Grigory Ryzhakov – Russian Writer

What Is Threatening Coral Reefs?

A coral reef, Papua New Guinea (photo by Brocken Inaglory, via Wikimedia Commons)


We often hear about the climate change and the never-ending discussion of it keeps making headlines in the media. There seem to be a consensus amongst climatologists – average atmospheric temperatures have been rising in the past several decades and the Arctic ice cap reducing. It’d be wrong to just call it global warming, as the local temperatures fluctuate so much that certain parts of the world could actually become cooler.

The central battle has been going on over one issue – to what extent climate change is caused by human activity. The problem is that our planet’s geochemical cycles are so complex, it’s hard to dissect the human factor and natural causes. Of course, the industrial emissions of carbon dioxide contribute to the greenhouse gas effect, and the CO2 levels have been on the rise ever since the records started.

One would say, okay, natural or human-driven, the changing climate will always shape our habitat, we just need to constantly adapt to that. The problem is that such changes normally occur over a long period of time , while the recent ones are fast.

What is ocean acidification? Click on the picture to find out more

One of the most terrifying yet not much talked about consequences of the climate change is the decline of coral reefs. What is the connection between them and raising temperatures and CO2 levels?

Corals are colonial animals belonging to the group called Cnidaria and they are related to sea pens, jellyfish and sea anemones. Many reef-building species of corals use calcium carbonate, the limestone, to build their exoskeleton, enabling a small colony to grow into massive reefs. Corals are trees of the sea: they create habitats for other animals, including fish, and are indispensable for survival of many tropical marine ecosystems. With corals dead, the reef slowly becomes desolate.

Corals are also important regulators of the climate. They assimilate CO2 from seawater to build their exoskeleton, thus lowering the global greenhouse gas levels. Unfortunately, with CO2 levels raising too fast, there is an increasing acidification of the ocean – corals are less capable of taking CO2 at lower (acidic) pH and they suffer as a consequence.

Corals are symbiotic animals: the microscopical algae called zooxanthellae live in their cells, photosynthesise, and supply the coral with sugar and extra energy for growth. When corals are stressed as a result of pollution, increased water temperature or the pathogen attack, they lose their symbionts (and their colourful pigments), and eventually die. This process is called coral bleaching.

In the past few decades coral bleaching became a major problem. The coral reef decline is estimated to cause over 100 billion dollar loss for the world economy in the next 50 years. Add to this impacts on oceanic ecosystems and the global climate. The reefs are currently reducing by twenty percent each decade. It’s just unthinkable – the coral variety that took 600 million years to evolve could be wiped out in less than a century.

Is controlling the CO2 emissions sufficient to stop this disaster from developing? We don’t know.

The coral biology is not very well studied at molecular level. We don’t know much about the molecular triggers of the bleaching process. And that’s where, in my opinion, many research efforts should be directed.

Now in addition to acidification, other geochemical factors like warming water and deoxygenation devastate marine habitats, e.g. provoking massive expansion of jellyfish and toxic algal blooms. A brief report on that has been recently featured in New Scientist.

A sponge Haliclona permollis (photo by jkirkhart35 [CC-BY-2.0 (], via Wikimedia Commons)

We more and more often see devastating reports in the media on the nature’s state, but sometimes we discover things that amaze us and give us hope in nature’s ability to self-regulate and sustain its habitats. Sponges, basal colonial animals, were found to be key organisms recycling organic matter and critical for the survival of coral reefs. A group of scientists used dye-labelled organic matter to track its accumulation in the coral reef ecosystem and found sponges to be primary consumers of this matter –  sponges took in the labelled organic molecules dissolved in the seawater and then shed their dead cells into detritus, as an insoluble matter, that could be used as food by other reef’s organisms. Sponges turned out to recycle around ten times as much organic matter as bacteria did. Now the researchers are looking at ways of using sponges in coral reef preservation programmes.

Oceanology has always been in fashion. It is sad that the fashion is driven by the concern for the ocean’s future rather than wonder at planet’s biggest aquarium and its spectacular and mysterious life.



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