Australasian Science Magazine

During the bleaching event there was a sharp contrast between corals that had C2 algae (left) and type D algae (right)

By Alison Jones

Not only are corals jeopardised by warmer waters but their growth is constrained as they change from heat-sensitive to heat-tolerant symbiotic algae in order to survive.

Until recently, scientists were hoping that coral reefs could adjust to climate change by gradually acclimatising to warmer conditions. This hope was based on the idea that most of the corals that make up tropical reefs could increase the amount of heat they can withstand by changing the type of algae within their tissue.

This “shuffling” occurs when the coral rejects its normal algae and new algae take their place. The new type of algae can help the coral withstand water that is 1–2°C warmer than normal. Although shuffling gives us hope that reef corals may be able to cope with climate change, the short-lived nature of the shuffling, the effects of algal type on the coral and the pace of warming, taken together, suggest that this adjustment will not be without a cost.

Corals are tiny animals called polyps that resemble jellyfish but are smaller and in contrast grow upside-down within a limestone skeleton cup. They live in a partnership with tens of thousands of even smaller single-celled plants called zooxanthellae.

The polyp acts as the host for the tiny olive-green algae, which in turn make food for their host using sunlight and water through photosynthesis. The algae supply most of the energy required by the polyp to grow its skeleton, regenerate and produce eggs and sperm for reproduction. In turn, the waste products from the polyp are used by the algae for survival. Tropical reefs, which are made up mainly of these types of corals, are highly productive because of the successful partnership between these two organisms.

There are many types of marine algae living within the corals on tropical reefs, and it is now thought that most of these corals can host more than one type of algae, often at the same time, and that corals can change algae types as their environment changes. Different types of algae are more heat-tolerant than others. The processes of heat-tolerance in the algae are similar to those used by plants. Because of this, we suspect that just as heat-tolerant plants grow much slower, heat-tolerant algae might also grow more slowly.

To investigate this, Dr Ray Berkelmans and I decided to compare the growth of corals with heat-tolerant algae and those of heat-sensitive algae. In Queensland’s Keppel region one of the main coral species normally has either one or the other of these types, making it an ideal location for our study.

Growth rates of corals were initially measured in the field and in aquaria in the laboratory. Just as we suspected, corals with a heat-tolerant algae called type D grew about 30% slower than those with the heat-sensitive type called C2.

During summer at the study site, water temperatures rose to 29°C and remained at that temperature for over 2 weeks. Colonies with type D algae appeared completely unaffected by the warm conditions whereas colonies with type C2 lost all their algae and appeared bleached. This appearance resulted when the corals discarded almost all of their type C2 algae.

These bleached corals took 3 months to recover to their normal colour. When they had recovered, many of them now had type D algae. Many had changed back to C2 or mixtures of types within 6 months.

We continued to measure the growth rates of the corals during this time. All the corals were growing much more slowly regardless of their type of algae or whether they had appeared bleached. Not only were the bleached corals struggling to grow because of their new heat-tolerant type D algae, they were now also struggling with the direct effects of being in such warm water for such a long period time and because they had lost their food source.

The most surprising part of this research for us was finding that even corals that had type D algae before the bleaching, and showed no evidence of being affected by the warm water, had 50% slower growth for at least 6 months afterwards. This means that either:

• the direct effects of the heat on the coral polyp affects growth;

• photosynthesis in the type D algae is affected by the heat but the algae are not expelled;

• despite being retained, the type D algae are not supplying the products of photosynthesis to their host for growth following bleaching; or

• the higher respiration rates of the stressed algae reduce photosynthesis and therefore the amount of energy that is available to the coral for growth.

Thus the growth of the faster-growing C2 corals is put in jeopardy, first because of the direct impacts of the heat stress, second because they temporarily lose their source of food, and third because they have recovered with a photosynthetically less-efficient type D algae.

To recover to their full growth rate, the corals must change back to their normal C2 algae, which then places them once again in jeopardy of bleaching. While permanently hosting D algae still represents a way for corals to cope with bleaching without temporarily losing their food source, our study shows that there is a cost in terms of their growth.

In addition, the study demonstrated that corals prefer to have the more efficient but heat-sensitive C2 algae, and any changes following the bleaching were only temporary. If another warm summer were to occur in the year following such a bleaching event, the corals may not have enough time to fully recover to their original growth rates.

Bleaching events on the Great Barrier Reef have typically been 2–4 years apart, but predictions of air and sea temperature rises of between 2°C and 4°C, and more frequent and extreme weather, suggest that such bleaching might occur almost every year on reefs in the future. If this is the case, corals that change to D-type algae may increase their chances of survival for the next event, but they will definitely grow more slowly.

The study shows that none of the corals have had time to fully recover before the next summer.

Will Reefs Fly South for the Summer?
Some scientists have suggested that, as conditions become too warm at lower latitudes, corals will find refuge in higher latitudes in the cooler southern waters. So why don’t corals just pack up and move south?

The site of our study at the Keppel Islands in the southern Great Barrier Reef is considered to be close to the southern limit of strong coral growth. Reefs start to change dramatically further south.

While corals can grow in cooler waters such as the southern Great Barrier Reef, there are limits to how well they can do so. The cooler temperatures make it harder for them to compete with soft corals and macro-algae, and growth rates are typically much slower.

Other than the factor of which type of algae a coral hosts, environmental factors that determine coral growth rates include conditions such as temperature, light levels, water quality, food, depth and ocean acidity. Many of the same factors that prevent strong coral reef growth south of the Keppels may make it highly unlikely that reefs will move south.

Even if they are able to shift south to cooler waters, the rate at which air temperatures are becoming warmer may be too fast for this to occur.

Ocean Acidification and Coral Growth
Another factor that governs coral growth is the pH or acidity of the surrounding seawater. As more carbon dioxide saturates the air, more becomes dissolved in the ocean, which decreases the pH.

The growth of the limestone skeleton of corals and other invertebrates depends on the amounts of calcium and carbonate ions dissolved in seawater and on the pH. If the surrounding water is saturated with these ions and the pH is too low, the formation of calcium carbonate to form the limestone skeleton is discouraged. Much like placing a piece of chalk into a glass of lemon juice, increasing acidity will discourage the formation of the coral’s protective skeleton.

The acidity of the oceans therefore further complicates the effects of increasing temperature on coral growth. Only time will tell if corals, already struggling to build their skeletons on reefs in the warmer northern part of the Great Barrier Reef, will find things easier in the south.

Transformation Rather than Adaptation of Coral Reefs
It is true that millions of years ago the oceans were much more acidic than they are today, and some corals may have survived these conditions by losing their skeletons altogether. However, the rate at which atmospheric carbon dioxide and the acidity of the oceans changed then was much slower than the rate at which changes are occurring now.

Corals may not be able to adjust quickly enough to survive current rates of climatic change. Reefs may be able to adjust to some extent as the climate changes, but it is more likely that many of the hard coral species will struggle to compete with the organisms that don’t need to calcify skeletons.

Some of the corals that build present-day reefs could eventually be extinct as conditions change too much for them to survive, and reefs may eventually look vastly different.