Microbes linked to coral reef decay

Micro-organisms thrive in environments ranging from ocean abysses and polar ice caps to thermal springs. They can adapt to all kinds of habitat owing to their metabolic versatility. Under favourable conditions such as warm waters microbes propagate, building monumental edifices – microbialites – of organic and sedimentary material in a variety of shapes such as veils, domes, pompons or mats. Microbialites have existed on the Earth’s surface for 3.5 billion years and are believed to have played a major role in forming the planet’s current atmosphere.

Scientists can’t fully explain the surge in microbialites observed over the last 20 years, but have linked the edifices with the creeping decay of coral reefs. This trend, now seen in most regions of the world, is causing great concern for one because microbial structures grow rapidly and some of the cyanobacterial species involved are potentially toxic.

The death of coral reefs has been linked to climate change (see related story), because reefs only thrive within a narrow temperature range, but their decline could also be due to other natural or human activities. Scientists are now trying to establish clearer cause-and-effect mechanisms to determine whether microbialites play a part in, or would make good indicators of, climatic disturbances in reef systems.

An international consortium of scientists, including researchers at New Caledonia’s Institute for Developmental Research (IRD), are working on the identification of microbial communities and the environmental factors that are causing them to spread.

Microbialites have been found to be sensitive to seasonal variations, developing more rapidly during the hot humid season. In the reef systems of New Caledonia and French Polynesia, a rise in sea surface temperature stimulates their growth. If climate change is indeed warming ocean waters, this could explain the spread of these unusual structures.

Research is also focussing on the role played by microbialities in the biogeochemical cycles of reef systems. A team of scientists have been observing changes in New Caledonian reefs, where microbialites have been built directly on coral regardless of whether the coral is alive or dead, resulting in a decreased availability of light and minerals. In some cases the researchers have found that where microbialites have developed, necrosis of coral colonies has occurred, suggesting these structures could be responsible for the irreversible degeneration of the reefs.

Whether or not microbial domes are directly responsible for coral decay, scientists warn that the volume of microbialites intruding into coral reefs has increased noticeably in the past few years, making it imperative to put in place surveillance networks.