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Corals obtain energy in two ways: firstly, through photosynthesis by their symbiotic algae, and secondly by taking up small food particles such as plankton directly from the water. In scientific terms, this process is known as “heterotrophy.” In a study published in Communications Biology, an international research team including Bremen-based scientist Marleen Stuhr (ZMT) reports that the contribution of heterotrophic food intake to energy production in corals has so far often been significantly underestimated. The underlying reason is rooted in the measurement methods themselves.
Coral reefs provide a livelihood for millions of people – through fisheries, coastal protection, or tourism. But the climate crisis is putting increasing pressure on reefs worldwide. Recent scientific publications, such as the Global Tipping Points Report, warn that these essential ecosystems may soon reach a tipping point.
Researchers are trying to find out how corals can build greater resilience against rising temperatures and ocean acidification. This requires a detailed understanding of how corals feed, the role of direct plankton uptake, and the contribution of their algal symbionts.
Until now, the heterotrophic share of coral nutrition has mainly been measured by analysing carbon isotopes in coral tissue. Researchers from the University of Rhode Island and the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen have now shown that this method alone does not provide the complete picture.
Much of the carbon derived from heterotrophic food does not remain in coral tissue, but is excreted or quickly respired. By contrast, nitrogen isotopes and certain fatty acids are far more effective as markers. These substances directly enter the tissue of the coral and remain detectable for extended periods.
In their experiments, conducted in 2019 at the Interuniversity Institute for Marine Sciences (IUI) in Eilat, Israel, the team worked with the reef-building stony coral Stylophora pistillata, a widely studied species common in tropical reefs.
Over the course of a 22-day experimental series, the researchers systematically examined the coral’s feeding behaviour by exposing them to different feeding scenarios: Some colonies were left unfed, some received food twice a week, another group was fed six times a week, and another group had been bleached beforehand and was then fed intensively. Freshly cultured brine shrimp larvae (zooplankton) served as the food of choice.
“We then measured physiological parameters such as photosynthetic performance, algal symbiont density, chlorophyll content, growth, and protein reserves to make the effects of feeding visible,” explains Marleen Stuhr from ZMT.
+++Markers reveal nutrient pathways+++
Methodologically, the researchers applied three complementary approaches: they analysed stable carbon and nitrogen isotopes, examined fatty acid profiles, and directly measured the quantity of prey the corals consumed.
“Through this combination, we were able to determine which markers are best suited for capturing the contribution of heterotrophic nutrition,” says first author Connor Love from the University of Rhode Island.
The experiments showed that Stylophora pistillata absorbs nutrients from food not evenly but selectively. Nitrogen is incorporated into the tissue far more extensively than carbon. Carbon, by contrast, is often respired or released as mucus.
“As a result, many conventional methods that rely on carbon isotope analysis significantly underestimate the true share of heterotrophic nutrition,” says Marleen Stuhr. “At the same time, we demonstrated that nitrogen isotopes and specific fatty acid biomarkers are very reliable indicators of food intake – both in the coral and in the symbionts.”
The findings highlight that previous standard methods are not always meaningful and that combining several markers gives a more realistic picture of nutrient uptake.
“If only part of the nutrients taken up are detectable in tissue, an essential contribution to nutrition is overlooked,” says Connor Love. “Using more robust markers such as nitrogen isotopes and fatty acid profiles will allow us to trace more precisely how corals balance their energy management between autotrophic photosynthesis and heterotrophic feeding.”
The researchers add that the tested biomarkers will help experimental studies, monitoring programmes and reef restoration efforts to better assess whether – and how much – corals take up nutrients in addition to photosynthesis, for example to compensate for stress. The experiments also showed that feeding improved certain physiological parameters but could not fully offset the consequences of coral bleaching.
+++Implications for coral reef protection+++
The study shows that commonly used methods significantly underestimate the contribution of heterotrophic nutrition in corals. This creates the potential risk of reef resilience being misjudged in ecosystem models or restoration projects.
The findings are crucial for understanding how corals respond to environmental stress and survive in nutrient-poor marine environments. When ocean temperatures rise or after bleaching events, the ability to take up additional food may be critical. A better understanding of feeding strategies also improves predictions of coral survival in the climate crisis.
“Reefs are highly complex and productive ecosystems whose stability depends largely on the nutrition of individual corals. If corals can compensate for energy and nutrient losses through increased heterotrophy, they may better withstand stressful events such as heat-induced bleaching,” says Marleen Stuhr. “More accurate assessments of reef resilience support decision-making in coastal protection, fisheries, and tourism.”
Connor Love adds: “For reef protection, our study sends two clear messages: first, corals likely rely more on heterotrophic nutrition than previously thought, which connects the survivorship of reefs to ocean plankton dynamics. Second, we found a reliable suite of biomarkers that serve as a valuable tool for assessing the nutritional status and resilience of corals to inform ecosystem models, management strategies, and restoration efforts.”
Dr Marleen Stuhr | AG Geoecology und Carbonate Sedimentology
Email: marleen.stuhr@leibniz-zmt.de
Dr Connor Love | Biological Oceanography | University of Rhode Island
Email: connor.love@uri.edu
Love, C.R., Stuhr, M., Fox, M.D. et al. Selective nutrient incorporation may underestimate heterotrophy of a mixotrophic reef-building coral. Communications Biology 8, 1285 (2025). DOI: 10.1038/s42003-025-08621-8, https://doi.org/10.1038/s42003-025-08621-8
Heterotrophic feeding in the experiment: A bleached coral (Stylophora pistillata) takes up small fo ...
Quelle: ZMT
Copyright: ZMT
The reef-building stony coral Stylophora pistillata in a reef of Eilat in the Gulf of Aqaba, norther ...
Quelle: Marleen Stuhr, ZMT
Copyright: Marleen Stuhr, ZMT
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