From the Field to the Lab: Hannah Manns’s Contribution to Understanding Heat Stress and Calcification in Reef Ecosystems

Blog written by Larissa Roy, with information from Hannah Manns (Early Career Researcher from the University of Konstanz) and edited by Jessica Hargreaves.

Coral bleaching seems to be happening every year, but what does this term mean?

Coral bleaching occurs when corals lose their photosynthetic microalgae (symbionts), a process triggered by several factors, the major one being extreme rising ocean temperatures. Symbionts are organisms that live in close relationship with corals, providing energy to the coral through photosynthesis. Without them, corals lose their primary source of energy, which can lead to starvation (as they have no food source), slowing their growth, reducing reproduction, and eventually leading to mass mortality.

Do all corals bleach at the same temperature?

Surprisingly, the SPP 2299 TRACE project based in the Voolstra lab at the University of Konstanz, with researcher Hannah Manns, tells us that the answer is likely no! Each coral species is unique and seems to be reacting differently to environmental stressors.

Hannah’s Background and Key Role in the TRACE Project

What does it take to study coral resilience?

For Hannah, a German and European Scientific Diver with a Master’s degree in Marine Environmental Sciences from the University of Oldenburg, it means combining fieldwork and lab expertise. Since April 2022, she has been working as a lab and field technician in the Voolstra Lab at the University of Konstanz, contributing to projects on heat stress resistance, resilience and calcification, while also supervising Coral Bleaching Automated Stress System experiments (CBASS; Voolstra et al., 2020).

In addition to her scientific expertise, her certification as a scientific diver enables her to access coral colonies directly in their natural habitats, giving her an invaluable advantage in collecting first-hand, high-quality data for the TRACE project. Through her work, she plays a key role in both field and laboratory experiments, including the operation and supervision of the CBASS experiments, which are central to the project.

Figure 2: Set-Up of the CBASS System

What makes the TRACE project unique in understanding coral bleaching patterns?

The project examines how coral bleaching varies between both different locations of coral reefs and also different species of coral in the same reefs. Commonly it is thought that coral reefs bleach uniformly when ocean temperatures rise by 1-2°C. However, the TRACE project is monitoring coral thermal resistance and growth over seasonally varying conditions across different reefs as well as across different coral species in the same reef to uncover whether difference in the reef location, and coral species influence how reefs respond to bleaching. One of the main ways they do this is to use the CBASS.

How does the CBASS system mimic natural heat stress conditions?

CBASS is a key tool in the TRACE project. What the system does is it mimics natural heat and heat stress conditions of a daily cycle, offering a controlled environment where researchers can precisely regulate temperature and light exposure to trigger stress in coral symbionts.

Basically, what happens is the team can take small samples of a coral off the reef, and place them in different tanks of the CBASS set up (like you can see here). Hannah can then individually increase the temperature in the tanks and see what increase of temperature makes the coral bleach. Using CBASS, researchers can identify heat-tolerant coral populations, to better understand regional resilience patterns and guiding conservation efforts.

But what do scientists actually quantify to determine a coral’s ability to withstand heat stress? One key metric that Hannah measures is called ED50. The ED50 is the temperature at which corals lose 50% of their photosynthetic efficiency, a parameter that is used as a proxy for the thermal tolerance of corals. By using the data of the CBASS the team can determine the ED50 value (thermal tolerance) for different coral species, enabling comparisons of corals thermal tolerance.

Are there any trade-offs between thermal tolerance and growth in corals?

Some corals host heat-resistant symbionts, allowing them to withstand higher temperatures (Baker et. al., 2001). However, this adaption comes at a cost, as more energy is allocated to maintaining thermal tolerance rather than growth of the coral skeleton (Jones & Berkelmans, 2010). Reef growth and skeletal development are influenced by temperature, ocean acidification, and bioerosion (Cantin et al., 2010; Dove et al., 2013; Perry et al., 2018). Corals that prioritize thermal tolerance often give less energy to calcification, potentially slowing reef growth (Cantin et al., 2010; Jones & Berkelmans, 2010), although outcomes depend on environmental conditions and species interactions (Darling et al., 2012; DeCarlo et al., 2017). Faster-growing corals, such as branching Acropora species, are more vulnerable to heat stress due to trade-offs between rapid growth and stress tolerance (Darling et al., 2012; Loya et al., 2001). In order to explore potential trade-offs between thermal tolerance and growth, a further comparison of ED50 values with calcification rates has been made, but these results are still under analysis.

How does collaboration strengthen marine research like the research in the TRACE project?

Collaboration is the key to successful marine research. When scientists tackle complex challenges like coral bleaching, it’s essential to bring together diverse expertise and perspectives. As the research spans lots of different disciplines, from fieldwork specialists and divers, to lab analysis, to data interpretation and climate and biological modeling. Teamwork is necessary and makes projects like this successful. The synergy increases the scope and accuracy of findings, and fosters innovation and shared learning within the scientific community. Some of this collaboration coming from the DFG Priority Programme “Tropical Climate Variability & Coral Reefs” (SPP 2299), which strives to promote a collaborative community.

Why does this Research Matter?

1. Baker, A. (2001). Reef corals bleach to survive change. Nature 411, 765–766 https://doi.org/10.1038/35081151
2. Cantin, N. E., Cohen, A. L., Karnauskas, K. B., Tarrant, A. M., & McCorkle, D. C. (2010). Ocean warming slows coral growth in the central Red Sea. Science, 329(5989), 322–325. https://doi.org/10.1126/science.1190182
3. Dove, S. G., Kline, D. I., Pantos, O., Angly, F. E., Tyson, G. W., & Hoegh-Guldberg, O. (2013). Future reef decalcification under a business-as-usual CO₂ emission scenario. Proceedings of the National Academy of Sciences, 110(38), 15342–15347. https://doi.org/10.1073/pnas.1302701110
4. Loya, Y., Sakai, K., Yamazato, K., Nakano, Y., Sambali, H., & van Woesik, R. (2001). Coral bleaching: The winners and the losers. Ecology Letters, 4(2), 122–131. https://doi.org/10.1046/j.1461-0248.2001.00203.x

5. Perry, C. T., Alvarez-Filip, L., Graham, N. A. J., Mumby, P. J., Wilson, S. K., Kench, P. S., … & van Woesik, R. (2018). Loss of coral reef growth capacity to track future increases in sea level. Nature, 558(7710), 396–400. https://doi.org/10.1038/s41586-018-0194-z
6. Jones, A. M., & Berkelmans, R. (2010). Potential costs of acclimatization to a warmer climate: Growth of a reef coral with heat tolerant vs. sensitive symbiont types. Ecology Letters, 13(12), 1565–1572. https://doi.org/10.1111/j.1461-0248.2010.01558.x
7. Darling, E. S., Alvarez-Filip, L., Oliver, T. A., McClanahan, T. R., & Côté, I. M. (2012). Evaluating life-history strategies of reef corals from species traits. Ecology Letters, 15(12), 1378–1386. https://doi.org/10.1111/j.1461-0248.2012.01861.x
8. DeCarlo, T. M., Cohen, A. L., Wong, G. T. F., Shiah, F.-K., Lentz, S. J., Davis, K. A., … & Lohmann, P. (2017). Community production modulates coral reef pH and the sensitivity of ecosystem calcification to ocean acidification. Journal of Geophysical Research: Oceans, 122(1), 745–761. https://doi.org/10.1002/2016JC012326