In a study published in Communications Biology, an international research team that includes Bremen-based scientist Marleen Stuhr (ZMT) has reported that the role of heterotrophic food intake in energy production within corals has often been significantly underestimated. The reason for this underestimation lies in the measurement techniques used.
Previously, the heterotrophic component of coral nutrition was primarily assessed by analysing carbon isotopes found in coral tissue. However, researchers from the University of Rhode Island and the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen have demonstrated that relying solely on this method does not give a complete understanding.
A significant portion of the carbon obtained from heterotrophic food does not stay in the coral tissue; instead, it is either excreted or rapidly respired. In contrast, nitrogen isotopes and specific fatty acids serve as much more effective markers. These substances are absorbed directly into the coral tissue and can be detected for longer durations.
During their experiments conducted in 2019 at the Interuniversity Institute for Marine Sciences (IUI) in Eilat, Israel, the team focused on the reef-building stony coral Stylophora pistillata, a species that is extensively studied and commonly found in tropical reefs.
During a 22-day experimental series, the researchers carefully studied the feeding behavior of corals by subjecting them to various feeding conditions: some colonies were not fed at all, others were given food twice a week, another set was fed six times a week, and yet another group, which had been bleached prior, was fed intensively. The preferred food was freshly cultured brine shrimp larvae (zooplankton).
“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 Stuhr from ZMT.
The researchers used three complementary methods: they analysed stable carbon and nitrogen isotopes, looked into fatty acid profiles, and directly measured how much prey the corals consumed.
“As a result, many conventional methods that rely on carbon isotope analysis significantly underestimate the true share of heterotrophic nutrition,” says 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 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 note that the biomarkers they tested will assist experimental studies, monitoring initiatives, and reef restoration projects in more effectively evaluating whether—and to what extent—corals absorb nutrients beyond photosynthesis, such as to alleviate stress. Additionally, the experiments revealed that while feeding enhanced some physiological aspects, it was not enough to completely counteract the effects of coral bleaching.
[image: Stylophora polyps up close – image by J Clipperton]
