Corals living on reefs near populated coastal areas have it rough—particularly near rivers that flush large plumes of sediment from land into the coastal waters.

As if being buried alive weren’t bad enough, microbial activity within the sediment that settles onto coral colonies may give them less than a day to live.

ANALYSIS: Coral Suffering Before Climate Change

A team of marine microbiologists now thinks they know why terrestrial run-off can be such a quick coral killer. Led by Miriam Weber of the HYDRA Institute in Italy and the Max Planck Institute for Marine Microbiology in Bremen, Germany, the team conducted laboratory experiments and intensive monitoring of sediment-laden corals in Australia’s Great Barrier Reef.

Their experiments revealed a deadly chain reaction that they spell out in a recent report in the Proceedings of the National Academy of Sciences:

Phase 1: A thin layer of sediment smothers the corals.

Blocked off from the light, symbiotic algae living in the corals will stop producing the oxygen the corals need—even if the sediment layer is only two millimeters thick.

Phase 2: Lack of oxygen and acidification damage small areas of coral tissue irreversibly.

If the sediments contain even a smidge of organic matter, microorganisms start decomposing it right away, quickly reducing oxygen concentrations beneath the sediment film to zero.

Acidification is an immediate problem as well: Other microbes digest larger carbon compounds via fermentation and hydrolysis, both of which lower the ambient pH.

“We were amazed that a mere 1% organic matter in the sediments is enough to trigger this process,” Weber said in a press release.

Phase 3: Hydrogen sulfide kills the remaining cells.

A third type of microbe begins digesting the dead patches of coral tissue, producing hydrogen sulfide, a compound that is highly toxic for the remaining corals. The process gains momentum and kills the remainder of the sediment-covered coral surface in less than 24 hours.

ANALYSIS: What’s the Coral Sea Without Coral?

Such were probably the three phases of death for this section of reef off the coast of North Queensland, Australia:

That’s the harsh reality, folks. All the more reason to promote strong management practices and regulations that minimize the nutrient pollution—topsoil, fertilizer, animal waste and sewage—that washes into tropical coastal seas.

PHOTOS: The Unexpected Beauty of Coral Crustaceans


Top: Intact reef located in coastal waters of the far north of Australia’s Great Barrier Reef. Natural vegetation dominates the nearby land.

Bottom: An impacted reef close to an urbanized coastal area in North Queensland, Australia.


Miriam Weber/HYDRA Institute/Max Planck Institute for Marine Microbiology, Bremen,