Discovering Archaea Associated With Corals
[Christina Kellogg]

Many studies have shown that coral mucus hosts a diverse bacterial community. Archaea (formerly called archaebacteria) are the third domain of life. Superficially, they look like bacteria, but genetically and biochemically, they are very different. For years, archaea were found only in extreme environments, such as hot springs or deep-sea hydrothermal vents. More recent studies, however, have uncovered archaea in temperate coastal waters. Archaea have also been detected in marine sediment and as symbionts in sponges. Archaea remain difficult to cultivate in the laboratory, so most current research is based on detection of archaeal DNA sequences, which are used to identify different types of archaea.

Left: The molecular "Tree of Life" consists of three domains derived from 16S rDNA genetic data. 16S rDNA is the gene that codes for ribosomal RNA, a key part of cellular reproduction. Eukarya includes plants, animals, and fungi. [larger version]

Above: Sequencing means using a chemical reaction to determine the order of the four nucleotide bases (A, G, C, T) that make up DNA. Each "species" of microbe will have a unique combination of bases, sort of like a bar code. In this example of a DNA sequence chromatogram, each of the four nucleotide bases is coded in a different color. [larger version]

To determine if there were archaea associated with tropical corals, mucus was collected from three species of reefbuilding 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, purified, and analyzed by molecular techniques. The resulting archaeal 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 related to archaea previously detected in marine waters. The remaining sequences were similar to archaea detected in marine sediment and hydrothermal vents. A few sequences were distinctive enough to be considered novel. 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. Finding archaea, with their varied (and, for the uncultivated specimens, mostly unknown) metabolic capabilities, adds to the likelihood of unique biogeochemical processes occurring in the microbial communities associated with corals. This work was done in cooperation with the Virgin Islands National Park and was published in the journal Marine Ecology Progress Series (Kellogg, 2004) article "Tropical Archaea: diversity associated with the surface microlayer of corals."

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