill of sediment over the marine habitat.” Another problem in the BVI is the diminishing wetlands which act as buffers and filters from runoff and pollution. “About 85% of our wetlands have been destroyed and replaced by building sites,” Dr. Jarecki said. “They’re cut down, filled in and built up.”
The lack of protection from pollution runoff and other stressors will inhibit our reefs from recovering when another bleaching event occurs. “By increasing susceptibility to disease and increasing mortality, we decrease diversity of corals and different organisms on the reef, we degrade the habitats for fish and other organisms,” said Dr. Jarecki. She proposed that the challenge to the BVI is how to locally manage the global crisis that is coral bleaching. “We can’t protect our reefs from things that are happening globally, such as increased sea surface temperatures, but we can promote resiliency—the ability of coral reefs to resist or tolerate or recover from bleaching events.” The best way for us to promote resiliency is to minimize the local stressors and maximize water quality by preserving the wetlands and using sediment control measures when building and installing proper sewage systems. There is hope, according to Dr. Jarecki, because even if one reef is bleached, it can recover under the right conditions, often assisted by healthy, nearby reefs that can provide larvae to reseed the damaged reef, allowing the zooxanthellae to return to feed their host corals, turning them back to their golden brown hue.
