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Coral Mucus Goes Mainstream—New Discoveries in Mucus-Hosted Microbial Communities

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Diploria labyrinthiformis
Above: Diploria labyrinthiformis, a type of brain coral, was one of the three reef-building species analyzed for coral-associated archaea.

Coral mucus (or "coral snot," to the more irreverent) is just what it sounds like—mucus that corals secrete. Hard corals, soft corals, tropical corals, and deep-water corals (and some other related organisms) all produce mucus. Corals create a mucus layer around them and use it like a slipcover; when too much dirt (sediment) collects on the mucus layer, the coral sloughs it off and makes a new one. Some corals also use the sticky mucus to catch food. The mucus is also loaded with microbes, not unlike our skin.

Who cares about coral mucus? More people than you would think. The cover story of the March 4, 2004, issue of Nature was a paper that quantified the mucus some corals release and calculated its contribution to the nutrient and energy budget of the reef environment.

It has been known for several decades that coral mucus hosts a concentrated bacterial community relative to the overlying seawater. Microbial-ecology studies continue to examine the microbial communities of corals in an effort to better understand the whole organism (the sum of the coral animal, the photosynthetic algal symbiont, and the associated microbes). It is imperative to understand the relations between microorganisms and healthy corals in order to tease out what goes wrong and allows or causes coral diseases.

The microbial ecology of corals is just beginning to be characterized. It has been speculated that coral-associated bacteria benefit the coral by fixing nitrogen, breaking down waste products, cycling basic nutrients back to the zooxanthellae (photosynthetic algal symbionts), and warding off other potentially harmful microbes by producing antibiotics or just occupying the available space. Changes in the bacterial communities in coral mucus echo changes in the health of the coral, suggesting that bacteria are at least tuned to their host's metabolism and at most may have an active role in maintaining the overall health of the organism.

Tree of life
Above - Tree of Life: The molecular biologist’s “Tree of Life” is constructed by comparing sequences of a universal gene (for example, the ribosomal RNA gene, which is present in all forms of life, because its product is essential to cellular function). The root of the tree is a presumed common ancestor; the branches are the three domains: Bacteria, Archaea, and Eukarya (which includes fungi, plants, animals, and us!).

Knowing that coral mucus hosts a diverse bacterial community, I wondered whether it also contained archaea. Archaea (formerly called archaebacteria) are the third domain of life. Superficially, they look like bacteria, but on a genetic level, they are as different from bacteria as we are. For years, archaea were found only in extreme environments, such as hot springs or deep-sea hydrothermal vents. More recent molecular-biological studies, however, have uncovered archaea in plain sight; in temperate coastal waters, archaea can account for as much as 30 percent of the microbial plankton. Archaea have also been detected in marine sediment and as symbionts in sponges. Archaea remain difficult to cultivate in the laboratory, however, and so most current research is based on detection of archaeal DNA sequences, which are used to identify different types of archaea.

In July 2002, I collected mucus from three species of reef-building corals in the U.S. Virgin Islands: Montastraea annularis complex, Diploria strigosa, and Diploria labyrinthiformis. Total microbial DNA was extracted from the coral-mucus samples, and polymerase chain reaction (PCR) was performed with archaeal-specific primers. The resulting DNA fragments were sequenced and identified by comparison to known sequences in GenBank, an international repository for sequence information. A diverse archaeal community was detected in each of the mucus samples. Most of the coral-associated archaeal sequences were most closely matched to archaea previously detected in marine water-column samples; the remaining sequences were most similar to those of archaea detected in anoxic sediment and hydrothermal vents. Some archaea that have rarely been detected were identified from the coral mucus, as well as a few sequences that were different enough to be considered novel archaeal types. Unlike previous bacterial studies, which found specific associations between certain bacterial and coral species, this study found that archaea seem to be generalists, with the main types detected being observed in all three coral species tested.

map of the Caribbean
Above: Coral mucus samples were collected from a reef in Hawksnest Bay, St. John, inside the Virgin Islands National Park.

This work was one of two simultaneous, independent studies to detect the presence of archaea associated with tropical reef-building corals. The detection of archaea, with their varied (and, for the uncultivated specimens, mostly unknown) metabolic capabilities, adds to the likelihood of unique biogeochemical processes. These processes and their hosts must be identified in order to integrate microbial functions with the biology of the coral animal and algal symbiont and begin to study the ecology of the synergistic system as a whole. The paper describing this work was published on June 8 in the journal Marine Ecology Progress Series (v. 273, p. 81-88).

Related Sound Waves Stories
Diving into Coral Disease
September 2003
African Dust Microbiology in the Caribbean
September 2002

Related Web Sites
St. Petersburg Science Center
U.S. Geological Survey (USGS)
Nature Highlights - March 4, 2004
Nature Publishing Group
National Center for Biotechnology Information

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in this issue: Fieldwork
cover story:
Benthic Habitats in Glacier Bay

North Carolina Submarine Groundwater

Mucus-Hosted Microbial Communities

Gulf of Maine Mapping Initiative

Forensic Geology Assists Investigation

Submarine Groundwater Discharge

USGS Participates in Marine Quest X

Caribbean Tsunami Hazard Workshop

USGS wins Blue Pencil, Gold Screen Awards

June Publications List

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