Coral reefs are complex underwater structures deposited by coral polyps in a symbiotic relationship with algae called zooxanthellae. Together they produce oxygen that helps sustain ocean life and absorb carbon dioxide from the atmosphere. In fact, the oceans are described as Earth’s “lungs”, producing half of the world’s oxygen and absorbing 30 percent of human-made carbon dioxide.
But if corals and their symbiotic algae are stressed, they can expel the zooxanthellae and lose this vital oxygen production process. And if the corals aren’t able to take in enough oxygen, they will die.
In the past, researchers have measured critical oxygen levels in the ocean that can cause coral bleaching and, in extreme cases, coral death. But in the case of corals, the precise concentration threshold has not been well understood. Now, two independent teams of scientists at STRI’s Bocas del Toro Research Station in Panama have used different methods to measure coral respiration in the presence of varying concentrations of oxygen.
These results indicate that corals do not use up a large proportion of their oxygen during the daytime when algal photosynthesis is active and oxygen supply is high. In contrast, during the night, corals consume more oxygen than they produce and can deplete their oxygen levels to critically low levels that can cause coral bleaching and, in severe cases, death. Oxygen consumption rates of coral spat increased under low pH conditions that mimic future ocean acidification and in the presence of sulfide, which is known to increase coral respiration and oxygen demand.