Ecological virgin land in the Red Sea: Of sweet jellyfish and nutritious mucus trails

In four several-week-long expeditions in as many years, LMU researcher Dr. Christian Wild and his work group CORE (Coral Reef Ecology; www.palmuc.de/core) of from the GeoBio-Center and the Department of Earth and Environmental Science studied key biogeochemical processes in coral reefs in the Northern Red Sea - entering an underwater landscape that has never been researched at such length before. "There were huge gaps in biogeochemical knowledge, because hardly any research of this kind had been conducted before," says the geobiologist. "Also, the coral reefs in the Red Sea are the most common type of reef in the world, the fringing reef. So, the findings we made, and have published in several current publications, apply equally to many other reef systems." Among other things, Wild's team has shown that different marine organisms covering the seabed have a different influence on oxygen availability in the coral reef. This can be bad for the oxygen-sensitive reef inhabitants, as a follow-up study has revealed. "In the Red Sea, we also discovered an unknown material cycle that keeps trace elements such as nitrogen, phosphorus and iron within the very nutrient-poor ecosystem," reports Wild. "We also showed that the upside-down mangrove jellyfish Cassiopea - which is also prevalent in coral reefs and lies on the seabed with its bell down and its tentacles pointing upwards - releases relatively large quantities of sugars and mucus, which serve as food for some other reef organisms and thus bring about an energy flow from the seafloor into the water column above coral reefs that has never been characterized before."

Over the last four years, Wild and his team have spent several months in and around the Northern Red Sea researching the little-investigated biogeochemical processes in the coral reefs. The most prevalent reefs here are fringing reefs, which grow parallel and close to the coast and often stretch many miles along the shoreline of the mainland. Fringing reefs are the most common type of reef in the world, occurring not only in the Red Sea, but also in the Indian Ocean, South East Asia and the Caribbean. "So, our results can also be applied to many other reefs," Wild emphasizes. The most useful results were obtained from so-called in-situ data loggers. These watertight sensors, each equipped with its own microchip, measure certain parameters in the seawater at short intervals and store this information in memory. After a set period, the loggers are taken back on land and read out, thereby delivering high-resolution data directly from the field.

The measurements from these devices have revealed, for example, that different marine organisms covering the seafloor - the benthic community - have a different influence on the oxygen availability in the coral reef. The more algae are present in a coral reef, the lower are the mean oxygen concentrations in the water directly above that reef. This is probably because the reef algae release larger amounts of sugars and other solute organic substances. These organic nutrients are very quickly broken down by microorganisms, which leads to a reduction in oxygen concentration. "Our supposition that this could negatively affect corals and other oxygen-sensitive reef organisms was in fact confirmed in a follow-up study," Wild reports. "We showed that corals are particularly damaged if they are in direct contact with reef algae."

Yet the corals themselves produce their own organic material: It turned out, for example, that all dominant stony corals release mucus into their surrounding water, which then remains attached to the surface of the corals for anything up to 80 minutes. "We know from our comparative studies at the Australian Great Barrier Reef that this is a very long time," says Wild. The sticky mucus threads - much like fly paper - collect tiny particles such as algae fragments, zooplankton and tiny sand grains from the water column, and become very heavy as a result. When the enriched mucus threads finally break off, they sink down to the seabed nearby - no more than a few meters away - where they are quickly broken down by microorganisms in the water column and reef sands. "This releases regenerated nutrients such as nitrogen, phosphorus and iron, which are then very quickly taken up again by photosynthesis-driven organisms, the so-called primary producers. This keeps these trace elements within the extremely nutrient-poor coral reef ecosystem." says Wild. "With this study, we have characterized a previously unknown, tightly closed material cycle that revolves around the release of mucus by corals in the Red Sea."

Finally, the jellyfish Cassiopea is the key to yet another link between the nutrient chains in the reef, which the researchers have newly detected. This mangrove-dwelling jellyfish is also often found in coral reefs, and lies "upside down" on the ocean floor, with its bell at the bottom and its tentacles pointing up. The sugars and mucus released by Cassiopea are taken up by microorganisms and small mysid shrimps, as Wild's team revealed in experiments using stable isotopes as markers. Their study revealed a previously uncharacterized flow of energy from the seafloor to the water column in the coral reef ecosystem, and once again draws our attention to how complex the interdependencies are in such habitats.

"We now also want to understand the chemical composition and the dynamics of organic material release by reef algae in the Red Sea," Wild continues. "We are most interested in working out how the availability of inorganic nutrients such as nitrates and phosphates, correlates with sugar release by algae and a subsequent stimulation of microbial activity. What we see now is, namely, that mass tourism and mariculture are forcing ever more fertilizers into the coral reefs near the coasts, which promote not only the growth but also the metabolism of reef algae, and that this could lead to a phase shift for several reasons. That is the transition from coral-dominated to algae-dominated reefs, which has already been reported from many other coral reefs. (suwe)

Publications:
"Benthic community composition affects O2 availability and variability in a Northern Red Sea fringing reef", Niggl W, Haas AF, Wild C
Cover article in Hydrobiologia

Contact: Dr. Christian Wild
Phone: +49 (0) 89 / 2180 - 6706
Fax: +49 (0) 89 / 2180 - 6601
E-mail: c.wild@lrz.uni-muenchen.de