(CORDIS) — Coral reefs, diverse ecosystems often described as ‘rainforests of the sea’, are feeling the effects of human activity, and the prognosis is not good. A new study led by the Max Planck Institute for Marine Microbiology in Germany has found that industrialisation, deforestation and intensive farming in coastal areas are playing havoc with the conditions of life below the surface. The findings show that oxygen depletion and acidification of the environment trigger a chain reaction that results in coral death.
Coral reefs are found in tropical shallow coastal regions on both sides of the equator. Coral polyps build the carbonate skeletons that create the fascinating, colourful reefs over hundreds to thousands of years. Photosynthesis of the symbiotic algae inside the polyps generates oxygen and carbohydrates from carbon dioxide and water. This in turn makes polyps grow.
Scientists have been studying the process of coral bleaching for the last 30 years. They have observed higher temperatures that induce the algae to produce toxins. Polyps, for their part, act by forcing out the algae. The result? The coral reefs lose colour, and end up looking like they’ve been soaked in bleach. Corals can only survive for a few weeks if symbiosis – a close link between two or more different organisms of different species that may benefit each member – is absent.
‘Our idea was that a combination of enhanced deposition of sediments with elevated organic matter load and naturally occurring microorganisms can cause the sudden coral death,’ said Miriam Weber of the Max Planck Institute for Marine Microbiology. ‘To get a handle on the diverse physical, chemical and biological parameters, we performed our experiments at the Australian Institute of Marine Science (AIMS) in Townsville [on the north-eastern coast of Australia, adjacent to the central section of the Great Barrier Reef] under controlled conditions in large containers (mesocosms), mimicking the natural habitat.’
The researchers discovered that in the first phase, light is blocked and algae will stop photosynthesis when a two-millimetre layer of sediment enriched with organic compounds covers the corals. In the second phase, organically enriched sediments lead to the digestion of the organic material by microbial activity, which in turn diminishes the concentration of oxygen underneath the sediment film to zero. The pH drops when other microbes take over digesting larger carbon compounds via fermentation and hydrolysis. In the third phase, coral tissue is irreversibly harmed by the lack of oxygen and acidic conditions. Microbes absorb the dead material, generating hydrogen sulphide, a highly toxic compound. Any coral that has not yet been wiped out, will be, within 24 hours after this process unfolds.
‘First we thought that the toxic hydrogen sulphide is the first killer, but after intensive studies in the lab and mathematical modelling we could demonstrate that the organic enrichment is the proximal cause, as it leads to lack of oxygen and acidification, kicking the corals out of their natural balance,’ Dr Weber explained.
‘Hydrogen sulphide just speeds up the spreading of the damage. We were amazed that a mere 1% organic matter in the sediments is enough to trigger this process. The extreme effect of the combination of oxygen depletion and acidification are of importance, keeping in mind the increasing acidification of the oceans. If we want to stop this destruction, we need some political sanctions to protect coral reefs.’
Experts from Australia and Italy contributed to this study.