Too much algae and too many microbes threaten coral reefs
April 25, 2016 - 10:07 AM
Coral reefs, the world's most productive and diverse marine ecosystems, rely on a masterful recycling system to stay healthy.
Corals and algae release nutrients that support a complex and
efficient reef food chain. But when the system gets out of whack, the
cycle breaks down and endangers the reef's health.
Threat of microbialization
A new study explores how a process called microbialization destroys
links in this delicate food chain. The scientists, including Forest
Rohwer of San Diego State University (SDSU) and Craig Nelson of the
University of Hawaii, published their findings this week in the journal Nature Microbiology.
"This well-documented study shows that human activities are affecting
coral reefs in very subtle ways," said David Garrison, program director
in the National Science Foundation (NSF) Division of Ocean Sciences,
which funded the research.
Millions of people around the world depend on coral reefs for
productive fisheries, and reefs play an important role in global
Overfishing the waters near coral reefs, however, removes the primary
algae-eaters from the environment, allowing populations of fleshy algae
to explode. In areas with large human populations, pollution often
exacerbates the problem by stimulating these algae.
Harmful microbes endanger reef ecosystem
Fleshy algae on reefs release copious amounts of nutrients known as
dissolved organic carbon, which microbes eat. The researchers theorized
that when increased levels of algae produce meals for microbes, there
are also higher levels of potentially harmful microbes throughout the
These microbes then endanger corals by depleting oxygen from the
environment or by introducing diseases. As the corals die off, the algae
have even more space to take over, leading to further coral mortality.
When reefs are dominated by fleshy algae, "most of the energy in the
ecosystem goes to the microbes," said the study's lead author, Andreas
Haas, a biologist at SDSU. "It doesn't support the variety of reef
organisms that make up a healthy system."
Sampling corals worldwide
Haas and co-author Mohamed Fairoz, of the Ocean University of Sri
Lanka, collected more than 400 water samples from 60 coral reef sites
across the Indian, Pacific and Atlantic Oceans.
Back in the laboratory, they tested these samples for evidence of
microbialization of algae-dominated reefs, looking for more microbes
with more potential to harm reef organisms.
They analyzed the abundance of microbes throughout the samples and
found that reef sites with more algae had more harmful pathogens, or
The study's results, the scientists say, support the idea that
microbialization linked with increasing algae on coral reefs can
decimate the reef ecosystem through microbial takeover.
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