Overgrowth Of Algae Endangers Coral Ecosystems

April Flowers for redOrbit.com – Your Universe Online

Overfishing and nitrate pollution can destroy coral reefs by allowing an overgrowth of algae that brings with it unwanted pathogens, chokes off oxygen and disrupts helpful bacteria. These are the findings of a new study out of the Oregon State University, published recently in PLoS One.

Large algal species, or “macroalgae,” are big enough to essentially smother corals. These macralgae grow at accelerated rates when sewage waste enters the water and increases nitrate levels, which acts as a sort of algae fertilizer. This accelerated algae growth becomes an even bigger problem when the large fish populations that are the most effective at removing the algal buildup are reduced through over-fishing.

The growth rate of corals has been observed to sink by as much as 37 percent in the presence of macroalgal competition, and other researchers have documented cases of hypoxia, or lack of oxygen.

Specialists in the field refer to this phenomenon as “the slippery slope to slime.”

“There is evidence that coral reefs around the world are becoming more and more dominated by algae,” said Rebecca Vega-Thurber, an OSU assistant professor of microbiology. “Some reefs are literally covered up in green slime, and we wanted to determine more precisely how this can affect coral health.”

The new study found that higher levels of algae cause both a decrease in coral growth rate as well as an altered bacterial community. The algae can also introduce harmful pathogens to the coral and at the same time reduce the levels of helpful bacteria. The useful bacteria help to feed the corals in a symbiotic relationship and also produce natural antibiotics that can help protect the corals from other pathogens.

One algae in particular, Sargassum, was found to be a vector, assisting potentially harmful microbes to enter the coral ecosystem.

Thousands of species of algae exist, and coral reefs have evolved with them in a relationship that often benefits the entire tropical marine ecosystem. When the system is in balance, some algae grow on the reefs, which provides a food source for a variety of fish species. The growth rate of this algae is typically regulated by the availability of certain nutrients, in particular nitrogen and phosphorus, and tropical fish species like the parrot fish consume large amounts of the algae, thus keeping its growth in check.

However, when algal growth is over-stimulated by nutrients in sewage waste water from coastal areas, and overfishing simultaneously curbs algal consumption at the same time, then this delicate balance can be disrupted.

“This shows that some human actions, such as terrestrial pollution or overfishing, can affect everything in marine ecosystems right down to the microbes found on corals,” Vega-Thurber said. “We’ve suspected before that increased algal growth can bring new diseases to corals, and now for the first time have demonstrated in experiments these shifts in microbial communities.”

At some high-value coral reefs, the problem is being directly addressed through the mechanical removal of algae. This process is costly, however, and researchers say that the best long-term solution to algal overgrowth is to reduce pollution and overfishing so that the ecosystem’s natural balance can be restored.

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