By Moritz Maschek

Image by Moritz Maschek: A close-up daytime image of a colony of large coral polyps. These polyps are closed and not feeding, yet gaining energy from sunlight via the zooxanthellae. 

Image by Moritz Maschek: A close-up daytime image of a colony of large coral polyps. These polyps are closed and not feeding, yet gaining energy from sunlight via the zooxanthellae. 

Like trees in a rainforest, coral breathes life into the ocean where there would otherwise be very little. Coral reef systems are made up of a network of calcium carbonate structures secreted and built by hard corals in warm, shallow ( less than 60m) and sunlit waters. They provide the fundamental habitat, structure and food source for thousands of marine species. There are approximately 2,500 species of coral, roughly 1,000 of which make up the physical structure of a coral reef. Each coral species has a unique growth form, pattern and colour; some so intricate and perfectly designed it’s hard to believe they’re not man made.

To understand how coral works, we must break it down a little further. A single coral structure (e.g. a brain coral) consists of a colony of genetically identical living organisms called polyps. Coral polyps (see image) are small sessile organisms with stinging cells most closely related to jelly fish and anemones. Each polyp creates a calcium carbonate mould for itself to live in, similar to a Gastropoda snail making its shell. The polyps feed mostly at night by trapping small organic particles in the water column using their tiny stinging tentacles. However, this is not their main source of energy. Coral polyps have formed a remarkable symbiotic relationship with microscopic single celled organisms called Zooxanthellae. Zooxanthellae belong to a diverse group of simple algae-like plankton called dinoflagellates which use sunlight for energy (photosynthesis).They can be free living or found in large numbers inside the tissue of a host; in this instance the tissue of the coral polyp. The zooxanthellae gives the coral its beautiful colour, as well as up to 90% of its required energy. In return, the coral polyp with its stinging tentacles offers a safe and stable environment for the zooxanthellae to live in, with ideal sunlight and CO2 for photosynthesis to take place. This delicately balanced relationship is highly beneficial to both parties, working together in perfect harmony as one tight unit. With the right amount of sunlight, the zooxanthellae can provide enough energy for the polyps to reproduce asexually; a complicated process in which they clone themselves to form more genetically identical polyps that add to the colony. As the colony grows, more calcium carbonate is secreted to accommodate the new polyps, in turn expanding the reef system and providing new habitat and more tissue for new zooxanthellae to occupy. 

In summary; what we commonly refer to as ‘a coral’ is a complex living organism made up of part animal (the polyp), part planktonic algae-like cells (zooxanthellae) and part mineral (calcium carbonate). In order to function, reproduce and grow, coral relies on its remarkable symbiotic relationship; a perfect balance of give and take which ultimately supports 25% of all marine life. 

The balance of life in tropical reef systems is facing increasing threats from human industrial activity. Right now, Australia’s Great Barrier Reef and many other tropical reefs around the world are experiencing one of the worst ever incidents of coral bleaching. A primary cause of this bleaching is prolonged higher-than-average ocean temperatures. Bleaching is a stress-response to these temperature changes., and just a 1 to 2 degree increase over a four week period is enough to trigger a mass bleaching event. The chemical explanation for bleaching is not fully understood, but the current understanding is that high temperatures create complex chemical changes that inhibit the photosynthetic pathways of the zooxanthellae, forcing the polyps to reject and expel the zooxanthellae. Without the zooxanthellae the coral completely loses its colour, turning an eerie pale white and becoming extremely vulnerable. The once colourful polyps become transparent and the white calcium carbonate structure can be seen through the polyps. At this stage, the coral is still alive, however without the zooxanthellae it is relying only on nighttime feeding for energy. The coral becomes week and more susceptible to disease, and will not have enough energy to grow or reproduce. If water temperatures return to normal, resilient species can regain their zooxanthellae and fully recover whilst less resilient species will die. However, if conditions stay unfavourable for too long, the majority of coral will eventually die and algae will take over. The lifeless structures will remain but most of the amazing biodiversity will disappear. 

The current mass bleaching event on the Great Barrier Reef (GBR) and elsewhere is being closely monitored by scientist who are already reporting the worst bleaching ever seen on northern parts of the GBR. An aerial study using planes and helicopters has shown the devastating extent of the bleaching (see right image). An area of 600kms of reef was surveyed and 60% of the reef was found to have bleached. The GBR Marine Park Authority has raised the highest possible alert or ‘Level 3’, indicating severe regional bleaching in the northern quarter of the marine park. The bleaching event is partly linked to the El Niño weather cycle, which raises temperatures in the region, on top of climate induced warming caused by carbon emissions.

Our continued reliance on fossil fuels spells the end for our iconic Great Barrier Reef. With ocean temperatures rising, we will be losing a significant amount of pristine reef each time these bleaching events occur.