A new study led by the University of Southampton in the UK has revealed why coral reefs can thrive in seemingly nutrient poor water, a phenomenon that has fascinated scientists since Charles Darwin. The research shows that corals farm and feed on their photosynthetic symbionts -- microscopic algae that live inside their cells. This vegetarian diet allows the corals to tap into a large pool of nutrients that was previously considered unavailable to them. Effectively, they are eating some of their symbiont algae to get the nutrition they need to survive.
By performing a series of long-term experiments at the University of Southampton's Coral Reef Laboratory, the scientists demonstrated that corals actually digest some of their symbiont population to access the nitrogen and phosphorus that symbionts absorb from the water. Where there is sufficient dissolved inorganic nutrients in the water, this mechanism allows corals to grow quickly, even if they do not receive any additional food. Results from fieldwork in remote coral reef atolls in the Indian Ocean support the lab findings, demonstrating that this mechanism boosts coral growth in the wild at the ecosystem level.
The scientists also analysed corals growing around islands in the Indian Ocean, some with seabirds on them and some without, to show that corals have potential to farm and feed on their symbionts in the wild. The reefs around some of these islands are supplied with substantial amounts of nutrients that come from 'guano', the excrements of the seabirds nesting on the islands. On other islands, the seabird colonies have been decimated by invasive rats. Accordingly, the associated reefs receive less nutrients. The scientists' new findings suggest that while coral animals may endure brief periods of starvation by feeding off their symbionts, some coral reefs might be at risk of starvation in response to more prolonged nutrient depletion brought on by global warming in some areas.
