• Algae growing on coral tips. Justin Marshall/coralwatch.org, Author provided. (The Conversation)
Bleached corals - such as those in the Great Barrier Reef - are weaker and more susceptible to a microbial takeover fuelled by algae.
By
Kemal Atlay

27 Apr 2016 - 12:57 PM  UPDATED 27 Apr 2016 - 12:57 PM

The world’s coral reefs, which are already weaker due to mass bleaching events, also face the threat of destructive microbial growth as a result of fleshy algae colonisation.

Once fleshy algae gain foothold in a reef, it encourages the growth of disease-causing microorganisms that can kill coral.

To study the destruction of reef ecosystems, an international team of researchers collected water samples from 60 different coral reef systems in the Indian, Caribbean and Central Pacific oceans. The study was published yesterday in Nature Microbiology.

Once a reef becomes dominated by fleshy algae, an energy imbalance is created where “most of the energy in the ecosystem goes into the microbes,’ says lead author Dr Andreas Haas, a marine biologist from San Diego State University. “It doesn’t support the variety of reef organisms that make up a healthy system.”

Dr Guillermo Diaz-Pulido, an algae and coral reef expert from Griffith University, says this finding highlights a major problem that could further decimate the already weakened, bleached corals in the Great Barrier Reef.

“More algae means more microbes, more microbes means less corals, and less corals means more algae, so we get into this very dangerous loop of processes,” Diaz-Pulido told SBS Science.

“Because there is much more algae in the reef, the algae and the microbial community associated with it are going to use all the organic carbon that is produced by the algae, which means there might be less organic carbon available for the reef ecology.”

According to the study, the presence of fleshy algae in the reef creates a feedback system that can kill corals and favours further growth of algae. The self-sustaining cycle is called DDAM and it consists of four main steps – dissolved organic carbon (DOC), disease, algae, and microorganisms.

The feedback system starts with algae producing dissolved organic carbon through photosynthesis, which provides a source of food for microorganisms. Increased microbial activity causes localised ocean acidification, and this can exacerbate the effects of bleaching and other diseases that can weaken and kill coral.

Once the coral is dead, new habitat is freed up for the fleshy algae to expand and colonise – this ultimately leads back to the start of the DDAM feedback system.

“It’s definitely a major source of concern for us because if the Great Barrier Reef turns into an algal reef, that would have major ramifications for a number of different industries,” says Diaz-Pulido.

“If that study is applied to the Great Barrier Reef, we will see increased microbe activity and microbial metabolism in reefs, which might then impair coral recovery [from bleaching] or it might impair the resilience of the reef.”

Coral bleaching occurs when abnormal environmental conditions, such as increased ocean temperatures and acidification, cause corals to expel their photosynethic algae companions called zooxanthellae that give the corals their rich colour. If the damaging conditions persist, the bleached corals may die.

In pictures: a close-up look at the Great Barrier Reef’s bleaching
Even though coral bleaching can look quite beautiful, it is still an environmental tragedy, writes marine biologist Justin Marshall.

According to Diaz-Pulido, tackling overfishing in the waters surrounding reefs is an easy way to limit algae growth, as it would ensure an abundance of algae-eating herbivorous fish.

“If we have healthy fish populations, it means that we will have plenty of herbivorous organisms that feed on the algae and therefore keep the algae in check,” says Diaz-Pulido.

Limiting nutrient run-off from fertilisers and reducing greenhouse gas emissions can also help to rein in algae growth as, just like any other plant, it relies on nutrients and carbon dioxide to grow.

Corals in trouble
Coral Bleaching Taskforce: more than 1,000 km of the Great Barrier Reef has bleached
Many corals in the reef's remote northern reaches are now expected to die as a result of warm waters linked to this summer's El Nino.
Western Australia’s coral reefs are in trouble: we mustn’t ignore them
Warming waters and other threats are seriously endangering our coral reefs, write CSIRO scientists