Human pathogen can cause coral disease

Despite the resilience of corals as a taxonomic group through geologic time, warming oceans, shifting seawater chemistry, overfishing, pollution, and disease currently threaten these habitat-building invertebrates with many coral reef ecosystems in a state of decline.  Researchers have identified a bacterium, Serratia marcescens as the cause of a disease called white pox in elkhorn coral (Acropora palmata).  White pox, more formally known as acroporid serratiosis, can lead to tissue loss and potentially the death of the coral colony.  What makes this especially interesting is that S. marcescens normally causes health troubles in humans–this is the first evidence of a human pathogen to a marine invertebrate.  Acropora palmata was once the dominant coral in the Caribbean, especially in the forereef and reef crest, shallow spots with high wave action.  Today, populations of this coral species has been decimated, reduced by up to 95% in abundance since 1980, and is now considered critically endangered by the IUCN.  Much of this decline is attributable to disease, along with other factors that compound this plight–for example, this species is particularly vulnerable to bleaching.

Previous work done in 2003 noted that S. marcescens was found in both untreated human waster and within A. palmata suffering from white pox, suggesting a relationship between the two.  In a new paper published this week in PLoS ONE, Dr Katheryn Sutherland and colleagues used Koch’s postulates, a standard method for showing disease causation, to  investigate the relationship between the two.  In short, fulfilling these postulates requires researchers to be able to isolate the suspected pathogen (S. marcescens) from the host coral and grown up in culture, the disease to manifest itself when a pure culture of the pathogen is introduced to the host, and isolated yet again from the experimentally-infected host (more on Koch’s postulates here).  The results show that S. marcescens is capable of causing white pox in this coral species speedily, with the coral losing tissue in as little as four days (see figure below).

While this disease is specific to this particular coral, the researchers also found that other coral species could possibly be acting as reservoirs for this pathogen while in seawater, given that the pathogen itself is not adapted well for life in the ocean.  Additionally, a coral predator, a snail, may act as a disease vector or reserve.

Improving wastewater containment and treatment in areas such as the Florida Keys can reduce this pathogen’s transmission, and efforts are ongoing in Florida to improve wastewater management, though this issue is occurring in the wider Caribbean as well.  This study shows an exception to the usual animal-to-human transmission model, but also that this pathogen, found in land-based mammals (us), can cause a disease in a marine invertebrate, jumping not only into a profoundly different environment but also into a much different animal, a colonial invertebrate rather than a vertebrate.  Responding to this issue would be obviously beneficial to corals themselves, but also to human health and for the economies that depend on reef habitats for tourism and resources.  The dynamics of this disease are yet another example that illustrate the interconnectivity of society, ecosystems, and economics.

 Figure:  Sutherland et al. 2011 (CC 2.5)

Sutherland, K., Shaban, S., Joyner, J., Porter, J., & Lipp, E. (2011). Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023468

Smaller corals potentially more resilient

“Professor Peter Mumby and Dr Laith Yakob from the University of Queensland report on their findings this week in the Proceedings of the National Academy of Sciences that small short lived corals which are taking over from large corals in some parts of the world are more resistant to disease.”

The authors warn that this finding, while seeming like a positive thing, actually has negative implications for the life coral reefs support.  Smaller corals mean less complexity, meaning less fish and associated invertebrates.

via Smaller corals take the heat › News in Science (ABC Science).

Plight of Staghorn Coral

Staghorn coral (Acropora cervicornis). Wikipedia Commons, photo taken by Alessandro Donà in Bonaire, 2007. Creative Commons Attribution ShareAlike 3.0 License

With any luck, we’ll be discussing a specific anthozoan (or maybe groups of anthozoans) each week.  Anthozoa is a class of marine organisms within the phylum Cnidaria, and consists of corals, anemones, and sea pens.  There are over 6000 anthozoans (that we know of—likely lots more, particularly in the hugely undersampled deep-sea), and most Cnidarian species in existence today are within this class.

Acropora cervicornis (staghorn coral) is a scleractinian coral species—meaning it is a reef-building coral that forms a calcium carbonate skeleton in the form of aragonite (for more on aragonite and ocean acidification, you can go here)—that occurs in pretty much all of the greater Caribbean.

A. cervicornis is among the faster growing corals in the Caribbean and provides reef framework that adds to habitat complexity on the larger coral reef ecosystem—helping to give habitat to all sorts of organisms found on reefs, including other invertebrates and reef fish.  But it’s probably not accurate to say that A. cervicornis is a major reef-builder throughout the Caribbean presently.  This species is listed by the International Union for Conservation of Nature as critically endangered.  Populations of this coral have declined over 80% in the past 30 years; the main culprit causing this enormous die-off is white-band disease (WBD).  While the cause is unknown, WBD causes tissue decay, eventually peeling away from the skeleton on afflicted colonies.  This exposed skeleton can be rapidly colonized by algae in short order, potentially causing other problems for the coral.  But not all A. cervicornis colonies are affected by WBD, with research indicating that 6% of  staghorn coral genotypes are resistant.  Larger mechanisms are undermining this and other corals worldwide as well:  altered ocean chemistry, increased thermal stresses via climate change and more intense El Nino/Southern Oscillation events, sedimentation, dominance of fleshy macroalgae due to grazer die-offs or overfishing…I could go on, but I think you get the picture.

A. cervicornis can reproduce asexually or sexually.  Asexual reproduction via fragmentation allows local propagation—this is one reason why hurricanes and other storms are really an integral facet of coral reef spatial ecology.  Storms and other mechanical stressors provide a means for branching corals to fragment and thus reach other local areas.  Despite this, reefs may have lower resilience due to anthropogenic stress and may not be able to recover from storms as quickly as they once were able.   Sexual reproduction through broadcasting larvae is needed for dispersal across any real distance and for the maintenance of genetic diversity.  A recent study published in the Public Library of Science (Hemond and Vollmer 2010) looked into the genetic connectivity of A. cervicornis in Florida and discovered a potential future genetic bottleneck.  The investigators, using mitochondrial DNA sequences, found that the A. cervicornis population in Florida was genetically diverse—the good news—but may be isolated from larval inputs from other populations in the Caribbean.   Acropora species within the Caribbean are known to have restricted gene flow and thus, reduced connectivity, among populations that are farther away than 500 km from each other.  However, these recent results indicate that the Florida Keys population appears to be isolated from even the Bahamas (<200 km), the Gulf Stream possibly acting as a dispersal barrier (remember, larval dispersal is dependent upon oceanographic conditions like currents).  In the here and now, these findings indicate the dependence of A. cervicornis in Florida on self-recruitment; conservation programs are called for in order to manage these populations as separate ‘unit’.  There is another, longer-term consequence of this lack of larval input.  Disease has reduced populations to a fraction of what they once were, and even with Florida’s relatively high diversity within this coral population, genetic drift may produce a future bottleneck, potentially putting the genetic diversity of this population at risk.

The following references and those cited therein were drawn upon for this post.  They would serve as nice starting points for more information.

Aronson, R., Bruckner, A., Moore, J., Precht, B. & E. Weil 2008. Acropora cervicornis. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on 10 February 2010.

Fautin, Daphne G. and Sandra L. Romano. 2000. Anthozoa. Sea Anemones, Corals, Sea Pens. Version 03 October 2000. http://tolweb.org/Anthozoa/17634/2000.10.03 in The Tree of Life Web Project, http://tolweb.org/

Hemond, E., & Vollmer, S. (2010). Genetic Diversity and Connectivity in the Threatened Staghorn Coral (Acropora cervicornis) in Florida PLoS ONE, 5 (1) DOI: 10.1371/journal.pone.0008652

Humann P, DeLoach N. Reef Coral Identification:  Florida, Caribbean, Bahamas. 2nd Ed. New World Publications. Jacksonville, FL. 2002.

Vollmer SV, Kline DI, 2008 Natural Disease Resistance in Threatened Staghorn Corals. PLoS ONE 3(11): e3718. doi:10.1371/journal.pone.0003718