Corals live in a symbiosis with algae, which gives them their bright colors and provides them with energy through photosynthesis. But the reef-building animals also feed on plankton themselves. A study now shows that the proportion of this direct form of nutrition has so far been significantly underestimated because common measurement methods do not fully capture the actual food intake. The results provide new insights into the energy balance of corals and can help to better assess their ability to adapt to climate change.
Coral reefs provide a habitat for countless fish, crabs, mussels and other sea creatures. They also make an important contribution to coastal protection in many regions and are a popular destination for tourists. The sedentary cnidarians live in a symbiosis with algae, which carry out photosynthesis and provide them with energy. In addition, the corals have a heterotrophic diet, meaning they eat food particles from the water around them, especially plankton. However, climate change is threatening these ecologically important reef builders. Rising sea temperatures cause the symbiotic algae to produce toxins and be rejected by the corals. As a result, the corals not only lose their bright colors and fade, but also their energy suppliers.
Feeding experiment in a seawater tank
But how important are photosynthetic algae for corals? How big is their contribution to the corals’ diet? A team led by Connor Love from the University of Rhode Island in Narragansett investigated this question. To find out how much plankton corals consume, Love and his colleagues kept stony corals of the species Stylophora pistillata in large seawater tanks and fed them at different intervals with the larvae of tiny crustaceans, i.e. zooplankton. Depending on the experimental group, the corals received zooplankton two or six times a week or had to survive without food at all.
In another group, the researchers first artificially triggered coral bleaching, removing the symbiotic algae, and then fed the bleached corals six days a week. On the one hand, Love and his colleagues recorded directly how much zooplankton the corals absorbed. On the other hand, they measured the carbon and nitrogen isotopes as well as the content of various fatty acids in the coral tissue in order to indirectly determine the source of these nutrients. “This combination allowed us to test which markers are best suited to capture the contribution of heterotrophic nutrition,” explains Love. In addition, the team recorded physiological parameters such as photosynthesis performance and growth of the corals to make the effects of feeding visible.
New indicators of coral nutrition
This showed that the corals only actually store some of the carbon they absorb with their food in their tissue. Another part is quickly excreted or exhaled. “If only some of the nutrients consumed can be measured in the tissue, a significant contribution from the diet is missed,” explains Love. However, conventional measurement methods have usually only analyzed the carbon isotopes in coral tissue. “This means they significantly underestimate the actual proportion of heterotrophic nutrition,” says co-author Marleen Stuhr from the Leibniz Center for Tropical Marine Research in Bremen.
According to the study, the nitrogen isotopes and fatty acids, on the other hand, provide a more reliable picture. They get directly into the tissue of the coral and remain detectable there for longer. “By using more robust markers such as nitrogen isotopes and fatty acid profiles, we will be able to understand much more precisely how corals balance their energy management between autotrophic photosynthesis and heterotrophic food intake,” says Love. This makes it possible to estimate more precisely how many nutrients the corals absorb in addition to photosynthesis and to what extent this makes them more resilient to environmental stress.
Corals can only compensate for bleaching to a limited extent
The bleached corals showed that they were able to at least partially compensate for the loss of their symbionts through food intake. Thanks to the zooplankton they consumed, they were able to continue growing without photosynthesis, albeit more weakly than their healthy counterparts. “Overall, feeding did not offset the negative effects of bleaching,” the research team reports. In addition, the bleached corals consumed less zooplankton than the intact corals for the same amount of food available. One explanation could be that the corals need energy to use their tentacles to swirl plankton into their mouths. Corals weakened by bleaching could be less effective.
“Our study means two things for reef conservation,” says Love: “First, corals are likely to rely more heavily on heterotrophic nutrition than previously thought, linking reef survival to plankton dynamics in the ocean. Second, we have found a reliable set of biomarkers that can serve as a valuable tool for assessing nutritional status, coral resilience and in reef monitoring.”
Source: Connor Love (University of Rhode Island) et al., Communications Biology, doi: 10.1038/s42003-025-08621-8