Harvesting light

Coral reef

21 November 2008 by Sebastian Hennige

To protect coral reefs we need to understand how they respond to stressful conditions - and that's proving more complex than previously thought. Sebastian Hennige explains.

Coral reefs are in trouble. Human activities which disrupt the fine ecological balance of reefs, such as overexploitation for food, cyanide fishing for the aquarium trade or changes in land use which can lead to eutrophication, or excessive nutrients in the water, harm reefs at a local level. But climate change threatens all coral reefs. And while some still think that climate change is something to worry about in the future, reefs are under threat now. This is partly because increased sea temperatures can cause reefs to die en masse.

2008 is the International Year of the Reef, and many organisations, commercial and academic, are doing their part to raise public awareness about the value of coral reefs, the threats they face and what we can do to protect them. But to protect coral reefs, we need to understand them.

The way a coral skeleton reflects unused light back to the zooxanthellae makes corals more efficient light harvesters than plants!

Corals may seem like simple organisms, but it takes a complex relationship between an animal host and the algae that live inside it -termed 'zooxanthellae' - to let them invest energy into forming calcium carbonate skeletons - the complex 'rock' structures which we all recognise.

When corals die, whether through direct or indirect effects of human activity, other organisms like algae replace them. Reefs lose their complexity and in turn cannot support as many animals such as fish or invertebrates. So keeping coral reefs healthy and associated biodiversity high is a good idea for many reasons, including as a potential food source and for ecosystem functioning and tourism. But how can we do this?

For efforts at management and conservation to be effective, we need a better understanding of the relationship between the coral animal and the zooxanthellae. This relationship is in essence simple: the algae get room and board - a place to live and nutrient-rich waste from the coral host - while the coral gets carbohydrates and sugars from the photosynthetic algae. Of course corals also feed themselves to attain essential nutrients in the same way an anemone or a jellyfish does - albeit on a much smaller scale - but the majority of energy for many comes from zooxanthellae.

Shedding light on fluorescence

So how can we assess the efficiency of this relationship in terms of carbohydrate production and benefits to the host? Well, the zooxanthellae, just like any other photosynthetic plant or algae, use light in a number of ways. Light fuels photosynthesis to produce carbohydrates, but if there is too much light for the zooxanthellae to process, the excess light energy can be lost as heat or re-emitted as fluorescence in order to protect cells from damage.

These processes are competitive; an increase in one means a decrease in another. So by using a specialised machine called a fluorometer, which measures the fluorescence emitted by corals without damaging them, we can assess photosynthetic efficiency and start to gain a better understanding of how hosts and zooxanthellae interact, and how different corals fare in different conditions.

Green fluorescent coral

Green fluorescent coral

But here the story gets more complicated. There are many different types of zooxanthellae, and there are big differences in how they process light and cope under thermal stress. Add to this the fact that multiple zooxanthellae types can be found within any one coral, that certain coral species will only associate with certain kinds of zooxanthellae, and that corals can increase or reduce feeding depending on zooxanthellae and local conditions, and you suddenly have a lot to account for!

The success with which different coral-zooxanthellae partnerships deal with thermal and light stress can be seen in coral bleaching. Coral bleaching is an obvious sign of environmental stress upon the coral, since colour is lost through either a reduction in chlorophyll a (the pigment associated with light absorption for photosynthesis) or by reduced zooxanthellae density. In the latter case, we don't know whether corals actively expel zooxanthellae, or whether their own tissue is lost from the skeleton along with the zooxanthellae.

But recent evidence suggests that not all corals bleach in stressful conditions. Some scientists think stressed corals can acquire resilient zooxanthellae types from the surrounding water - this is called the adaptive bleaching hypothesis. Perhaps a more likely scenario is that the algae types already within the coral undergo a 'shuffling' in dominance, so that the most suitable zooxanthellae type becomes the most numerous. Of course, this is hardly coral's answer to climate change - not all species have such flexibility, and those that do have their limits.

We are only now starting to understand how coral's internal light environment varies depending on the host, and how this affects zooxanthellae photosynthesis and thereby carbohydrate production. Evidence suggests that host fluorescent proteins (FPs), which give corals their colourful appearance, may also protect them from bright light. These FPs occur in the host animal, so this fluorescence is different from that produced by zooxanthellae. The colour and expression of these proteins varies between corals, causing differences in colour.

Reflective skeletons

But this is not the only way in which corals differ. Their shapes vary greatly across species and environmental conditions from branching to plate-like to boulders. The skeletons in all these growth forms act as light reflectors and let the zooxanthellae capture more light, a role we are only now starting to understand. The way the skeleton can reflect unused light back to the zooxanthellae makes corals more efficient light harvesters than plants!

Light harvesting may be one of the fundamental driving forces in the evolution of coral skeletons.

Researchers at the Universidad Nacional Autónoma de México believe that light harvesting may even be one of the fundamental driving forces in the evolution of coral skeletons. All this means that understanding the differences between algal and host types is crucial for understanding coral as a whole organism, or 'holobiont'.

At the recent Global Aquatic Productivity meeting in Eilat, Israel, scientists quantified differences between zooxanthellae types and incorporated them into experiments designed to reassess pioneering work from back in the 1980s, using the knowledge we now have of how these organisms function. We hope these experiments will further our understanding of how zooxanthellae and coral hosts are modified to optimise productivity over a range of environmental conditions, and how flexible these processes are.

By understanding these interactions in different environments and between different coral species, we can form a solid basis for management and conservation strategies. The recent 11th International Coral Reef Symposium saw the joint efforts of more than 2,500 scientists and conservationists from over 150 countries to decide how best to manage reefs for the future.

The message was clear: coral reefs are in danger of major changes, which will affect overall reef productivity, reef complexity and associated bio-diversity. Effective science-driven management, which takes account of how both resilient and sensitive coral species contribute to reef structure, will let us slow their rate of decline, as a genetically diverse, healthy reef will be more resilient than a reef harmed by human activities. But the question remains: in the face of future climate change, will this be enough?

Sebastian Hennige is completing a NERC-funded PhD in coral photobiology at the Coral Reef Research Unit at the University of Essex.