The future of corals – what X-rays can tell us

This summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO₂ is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?

What types of corals did you examine?

Tali Mass: These are coral larvae from colonies of the stony coral Stylophora pistillata from the Red Sea. We collected them ourselves during spawning nights in April 2020 at a depth of a few metres. They come from the reef next to the Interuniversity Institute of Marine Sciences in the Gulf of Eilat, Israel. We allowed these larvae to grow in our environment simulators aquaria system for several weeks and exposed them to different pH-conditions. Some tanks contained normal seawater, while others replicated conditions that simulated acidity predicted at the end of this century, assuming no climate protection measures are taken worldwide. This scenario, known as RCP8.5, is associated with significant acidification and global temperature increase of four degrees or more, expected to cause major global disruption. In this context, corals are giving us a window into a potential, bleak future.

How does such a complex skeletal structure grow?

Tali Mass: Each individual coral animal creates the structure by secreting mineral calcium carbonate The survival of the corals depends on the formation of a robust skeleton during these early stages of life, persisting into adulthood.

Paul Zaslansky: The skeleton consists of two different components: calcification centres, known as RADs, and fibre-like structures, known as TDs. RADs are made up of granular calcium carbonate, while TDs consist of densely packed, elongated crystals, primarily aragonite. Curiously, and despite much progress, much about the process of skeleton growth remains unclear. For example, it is widely assumed that the RADs formed first and then the TDs attach and grow outward.

Why do these details matter?

Tali Mass: Detailed knowledge of coral calcification is important for a basic understanding about coral biomineralization, but also for predicting the future and finding ways to intervene. With scientifically rigorous knowledge, we might be able to come up with protection strategies which we then also could test in the field. At the same time, it is simply fascinating to explore how such widespread and evolutionarily important complex skeletal architectures arise in nature. This might even be an inspiration for the development of new materials.

How could you examine the formation of the skeletons? 

Paul Zaslansky: At the BAMline at BESSY II, we found a way to directly visualize, in three dimensions, where and when coral mineral phases were formed. With contrast enhanced and absorption tomographic imaging and careful AI assisted analysis of the large data, we quantified the samples in great detail. The results were quite unexpected. To ensure that they were real, we validated our findings by combining various methods: high-resolution synchrotron X-ray μCT, scanning electron microscopy (SEM) and X-ray diffraction (XRD) and X-ray fluorescence (XRF) mapping. This allowed us to distinguish different mineral phases in order to decipher their composition and to understand their three-dimensional growth dynamics. We also used Monte Carlo simulations in order to interpret the results correctly.

What did you find out?

Paul Zaslansky: Indeed, we can now better describe, in a quantitative manner, the skeleton structure, including the mineral composition of TDs and RADs. Our data show that RADs and TDs form simultaneously, not sequentially as it was assumed. This sheds light on how corals adapt to the environment in which they grow. We observed significant differences in growth patterns under normal and acidified ocean conditions. Under severe acidification, which means a pH of 7.6, RADs become underdeveloped, reducing the stability of the skeleton. At the same time, we observed that both TDs and RADs have a higher density under acidic conditions than under normal conditions. This suggest that the coral animals may be adapting the crystals that they produce. This is fascinating.

What do you expect for the future? Will coral reefs still exist?

Tali Mass: Our results show that the effects of ocean acidification on coral skeleton formation are more complex than previously thought. Whether this will be enough to ensure their survival is highly questionable. So far, the corals in the Red Sea have been impressively resilient to heat waves, but this might change if global warming and acidification continue. We can clearly see that lower pH leads to less stability, which is definitely an additional stress factor. We urgently need effective climate protection measures to prevent the worst-case scenarios. 

Thank you for this conversation!

Info box: Stony corals

Stony corals spawn at specific times of the year. First, the spawn develops into a planula larvae, which floats in the ocean and then settles somewhere on an existing reef to become a tiny polyp. These polyps secrete a calcium compound from their foot disc, allowing the reef to grow. Corals filter microplankton, nutrients and trace elements from the water. However, they also depend on symbiotic algae, which they store in their outer tissue. It is these algae that give corals their intense colours. During marine heatwaves, the algae begin to produce toxic substances. As a result, the corals reject the algae, they “bleach” and die shortly afterwards. Global warming produces strong marine heatwaves much more frequently, which is a threat to the health of coral reefs. Additionally, climate change drives the acidification of the oceans. This significantly more acidic environment has damaging effects on the coral skeleton which in turn undermines and endangers the growth of corals.

Info box: BESSY II:

BESSY II is a synchrotron radiation source which generates extremely brilliant, intense X-ray pulses for materials research. A variety of methods are available to analyse samples, both in terms of their structural composition as well as the chemical and physical processes that occur during examination. At the BAM line imaging station, three-dimensional computer tomograms with submicron resolution and X-ray contrast enhancement can be collected within minutes.