Coral reefs are ecosystems of extraordinary diversity. Considered "the rainforests of the sea", they contain ~35% of described marine species despite only occupying 0.2% of the world's ocean. Although they are extremely important habitat providers and form large living structures (some reefs can be seen from space!) the coral animals themselves are small and very sensitive to changes in their environmental conditions. A stressed coral can indicate changes in the physical and chemical properties of the seawater and pressures on the whole ecosystem. Coral reefs are currently faced with an unprecedented mix of human-induced stressors, ranging from overfishing of herbivorous fish (which allows turf algae to overgrow the coral), to changes in the water quality from coastal development, dredging and agricultural runoff. However, two of the major threats are global issues resulting from the billions of tonnes of CO₂ we pump into our atmosphere each year. Firstly, global warming is heating the ocean at a record rate; in fact the world's reefs have recently (2015-2016) experienced the third global coral bleaching event ever recorded. And secondly, as well as acting as a sink for heat, the ocean also absorbs CO₂ gas, which is changing the chemistry of the seawater and driving ocean acidification. A warming and acidifying ocean are pushing corals to their limits and it is important now more than ever to communicate the plight of the world's coral reefs.

In recent years it has become essential to understand how multiple stressors will affect corals (read also the Break: Ocean acidification and its effects on coral reef growth). Of course there are always winners and losers when conditions change, therefore it is also important to investigate the variability in responses to stress among different species, corals from different environments and in different stages of their lifecycle.

Our research focussed on juvenile corals from the sub-tropical Houtman Abrolhos Islands in Western Australia. Juvenile corals are small (one-month-old recruits in this study were around 1 mm in diameter) and yet are immediately faced with the same natural pressures faced by adult corals on the reef. These include; overgrowth by other benthic plants and animals, predation, and damage from storms. It is therefore important for young corals to rapidly grow into more robust size classes. However, changes to their physical and chemical environment have been shown to reduce their growth rate. In this study we were interested in how high temperature and CO₂ affect not only the overall growth rate of young corals, but also the structure of the juvenile skeleton.

We cultured and grew corals from larvae to one-month-old juveniles under elevated temperature and CO₂ regimes simulating the conditions that will likely be experienced in our oceans by the year 2100. We then analysed the skeletons to both view and quantify the changes to the structure. Since coral recruits are so tiny, it has been difficult in the past to show how the skeletal structure changes under these stressful conditions. However advancements in MicroCT technology made it possible for us to reconstruct high resolution 3D images using a 3D X-ray microscope. This 3D approach allowed us to view the skeleton from any angle, look inside at internal structures, quantify changes in the volume of the skeleton and make cross sectional measurements.

We found that acidification caused reduced overall mineral deposition and structural deformities in the skeleton. The deformities included reduced vertical growth, reduced thickness of individual skeletal structures, over or undersized structures, as well as large gaps in the lattice structure of the skeleton. We also observed a more porous and fragile skeleton with a higher frequency of fractures in the corals grown under high CO₂.

Interestingly, temperature did not have a negative effect on skeletal growth. We expected temperature and CO₂ to have a cumulative effect; however temperature either had little effect or a mitigative effect against CO₂. We think this response could be unique to sub-tropical juveniles. It may be related to both the large dispersal distances possible in the larval phase (and therefore a potentially wide range of thermal environments experienced) and because the experiment was conducted in the sub-tropics, where there is a broader annual range in temperatures.

This study highlights how susceptible young corals are to ocean acidification. Reduced growth and structural robustness could reduce early survival rates, as young corals need to grow rapidly to maintain a position on the substrate. The reef-wide implication for reduced growth and survival of juveniles is reduced overall recruitment success on a reef. Successful recruitment is particularly important when a disturbance such as the recent global bleaching event occurs, because the yearly influx of new young corals are a vital part of reef recovery.

Next read: Nitrogen is becoming less available in ecosystems around the world by Rachel Mason

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