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| USGS Coral Microbial-Ecology Studies: |
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Introduction
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Above: 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: A coral reef in the south Pacific. Photo credit: Christina Kellogg, USGS. [larger version]
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Coral microbial ecology is the study of the relationship of coral-associated microorganisms to each other, the coral host, and to their environment. Just as we humans have beneficial bacteria living on our skin and in our intestines, corals also have co-habitating non-pathogenic (not disease-causing) microbes. These microbes include bacteria, archaea, and fungi—representing all three of the major domains of life. Archaea are prokaryotes (cells with no nucleus) like bacteria but are genetically and biochemically more similar to eukaryotes (organisms with cells that contain a true nucleus).
Where are these microbes? Coral-associated microorganisms are found in the
mucus, tissue, and skeleton of corals. Corals secrete a surface layer of mucus,
which functions to keep the coral surface clean of sediment and to capture prey.
Each species of coral has mucus that is biochemically unique. When shed, this
mucus provides a major nutrient source for the reef environment (Wild, 2004).
The tissue of the coral animal is where the symbiotic algae, known as zooxanthellae,
live. Endolithic microorganisms (bacteria, fungi, or algae that bore into the coral's
calcium carbonate skeleton) are a potential source of nutrients to the overlying tissues.
What can we learn from studying coral microbial ecology? Why is it important? Coral reefs in the Caribbean have been in decline for several decades. One of the most visible causes is disease. Coral diseases are not well understood, but the few that have been characterized are all caused by microorganisms. In order to understand the diseased state, we must first understand the healthy state. Until now, coral biology studies have been limited to studies of the coral animal, the algal symbiont, or the interactions of the two. Coral microbial-ecology studies contribute a missing piece of information to the study of overall coral biology. It has been speculated that coral-associated bacteria benefit the coral by fixing nitrogen, breaking down waste products, and cycling basic nutrients back to the photosynthetic algal symbionts (zooxanthellae). Bacteria may also ward off other potentially harmful microbes by producing antibiotics or by just occupying the available space. Relative to disease studies, it would be helpful to know if the infectious agents come from the normal microbial flora under certain conditions.
The field of coral microbial ecology is
relatively new. What do we know so far?
- A handful of coral pathogens (microbes
that are proven to cause a specific disease)
have been identified (reviewed by
Rosenberg and Loya, 2004).
- Coral-associated microbes are distinct
from those in the water column (Frias-Lopez, 2002).
- Different species of corals have different
bacterial communities (Rohwer,
2002).
- The coral-associated bacterial community
shifts when the coral is stressed
(Pantos, 2003).
continue to Global Climate Change - Microbial Communities as a Diagnostic Tool?
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