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

Ove Hoegh-Guldberg speaks on climate change in the ocean

From the 2011 National Council for Science and the Environment conference in Washington, D.C.  You can find out more about Dr Hoegh-Guldberg at his laboratory site.

Video:  John Bruno on Vimeo (cc).  Via SeaMonster.

The future of shallow-sea coral reefs

Coral reefs, in case you haven’t been keeping up, are increasingly threatened by overfishing, ocean acidification, and warming, among other human-derived factors.  Recently, much buzz has been created on the web (e.g., here, here, and here) in the wake of an updated assessment from the World Resource Institute and its partners concerning the global status of shallow-sea coral reefs.

I confess I have not had the chance to dive into the full report, but here are some key snippets from the  executive summary:

  • 60+ % of the world’s coral reefs are threatened by local sources, such as overfishing, destructive fishing methods and pollution
  • Overfishing is the “worst immediate threat,” affecting 55% of reefs.  Coastal development and land-based pollution, and maritime pollution and damage from vessels come in second and third, affecting 25% and 10% of reefs, respectively.
  • 75% of shallow-sea coral reefs are threatened by the aggregation of local and thermal stresses.
  • Despite that 25% of reefs are within protected areas, many of these are inadequate or only partial protected.

The report also offers map-based information for the current status of these ecosystems and predictions for 2030 and 2050, integrating both local and global threats.  WRI offers KLM files of these assessments/probable future scenarios here, which I quickly threw into Google Earth and centered the maps over southeast Asia, generally acknowledged to the ‘epicenter of marine diversity’.  Click on the images to embiggen; the cool-colored dots represent sites with low risk and the red – maroon dots represent sites that are increasingly threated1.

Today

 

2030


2050


Visit the World Resources Institute to check out the report, the summary, map files, and lots more (cc 3.0).  The executive summary also has a nice list of ways you can help.

 

 

1 If anyone knows how to import .kmz files into the far more figure-friendly Google Maps without wading into the land of ESRI and QGIS, and is willing to share, that would be excellent.

 

 


Lunar cycles and reproduction in the deep sea

Some biological patterns in marine species, particularly concerning reproduction, are related to the moon.  Shallow-ocean corals, for example, undergo mass spawning events (the synchronous release of eggs and sperm into the water column to combine), the timing of which, are set to the lunar clock.  Reef fishes, shallow-ocean echinoderms, mollusks and more, also time spawning events in respect to the phase of the moon.

The deep-sea, the largest biome on Earth, covering more than 326 million km2, has not been explored in terms of this lunar-synchronicity.  The dearth of photosynthetically-useful sunlight below 200 meters* would appear to make such moonlight-related cycles unlikely at best.

However, in a recent paper, Annie Mercier and her colleagues have shown that this may not be the case.  They demonstrate in both lab and field settings evidence of lunar periodicity in the reproduction of 6 deep-sea species, containing members from two different phyla:  Cnidaria and Echinodermata.

The researchers examined preserved samples of Phormosoma placenta (a deep-sea echinoderm) collected at various stages of the moon.  They found that despite being collected from between 700 – 1400 meters beneath the waves, physiological signs of recent spawning in both sexes coincided with the new moon.

Back in the lab, gamete and larval releases (reproduction events) were observed in captive specimens from 5 different species according to lunar patterns.  These specimens were collected between 100-1000 meters, with most species collected below 400 meters.  A minimum of 3 lunar months’ worth of data was compiled; some species actually repeated breeding periods in this timeframe.

The question remains if these animals are displaying internal rhythms that are kept in time by some sort of lunar cue, or if they are responding to something externally that follows the lunar period.  But what cues, or drivers, of a lunar period could be detectable at such great depths, where even sunlight wanes or is essentially eliminated?

Organic matter from surface waters falls into the deep sea; there is the possibility that these fluxes of sustenance may show lunar patterns.  Previous work has shown growth bands in some species of deep-sea corals that may correspond to monthly or lunar periods.  Other hypotheses include the idea that these animals can somehow directly perceive moonlight at great depths, or that deep tidal (related to lunar phase) currents exist.

In this study, internally brooding corals released larvae during the full or during the waning phase.  The 4 free-spawning species released gametes with the new moon.  The authors note that while this is opposite to the mass spawning events in shallow-ocean corals, which release during the full-moon, this may be due to the very different environmental and biotic factors in shallow areas versus the deep sea.

 

* This is the reason that little to no primary production occurs (that is, organisms producing chemical energy) in most, but not all, ecosystems known in the deep-sea.  Some deep-sea organisms are capable of undergoing chemosynthesis and can use inorganic chemicals, rather than sunlight as in photosynthesis, as an energy source.  However, even with a widespread lack of primary productivity and severe food limitation in most areas, diversity in the deep sea is among the highest on the planet.

Image:  Flicker user ZedZap (cc 2.0)

Sources:
ResearchBlogging.orgMercier A, Sun Z, Baillon S, & Hamel JF (2011). Lunar rhythms in the deep sea: evidence from the reproductive periodicity of several marine invertebrates. Journal of biological rhythms, 26 (1), 82-6 PMID: 21252369
Ramirez-Llodra E, et al. (2010). Deep, diverse and definitely different: unique attributes of the world’s largest ecosystem Biogeosciences, 7 (9), 2851-2899 : 10.5194/bgd-7-2361-2010

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).

Boom: the destruction and rebirth of a marine ecosystem.

In 1883, the world shuddered as the loudest known sound in human history echoed from its epicenter in Indonesia.  The noise generated by Krakatoa, a volcanic island in the Sunda Strait, was heard over 3,000 kilometers aways both to the east and west; the resulting tsuanamis produced waves over 30 meters in height and killed 36 thousand people.  The eruption altered weather globally. The volcanic dust suspended in the atmosphere halted a proportion of the solar radiation able to reach the Earth–in the year that followed global temperatures were reduced by as much as 1.2 degrees Celsius, not reverting back to normal until 1888.

From this volcanic destruction (two thirds of Krakatoa sank into the sea due to the blast), the island of Anak Krakatau-‘child of Krakatau’-was born, rising from Krakatoa’s crater less then 80 years ago.  This newly emerged island presents a unique opportunity to witness to the early stages of ecosystem development.

Eruptions on Anak Krakatau have been active recently, leading some to speculate whether or not another epic volcanic event is on the horizon.  Interestingly, fringing coral reefs have since formed on Anak Krakatau and nearby areas.  The recovery of the island’s terrestrial ecosystems has been subject to much attention and research.  However, the marine communities have largely been unstudied until the last decade.  In a work published this year in Coral Reefs, Dr. Starger and colleagues examined the genetic diversity in two reef-building corals, organisms within a marine ecosystem that was totally obliterated by the eruption in 1883.  Using microsatellites–short, repeating sequences of DNA–the researchers found that the genetic diversity in Pocillopora damicornis and Seriatopora hystrix, has largely recovered due to initial larval migration from other upstream sources.  Further analyses indicate that the coral populations within the Krakatau region may be self-recruiting at this point, and may even be providing larvae to other regions.

So these species of corals have recovered after an sudden, violent eruption.  What can we glean from this?  Coral reefs worldwide are in trouble, experiencing rapid decline, with mass mortality events projected.  In the face of such degradation, more data are needed for conservation, particularly in order to design effective marine protected areas (MPAs).  Connectivity is paramount in placing reserves.  Coral reefs can serve as larval sources, able to provide offspring to other areas, or sinks, which cannot.   The researchers show the repopulation of a destroyed marine ecosystem in the Java Sea from other sources, eventually becoming self-seeding.  This larval transport, especially in corals, may not permanently sustain reef populations over time, but can help to initally repopulate areas, for example, after a giant volcano pops its top.  Understanding the source-sink dynamics of these hugely-diverse ecosystems will likely be imperative in conservation planning.  Recovery of reef environments after disasters is dependent on healthy, nearby larval sources, giving importance to identifying and protecting these areas in networks of MPAs.

C. J. Starger, P. H. Barber, Ambariyanto, & A. C. Baker (2010). The recovery of coral genetic diversity in the Sunda Strait following the 1883 eruption of Krakatau Coral Reefs, 29, 547-565 : 10.1007/s00338-010-0609-2

Image 1:  Anak Krakatau. NASA EO-1 Team; Image 2:  Krakatoa, 1888 lithograph. Wikimedia Commons.

A cautiously-optimistic call to arms in coral reef science

Coral reef ecosystems are under assault from multiple stressors.  These range from local to global disturbances, some falling into the realm of naturally-induced. Some of these disturbances can be exacerbated through anthropogenic means.  Some of the most insidious threats to these ecosystems are completely human-induced, such as the current inextricable acidification of the global ocean.

There have been mechanisms proposed that corals could theoretically use to deal with a rapidly changing global environment, particularly a warming surface ocean.  Existing coral reef ecosystems are built around a symbiosis between the corals themselves and an intracellular photosynthetic dinoflagellate symbiont, within the genus Symbiodinium.  Shallow-sea corals are capable of capturing bits of organic matter and small plankton in the water column for subsidence, but these organisms garner most of their energy from their photosynthetic tenants.  Bleaching occurs when this symbiosis is compromised–one common mechanism is heat stress.  Other causes exist, but bleaching via heat stress is an increasing dominant phenomenon; stark bleaching predictions related to oceanic temperature anomalies were recently announced for most of the Caribbean.  Scleractinian corals (stony, hypercalcifying, reef-building corals) show variability in terms of which species bleach at certain thermal thresholds, as well as their geographic locations around the world.  This variability may be at least partially explained by the different symbiont types within the corals.  All types of coral symbionts are apparently not equal in how they can deal with warmer temperatures.  For example, evidence exists that type D symbionts are more abundant after severe bleaching episodes and the ensuing mortality1.  One proposed hypothesis states that  reef-building corals can switch out their existing internal symbionts in exchange for more heat-tolerant symbiont types.  A study in the Public Library of Science2 warns that this mechanism may not the panacea hoped for.  Colonies of a scleractinian coral, Porites divaricata, were bleached and then exposed to symbiont types not usually found associated with this species.  Some colonies were actually able to uptake these new symbionts–the first time that this environmental acquisition has been shown experimentally for scleractinian corals–however, this was an evanescent acquisition.  Only the original symbiont types proved to be stable during the bleaching recovery, implying that not all corals have the capacity to acclimate to a warming ocean by uptaking more heat-resistant symbionts.

However, a recent review of coral reef resilience3 by Dr Terry Hughes and collagues reveals guarded optimism for these heavily-impacted ecosystems, provided that action is taken.  In this context, resilience is the ability of an ecosystem to absorb and adapt to disturbances, without  ecosystem functions being profoundly altered.  These disturbances can vary in their rates of change; for instance, when long-term stressors are at a low level, reefs can be acted upon point (acute) disturbances of greater magnitude and speed and still recover.  The authors point out that natural dynamics are inherent in complex biological systems–Heron Island on the Great Barrier Reef loses almost all coral cover on a decadal basis due to cyclones, yet can recover due to their locally high resilience.  However, this is proving to be the exception, rather then the rule:  many reef systems have been pushed near the proverbial tipping-point, rending them ecologically unable to recover from acute, high-magnitude disturbances.  Hurricanes are enormously powerful disturbances, yet resilient reefs will recover relatively quickly.  However, even a gradual shift away from a coral-dominated ecosystem from long-term chronic impacts, say increased levels of pollution or sedimentation, can reduce resilience over time.  These types of local stressors can accumulate and produce community changes without acute, large disturbances.  Even some deep reef sites, below wave action and thus unaffected from hurricanes, have been gradually moved away from normally coral-dominated ecosystems, undergoing phase-shifts.  Unfortunately, these local impacts can be compounded by global impacts such as invasive species, warming, and ocean acidification. Phase-shifts refer to a change in the species assemblage to one that is not normally dominant.  Consider the Caribbean.  Coral cover has declined by ~80-90% since the late 1970s at sites throughout the region, while macroalgae cover has increased markedly.  In a nutshell, many grazing fishes were once present, keeping the macroalgae at bay and allowing for both the persistence of coral-dominated reefs and the settlement of coral larvae.  Once these grazer populations were exploited by overfishing, a sea urchin became the dominant grazer until it was decimated from a disease epidemic in the early 1980s.  This die-off was a pivotal event, and phase-shifts to algal systems become to become more common and many reefs are still operating under this alternative stable state today.  Atolls in the Pacific are thought to be much more resilient to these types of ecological changes, due to lower fishing pressure and less directly-induced human impacts overall.  On the other hand, the isolation of these oceanic oases means that these sites are much more self-dependent than other sites, due to reduced larval connectivity (e.g. coral reef organisms’ larvae have less success getting to isolated sites, simply due to distance and ocean currents).

So is there good news to be had?  These phase-shifts have been shown to be reversible in some cases.  Diadema antillarum populations in the Caribbean is recovering, albeit slowly and in spatially discrete locations, providing increased herbivory on some reefs–at these locations, the macroalae is being grazed back and coral-dominated systems are possible again.  This is not widespread currently, as most Caribbean reefs continue to be algal-dominated.  Another example of the reversibility of these trends involve No-Take-Zones (NTAs), a type of Marine Protected Area (MPA).  In the Bahamas, NTAs have resulted in higher densities of herbivorous fishes, contributing to declining macroalgal cover and more young corals settling on the reef.  MPAs have also been shown to increase coral cover on a global basis.

Rather than dealing with each threat to coral reefs individually, a common approach, they must be dealt with in an integrated, proactive fashion, rather than reactively after the damage is likely already done.  Hughes et al. stress the need for:   the enforcement of harvesting regulations, local education, integrative decision-making via better access to “international networks of expertise”,  and the confrontation of climate change by rapidly reducing greenhouse gas emissions.  Rapid climate change and ocean acidification will ultimately undo any management policies for coral reef ecosystems.  It is imperative for the state of coral reefs worldwide to be recognized for what it is: “a crisis of governance.”  Though the situation may be daunting, these systems can be saved.

[This article has been cross-posted, albeit with a different image 1, at The Urban Times, an online magazine.]

Images  1:  Close-up of a scleractinian brain coral. From Flickr user Laszlo Ilyes (cc) 2: Scleractinian coral colonies shown before (A) and after (B) one month of increased temperatures (Coffroth et al 2010).

References:

1. Baker AC, Starger CJ, McClanahan TR, & Glynn PW (2004). Coral reefs: corals’ adaptive response to climate change. Nature, 430 (7001) PMID: 15306799

2. Coffroth MA, Poland DM, Petrou EL, Brazeau DA, & Holmberg JC (2010). Environmental symbiont acquisition may not be the solution to warming seas for reef-building corals. PloS one, 5 (10) PMID: 20949064

3. Hughes TP, Graham NA, Jackson JB, Mumby PJ, & Steneck RS (2010). Rising to the challenge of sustaining coral reef resilience. Trends in ecology & evolution (Personal edition) PMID: 20800316