Infectious disease outbreaks are a major cause of coral loss and reef degradation. Evidence from paleontological studies and ecological monitoring indicate that coral disease prevalence, variety, and host range have all increased over the last 30 years. But what is the origin of coral pathogens? Even in the few cases where the causal pathogen has been identified, we don’t know where it came from, if it is new or newly introduced or whether it was always present on reefs, but only recently became virulent due to a mutation or environmental change.
Identifying the source of coral pathogens is a key goal of coral disease ecology with obvious importance for disease mitigation and reef management. Coral pathogens could originate in terrestrial habitats and be introduced to the ocean following deforestation and soil runoff. They could also be added via sewage outfalls, transported from faraway reefs in the ballast water of cargo ships, and might even be spread within and among regions by human travel.
A pair of exciting new studies clarifies the origins of sea fan aspergillosis, a major Caribbean epizootic caused by the pathenogenic fungus Aspergillus sydowii. In the first paper, Rypien et al. (2008) used molecular markers to determine patterns of relatedness among strains of Aspergillus sydowii collected from a variety of hosts and environments. Specifically, they tested four hypotheses:
1) The Endemic Marine Hypothesis predicts that corals are infected by fungus that is native to marine habitats and therefore phylogenetically distinct from nearby terrestrial isolates.
2) The Terrestrial Runoff Hypothesis predicts that isolates from diseased corals will be most closely related to terrestrial isolates from nearby landmasses.
3) The single-origin African Dust Hypothesis predicts that isolates will have reduced genetic diversity and allelic richness, with evidence of a recent bottleneck in coral disease-causing isolates. This hypothesis also predicts that isolates will be most closely related to terrestrial isolates from Africa.
4) The multiple origins African Dust Hypothesis predicts no evidence of a recent bottleneck, with disease-causing isolates being most closely related to terrestrial isolates from Africa.
Their results essentially refute all four hypotheses and reveal a pattern of global panmixia and multiple origins, suggesting that a single source of Aspergillus sydowii into reefs is unlikely. The results illustrate the opportunistic nature of the fungal pathogen and suggest that a diversity of isolates can cause aspergillosis.
Despite coming from very different geographic locations (Japan to North America to Europe) and different sources (diseased corals, diseased humans, dried fish), we found that all strains form a single well-connected global population. – lead author Dr. Krystal Rypien, currently a post doc at Scripps Institution of Oceanography
Dr. Rypien added; This has important implications for the management of disease, as it means that any isolate of this fungus has the potential to cause disease in coral, and that we are not dealing with a specialized group of pathogens. Interestingly, this is similar to a close relative, Aspergillus fumigatus, a common fungal pathogen of immune compromised humans. Given the global distribution of A. sydowii, and evidence for multiple introductions into marine systems, it seems that this pathogen has always been present in marine systems, and changes in environmental conditions and host immune status are likely to be more important in driving patterns of disease outbreak.
The second study (Rypien 2008) tested the widely-believed hypothesis that African dust plays a role in coral epizootics in general and sea fan aspergillosis in particular. Each year hundreds of millions of tons of dust is transported from the Sahara desert to the Caribbean. There are indications that the volume of dust has increased as the Sahara expands and atmospheric conditions and wind patterns change. The idea is that African dust can cause or exacerbate coral epizootics by depositing nutrients and trace elements that benefit pathogens or by transporting pathogens from terrestrial African habitats to Caribbean coral reefs. Past studies have indeed found Aspergillus spp. in dust samples collected from the Caribbean, but none have identified the fungi to species, which turns out to be a critical shortcoming.
Dr. Rypien collected dust samples from the Caribbean and Africa, isolated Aspergillus, and identified the isolates to species using standard colony-level and microscopic morphological characteristics. Despite yielding seven different species of Aspergillus and related taxa, there was no A. sydowii in airborne dust samples from Africa and the Caribbean or in sediment samples from Africa and the Cape Verde Islands.
The lack of A. sydowii in airborne dust and sediment samples suggests that African dust is an unlikely source of the marine pathogen A. sydowii. Given the high richness of fungi observed, even under selective growth conditions, identification of potential pathogens to the species level is critical.
The study doesn’t entirely refute African dust as a source of Aspergillus sydowii – it is nearly impossible to prove the absence of something – but it does cast doubt on much-heralded theory.
Rypien, K. L. 2008. African dust is an unlikely source of Aspergillus sydowii, the causative agent of sea fan disease. Marine Ecology Progress Series 367:125-131.
Rypien, K. L., J. P. Andras, and C. D. Harvell. 2008. Globally panmictic population structure in the opportunistic fungal pathogen Aspergillus sydowii. Molecular Ecology 17:4068-4078.
Amidst the current policy debate in Australia on climate change is a surreal argument that policies that will destroy the Great Barrier Reef (GBR) are acceptable and economically rational. Ross Garnaut was alive to the damage to the GBR when saying Australia should initially aim for a global consensus to stabilise greenhouse gases in the atmosphere at 550 parts per million. Garnaut (2008a: 38) was brutally frank in his supplementary draft report:
“The 550 strategy would be expected to lead to the destruction of the Great Barrier Reef and other coral reefs.”
His final report does not shy away from this conclusion (Garnaut 2008b).
The Australian and Queensland governments have always silently avoided this point when explaining the costs and benefits of their climate policies. Neither has ever stated a stabilisation target for the rise in global temperatures or greenhouse gases. To do so would expose them to the criticism that their policies will not save the GBR or a host of other ecosystems.
Garnaut’s frank admission reflects the findings of research of the impacts of climate change to the GBR since mass coral bleaching occurred globally in 1998 and 2002. Rising sea temperatures and increasing acidity of the oceans due to our use of fossil fuels are now well-recognized as major threats to coral reefs and the marine ecosystem generally in coming decades.
In relation to coral bleaching the IPCC (2007b: 12) found that:
“Corals are vulnerable to thermal stress and have low adaptive capacity. Increases in sea surface temperature of about 1 to 3°C are projected to result in more frequent coral bleaching events and widespread mortality, unless there is thermal adaptation or acclimatisation by corals.”
The findings of the IPCC suggest that a rise of 1°C in mean global temperatures and, correspondingly, sea surface temperatures above pre-industrial levels is the maximum that should be aimed for if the global community wishes to protect coral reefs. The range of 1-3°C is the danger zone and 2°C is not safe. Supporting this conclusion Ove Hoegh-Guldberg and his colleagues concluded in a review of the likely impacts of climate change to the GBR edited by Johnson and Marshall (2007: 295):
“Successive studies of the potential impacts of thermal stress on coral reefs have supported the notion that coral dominated reefs are likely to largely disappear with a 2°C rise in sea temperature over the next 100 years. This, coupled with the additional vulnerability of coral reefs to high levels of acidification once the atmosphere reaches 500 parts per million [CO2], suggests that coral dominated reefs will be rare or non-existent in the near future.”
The IPCC’s (2007a: 826) best estimate of climate sensitivity found that stabilising greenhouse gases and aerosols at 350 parts per million carbon dioxide equivalents (ppm CO2-eq) would be expected to lead to a rise in mean global temperatures of 1°C, stabilising at 450 ppm CO2-eq will lead to a rise of 2°C, and stabilising at 550 ppm CO2-eq will lead to a rise of 3°C.
Following on from a previous article at Climate Shifts, a recent article published in PLoS One shows that corals are proving to be even more non-conformist than previously thought. Zoe Richards and co-authors from the ARC Centre of Excellence for Coral Reef Studies found that ‘rare’ species of branching corals are able to cross breed with other branching corals to create hybrids, therefore avoiding probable extinction:
“Coral reefs worldwide face a variety of marine and land-based threats and hundreds of corals are now on the red list of threatened species. It is often assumed that rare coral species face higher risks of extinction than common species because they have very small effective population sizes, which implies that they may have limited genetic diversity and high levels of inbreeding and therefore be unable to adapt to changing conditions.
When we studied some particularly rare species of Acropora (staghorn corals), which you might expect to be highly vulnerable to extinction, we found some of them were actually hybrids – in other words they had cross-bred with other Acropora species. This breaks all the traditional rules about what a species is. By hybridising with other species, these rare corals draw on genetic variation in other species, increasing their own potential to adapt to changing conditions.
When we looked at the genetic history of rare corals, we found that they exhibited unexpected patterns of genetic diversity. This suggests that, rather than being the dying remnants of once-common species, they may actually be coral pioneers pushing into new environments and developing new traits by virtue of the interbreeding that has enabled them to survive there.
This is good news, to the extent that it suggests that corals may have evolved genetic strategies for survival in unusual niches – and may prove tougher to exterminate than many people feared. With such tricks up their sleeve, it is even possible that the rare corals of today could become the common corals of the future.” (Link)
We know the biomass of macroalgae on coral reefs is largely controlled by herbivory and that one of the most important groups of grazers are parrotfish. A new study published in PNAS (Berkepile and Hay 2008) indicates that the richness and composition of grazer species is also important. In a nutshell, different fish consume different seaweeds because of their differing chemical defenses. Similar work in other benthic marine systems has found that consumer species richness can be an important determinant of ecosystem functioning, yet this is the first such study on a coral reef.
Our study shows that in addition to having enough herbivores, coral ecosystems also need the right mix of species to overcome the different defensive tactics of the seaweeds. explained Mark Hay, the Harry and Linda Teasley Professor of Biology at the Georgia Institute of Technology.
Despite different species of parrotfish in the Caribbean having different feeding behaviors, bioerosion rates, and preferred diets, parrotfishes are often considered as a unified functional group when inferring their effects on community structure. However, we found that redband and princess parrotfish had considerably different effects on communities, suggesting that grouping all parrotfishes may blur important distinctions among species.
Despite their different feeding morphologies, ocean surgeonfish and princess parrotfish generated similar macroalgal communities dominated by upright brown macroalgae (e.g., L. variegata and Sargassum spp.). In contrast, despite their more similar jaw morphology, the communities generated by redband and princess parrotfish differed considerably in the abundance of upright macroalgae. Similar to the work of Bellwood et al., these results show that fishes with different feeding morphologies can have similar effects on community structure, suggesting that relying primarily on jaw functional morphology to construct functional groups or infer a species’ impact may be unreliable.
Working out of the underwater Aquarius laboratory off Key Largo Florida, Hay and co-author Deron Burkpile – who is now at Florida International University in North Miami – constructed 32 cages on the reef. Each cage was about two meters square and one meter tall and was sealed so that larger fish could neither enter nor leave.
The number and type of fish placed into each four-square-meter cage varied. Some cages had two fish that were able to eat hard, calcified plants; some had two fish able to eat soft, but chemically-defended plants; some had one of both types, and some had no fish at all
For the cages in which we mixed the two species of herbivores, the fish were able to remove much more of the upright seaweeds, and the corals in those areas increased in cover by more than 20 percent during ten months, Hay said.
The data we are seeing in Fiji [from similar experiments] suggests that diversity may be even more important there than it was in the Caribbean. There are a lot of different species doing a lot of very different things. These consumers are very important, and in areas where they are over-fished, the reefs are crashing.
Berkepile, D.E. and M.E. Hay. 2008. Herbivore species richness and feeding complementarity affect community structure and function on a coral reef. PNAS 105: 16201–16206
The recent Garnaut report states that “the solutions to the climate change challenge must be found in removing the links between economic activity and greenhouse gas emissions.” In order to successfully mitigate climate change impacts on both the environment and the economy, we need to go a step further and replace those links with avenues for sustainable economic activity. This can effectively begin with innovative designs for improving efficiency in energy production and usage.
Rather than compensating mining companies that are vulnerable to the new emissions trading scheme, the pledged compensation should be used to train employees of these companies with skills that will help them develop innovative designs for efficient energy usage to the commercialisation level. These high emission companies should begin investing in new technologies which could eventually be traded instead of coal to countries like China, in order to spread the improvements in carbon emissions to a global scale. Of course, this is the ten billion ton gorilla in the room that no one quite wants to recognise (at least not publicly!)
Credits to trade-exposed companies and low income households should only be considered to the extent that benefits are not initially received for their investment. Once benefits are realised, this monetary gain must be re-invested into future innovative solutions, thereby replenishing the funding for green solutions. Essentially, we need to amp up the green investment cycle. For example, in the above situation a mining company burdens the cost of training some employees and using their work hours for sustainable development avenues.
Once the company receives return on their investment, re-investment into development of sustainable technologies should occur to the extent of the original “loan” or government credit. Similarly, households given credits, for example, to install solar panels should be encouraged to re-invest the savings on their electricity bills into new innovative technologies. The establishment of this positive feedback loop should be a condition of receiving the credits in order to prevent the misuse of the credits or the undermining of carbon trading.
The missing links in the solutions to climate change are the real ideas that will drive the economy towards sustainable development. Treading softly on this issue is not an option – time is of essence. Another weak link in this much needed cycle is the fact that economic gain is our society’s key motivation and the environment is severely undervalued. The Garnaut Review states that environmental and social costs “are not amenable to conventional measurement”.
In other words, any cost-benefit analysis will not be accurate. Society’s real motivation needs to come from desire to maintain and conserve the environment for future generations. There is no adequate or accurate way to quantify this desire. And there is no way to ensure that that this desire is a top priority of world citizens. It seems that the best way to achieve this goal is to steer people’s actions economically. However, it is unlikely that the outcome will exhibit the same strength when motivated by monetary value.
A report released yesterday by the Queensland Premier Anna Bligh showed that water quality on the Great Barrier Reef is not improving, and that further action is needed to reverse the ongoing decline. As part of the Reef Water Quality Protection Plan initiated by the Australian and Queensland governments, the 2007 Water Quality Report is the first step in a four year process, addressing water quality issues such as catchment pressures, marine ecosystem health and land management practices affecting the Queensland coastline and Great Barrier Reef.
Some of the key findings of the report seem to confirm what scientists have previously observed: that over the last 150 years, the catchments adjacent to inshore reefs have been extensively modified for agriculture (e.g. sugar cane), cattle and sheep grazing, tourism, mining and urban development, leading to significant increased in sediments, nutrients and pesticides impacting upon the inshore Great Barrier Reef. From the report, monitoring of priority catchments has shown that:
- 6.6 million tonnes of sediment are discharged in the reef lagoon annually (four times higher than estimated pre-European settlement levels)
- 16,600 tonnes of nitrogen are discharged in the reef lagoon annually (five times higher than estimated pre-European settlement levels)
- 4,180 tonnes of phosphorous are discharged in the reef lagoon annually (four times higher than estimated pre-European settlement levels)
In response to the report, Premier Bligh called for a summit on reef water-quality issues in the next month:
“Work done to date as part of the Plan includes financial incentives to help farmers improve land management practices and targeting diffuse pollution from broadscale land use,”
“However, since 2003 many external factors have deteriorated including the effects of climate change, coral bleaching and ocean acidification.
“It has increased the urgency for more work to be done.
“I have discussed this matter with the Prime Minister and met with Environment Minister Peter Garrett.
“We agreed that the first step will be a joint Commonwealth-state reef water quality summit at Parliament House at the end of this month,” she said.
“The summit will bring together the best minds from the environmental and scientific fields to study the latest data and discuss what urgent action we need to take to prevent further damage to – or worse – the complete demise of the reef.” (Link to Media Statement)
The Environment Minister Peter Garrett also acknowledges the issue:
“We’ve specifically committed $200 million to reef rescue knowing that we need to provide additional resources, additional investment, and additional effort to safeguard what is one of our most important national and international natural resources and treasures” (Link)
I look forward to the proposed summit and applaud the Queensland government for taking such forward action in addressing water quality issues – it seems for Peter Garrett (pictured above left in typical Midnight Oil attire) there is no excuse!
Crown of thorns starfish (COTS – Acanthaster planci) are notorious throughout the Indo-Pacific region. COTS are voracious coralivores, and in outbreak proportions can eat vast areas of reef by exuding their stomachs and digesting coral polyps (read more). Having been diving in oceans around the world over the past few decades, i’ve often pondered the differences in colourations of COTS between reef regions, and whether they represented a single species. A recent paper published in Biology Letters by Catherine Vogler from Göttingen University and colleagues at the Smithsonian and University of California confirms that COTS aren’t a single taxonomic entity, and in fact represent a ‘species complex’ of upto four seperate species.
Different appearances of the Crown of Thorns starfish across locations, clockwise from Top Left: Madagascar (Image credit: Mila Zinkova), Thailand (Image credit: Jon Hanson) Okinawa, Japan (Image credit: Gary Hughes), Fiji (Image credit: Matt Wright)
Using a genetic approach, the researchers analysed DNA from over 237 starfish collected from reefs around the world. Their results strongly suggest that their are in fact four species of COTS, located in the Pacific Ocean, Red Sea, Southern Indian Ocean and Northern Indian Ocean).
Geographical distribution of the different species of crown of thorns (each colour represents a different species where sampled, piecharts indicte the frequency of each species per location)
It’s fascinating to think that the divergence of these species occured between the Pliocene (3.65 million years ago) and early Pliestocene (1.95 million years ago). More importantly though, this discovery may have fairly interesting implications for conservation biology. The researchers point out that whilst outbreaks of COTS are well-researched phenomena on the GBR and Indo-Pacific reefs since the early 1960’s, outbreaks in the Indian Ocean and Red Sea are much less severe. It seems that a better understanding of the genetic structure of COTS populations and identifying species boundaries may go a long way to explaining the intensity and magnitude of COTS outbreaks in different regions.
Reference: Vogler et al (2008) A threat to coral reefs multiplied? Four species of crown of thorns starfish. Biology Letters doi:10.1098/rsbl.2008.0454 (Link)