This is the second in a series of five referenced articles about shared characteristics between deep and shallow water corals
Special guest post by Christina A. Kellogg
Just as humans have beneficial bacteria living on our skin and in our intestines, corals have symbiotic microbes in their mucus, tissues, and skeletons. Unfortunately, there are also disease-causing microbes that can infect corals. These coral-associated microbes include all three of the major domains of life: Bacteria, Archaea, and Eukarya (fungi, and in shallow-water corals, algae) and also viruses (Rosenberg et al. 2007). Identifying and characterizing these coral-associated microbes is a relatively new field, in which most of the key information has been generated within the past decade and mainly from shallow-water species. So who are these coral-associated microbes and what do they do?
Algae: The best known microbial associates of shallow-water corals are zooxanthellae--the symbiotic dinoflagellates that photosynthesize and provide energy to the coral animal in the form of carbon compounds. Zooxanthellae were originally considered a single species, but molecular techniques have revealed that there are multiple genetically diverse groups that have different distributions, host-specificity, and stress tolerances (Reviewed in Baker 2003). It is generally believed that this symbiosis increases the coral's rate of calcification. However, there are azooxanthellate shallow-water corals (Marshall 1996) and deep-water corals (Bell & Smith 1999) with comparable calcification rates, which suggest that development of coral reefs may not be as tightly linked to light-enhanced calcification as previously thought. Moreover, it raises the question of how azooxanthellate corals (both shallow and deep) compensate without the dinoflagellates. Shallow-water corals may also contain endolithic algae. These algae bore into the coral skeleton, and can be seen as color bands in sectioned corals (see Fig 2). When a coral bleaches (i.e., loses its zooxanthellae due to thermal stress or disease), the endolithic algae can provide some nutrients to support the coral until it can reestablish its zooxanthellae (Fine & Loya 2002).
Bacteria: Multiple studies have shown that corals host complex and diverse bacterial communities that are distinct from those in the water column: Examples of shallow-water studies include Ritchie & Smith 1995, Rohwer et al. 2001, Frias-Lopez et al. 2002, Bourne & Munn 2005, and there are two recent deep-water studies, Penn et al. 2006 and Yakimov et al. 2006. It has also become clear that different species of corals have different bacterial communities (Rohwer et al. 2002) and that there is evidence of specific bacteria-coral interactions; where a particular bacterial species (represented by a unique 16S rRNA gene sequence) or group of species have been repeatedly found in association with multiple individuals within a coral species (Shallow-water: Rohwer et al. 2001, Bourne & Munn 2005, Webster & Bourne 2007; Deep-water: Kellogg 2007). The presence of coral species-specific bacteria makes it clear that these interactions are not random or passive.
Our understanding of the specific roles filled by coral-associated bacteria is just beginning, but it has been speculated that coral-associated bacteria benefit the coral by fixing nitrogen, breaking down waste products, and (in shallow-water corals) cycling basic nutrients back to the zooxanthellae (Shashar et al. 1994, Rohwer et al. 2001, Lesser et al. 2004). Bacteria may also ward off other potentially harmful microbes by producing antibiotics or just by occupying the available space (Dobretsov & Qian 2004, Ritchie 2006). Coral-associated bacterial populations are closely attuned to host metabolism and may change in number or composition in response to a change in coral health (Ducklow & Mitchell 1979, Pantos et al. 2003).
Archaea: Archaea are associated with shallow-water corals, but their functions remain unknown (Kellogg 2004, Wegley et al. 2004). A recent study found archaeal ammonia monooxygenase gene expression associated with five species of corals (Beman et al. 2007). This gene encodes the first step of a pathway that converts ammonia to nitrite or nitrate, so these archaea may be removing host waste products like ammonia and urea. Attempts have been made to detect archaea in deep-sea corals, but have not yet been successful (Yakimov et al. 2006, Kellogg 2007).
Fungi: Fungi are found in both shallow and deep coral species, but it seems that they are pathogens or part of the microbial complex that alters the coral after death (Freiwald et al. 1997, Bentis et al. 2000, Priess et al. 2000, Ravindran et al. 2001).
Scientists are interested in these microbial communities not only because they are integral to coral biology and ecology, but also because their study contributes to our knowledge of microbial diversity and biogeography. Corals are microbial landscapes hosting an amazing diversity of life (Knowlton & Rohwer 2003). While shallow-water corals are characterized by their photosynthetic algal symbionts, bacteria and archaea are beginning to be seen as symbionts in their own right. It remains to be determined whether bacteria and archaea play an even larger role in deep-sea corals to offset the lack of photosynthetic partners.
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Chris, a nice post. I'm looking forward to seeing you in July at ICRS - it should be a fun meeting (and our first coral outing!).
Thouhts on why you might not be finding Archaea yet in deep-water corals? Curious.
"Thoughts on why you might not be finding Archaea yet in in deep-water corals?"
I think it has to be a methodological shortcoming--either there is an inhibition of the PCR, or the species of Archaea are just different enough that the 'standard' archaeal primer sets aren't picking them up. As abundant as Archaea are in deep-sea sediments and the water column, and knowing they are present in shallow-water corals, I can't believe they aren't there...