Starving corals of oxygen

When fleshy algae take over coral reefs they stimulate bacteria to grow, leading to loss of oxygen in the water surrounding the reef.
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A lone colony of the coral species Diploria labyrinthiformis competing with small fleshy algae on a degraded reef flat on the island of Curaçao in the Caribbean. Image credit: Cynthia Silveira (CC BY 4.0)

Rising water temperatures, pollution and other factors are increasingly threatening corals and the entire reef ecosystems they build. The potential for corals to resist and recover from the stress these factors cause ultimately depends on their ability to compete against fast-growing fleshy algae that can rapidly take over the reefs.

Living on the fleshy algae, the coral and in the surrounding water are communities of bacteria and other microbes that help maintain the health of the coral reef. Both corals and algae modify the chemical and physical environment of the reef to alter the composition of the microbial communities for their own benefit. Algae, for instance, release large amounts of sugars and other molecules of organic carbon into the water. These carbon molecules are then taken up by the bacteria, along with oxygen, to produce chemical energy via a process called respiration. This could cause the levels of oxygen in the water to decrease, potentially damaging the corals and creating more open space for the algae.

Previous studies have revealed how communities of microbes on coral reefs use organic carbon, but it remains unclear how they affect the levels of oxygen in the reefs. To address this question, Silveira et al. used an approach called metagenomics to analyze the bacteria in samples of water from 87 reefs across the Pacific and the Caribbean, and also performed experiments with reef bacteria grown in the laboratory.

The experiments showed that bacteria growing in the presence of fleshy algae became larger and more abundant than bacteria growing near corals, resulting in the water containing lower levels of oxygen. Furthermore, the fleshy algae produced bubbles of oxygen that were released from the water. Silveira et al. developed a mathematical model that predicted that these bubbles, combined with the respiration of bacteria that live near algae, caused the loss of 67% of the oxygen in the water surrounding the reef.

These findings represent a fundamental step towards understanding how changes in the levels of oxygen in water affect the ability of coral reefs to resist and recover from stress.