Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|Reefs at low pH|
|Written by Tim Wijgerde|
Global warming is seen as the number one threat to the world's coral reefs, next to overfishing, pollution and eutrophication. It is widely accepted that anthropogenic CO2-emissions are the main cause behind the acceleration of global warming, although there is still some controversy. Ice core drillings have revealed that our planet has seen dramatic climate changes before1, along with increased temperatures and atmospheric CO2 concentrations. It is common knowledge that elevated sea surface temperatures may cause coral bleaching. That ocean acidification could be just as threatening, is not.
The CO2 equilibrium
Today, the planet's CO2 concentration is roughly 385 parts per million (ppm), although this value fluctuates (fig.1). This fluctuation is caused by the fact that the majority of the earth's vegetation grows on the northern hemisphere, resulting in a higher total photosynthesis during our summer. During the fall on the northern hemisphere, the CO2 level rises gradually.
Figure 1: Monthly mean CO2 concentrations in ppm as measured at Mauna Loa, Hawaii, at approx. 11,000 feet (3400m). Cycles in the graph can be clearly seen, caused by the summer to winter switch on the northern hemisphere (Courtesy of the National Oceanic and Atmospheric Administration, www.noaa.gov).
These fluctuations occur in our oceans as well, given the fact that our atmosphere is constantly 'dosing' CO2 into the seas. In short, when CO2 levels increase in the atmosphere, so too will they in the oceans. We call this process Henry's Law. As a consequence, the oceans have actually inhibited global warming by taking up vast amounts of CO2. Marine water is a natural buffer, which means that it can be regarded as a solution, able to retain a relatively constant pH value. This ability can be quantified, and it is called alkalinity, which is mainly determined by bicarbonate ions (HCO3-). When CO2 dissolves into marine water, the chemical reaction is this:
The paradox here is that CO2 produces alkalinity, which seems to be beneficial. However, for every bicarbonate ion which is produced, a proton is produced as well. These protons are the acidic atoms which cause a drop in pH level. This counterbalancing is responsible for a phenomenon which we call the law of constant alkalinity2.
The acidification of the oceans
Oceanic pH value has been measured over time (fig.2), and shows a significant decrease over the years.
Figure 2: a: History and future of atmospheric CO2 using two different climate models, IS92a and S650. Using the worst-case scenario model, IS92a, atmospheric CO2 will double over the next 100 years. b: History and future of oceanic pH levels at various latitudes. A significant decrease in oceanic pH can be expected, even using optimistic models (Modified from Orr et al, Nature, 2005).
Oceanic pH has never been lower than 7.6 in the past 300 million years3, and models predict that if current burning of fossil fuels persists this record will be broken10. It has to be said that these are long-term predictions, dealing with worst-case scenarios. Either way, a drop in marine pH is already noticeable, having decreased 0.1 compared to the pre-industrial period3.
The rise in atmospheric CO2 concentration will not be without consequences; many ecosystems will suffer from this. Including coral reefs. How exactly?
Marine organisms such as coral polyps have adapted to ocean water over millions of years. Temperature, pH and concentrations of all kinds of elements have been relatively stable for long periods of time. And with adaptation comes dependency. A dramatic change in ocean water composition can be disastrous. Many marine organisms build skeletons from calcium carbonate, CaCO3, in specific crystal structures (aragonite or calcite). Amongst them are coccolithophores (phytoplankton species), corals, foraminifera (unicellular animals), echinoderms, crustaceans and molluscs.
The problem which will present itself in the near future is of a chemical nature. Normal seawater has a pH of about 8.0 – 8.3, and is fully saturated with carbonate (CO32-) ions. This is the main reason why coral skeletons don’t dissolve in marine water.
As oceanic pH decreases, this carbonate saturation will decrease. When pH levels have decreased sufficiently, a shift in the CO2/HCO3-/CO32- equilibrium will occur, decreasing CO32- concentrations:
As a direct consequence, skeletons made out of calcium carbonate will start to dissolve, as more 'space' is given to the carbonate component. This exact process occurs in a calcium reactor, where we add CO2 to decrease internal reactor pH and allow crushed coral to dissolve! From this point, it is quite easy to start seeing the bigger picture here. Once this critical shift in equilibrium is reached, this will threaten the entire marine ecosystem.
Like coral reefs, coccolithophores (phylum: haplophyta, fig.3), a group of important phytoplankton species, are under threat as well. Just like cyanobacteria, diatoms and dinoflagellates, these algae produce carbohydrates using the sun's energy, a process which takes up CO2. A significant proportion of them is not consumed, but eventually sinks to the deep seas. The oceans therefore are a huge 'sink', or storage facility, for carbon on earth. This too has masked a rise in atmospheric CO2 concentration, and thus global warming.
Figure 3: Electronmicroscopic photographs of coccolithophore species. The calcite plates can be clearly seen. (Courtesy of Dr. Baumann, University of Bremen, Germany).
What makes these algae so special, is that they produce coccoliths, calcareous plates, as an exoskeleton (fig.3). Hence their name. These plates reflect the sun's beams, which can be captured on film (fig.4). Like anything building a calcium skeleton, coccolithophores will be under great threat as well if pH levels continue to drop.
Figure 4: Blooms of Emiliana huxleyi, a common cocco-lithophore, photographed off the coast of Newfoundland in the Western Atlantic, July 1999 (Courtesy of Toby Tyrrell and Norman Kuring (NASA/GSFC), the SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE).
The foundation of any ecosystem is formed by primary producers. These are species which convert inorganic compounds to organic ones, by harnessing the sun's energy or by using heat sources such as seen around so-called black smokers in the deep sea. All other species are dependent on these producers, feeding on them and each other, thereby creating a food-web. When we remove the foundation of anything, be it a building or an ecosystem, it completely collapses. The loss of coral reef and keystone planktonic species would be disastrous events, basically denying most of the life in our oceans.
As mentioned above, there is a critical threshold pH at which calcium carbonate is allowed to dissolve. Determining this value is very important, as it allows scientists to predict when reefs will start to collapse. This value has been found to lie around a pH value of about 7.4-7.66. Recently, experiments have been conducted with the Mediterranean corals Oculina patagonica and Madracis pharensis (fig.5). At a pH of 7.4, these coral's skeletons dissolve quickly; within 4 months 75% of the skeleton had dissolved. We call this phenomenon a negative net calcification, as more skeleton dissolves than is being precipitated by the coral. After exposure to normal sea water having a pH of about 8.2, recovery could be clearly seen (fig.5c).
Figure 5: Oculina patagonica, a Mediterranean coral species. A: control colony. B: Solitary polyps have formed after exposure to marine water at a pH of 7.4. C: Recovery by a positive net calcification after exposure to normal sea water at a pH of 8.2. D: Time series where the relative change in protein/polyp and total colony weight is depicted. Within 4 months 75% of the colony's skeletal mass dissolves (Fine and Tchernov, Science, 2006).
It is both exciting and comforting to see these species undergo this dramatic change, switching to a solitary, Zoanthid-like form, and eventually recover. An intriguing fact is that the world's oceans, and thus the coral reefs, have seen these dramatic pH drops before. Current insights indicate that corals first appeared during the Permian era7, 300 Ma ago (300 Mega annum, or 300 million years ago). Hermatypic, or reef-building corals are believed to have arisen during the Triassic period, 250-200 Ma ago. This is confirmed by geological studies. So-called 'reef gaps' have been found; layers in the earth's soil which lack fossilized coral8. This had led to the hypothesis that corals in the past have shifted from a fossilizing to a soft form8,9. This theory is confirmed by the above experiment.
Although the colony above cannot be seen having completely recovered yet, these experiments provide hope for the future. According to estimates, atmospheric CO2 concentrations will have to triple (about 1000 ppm) before the above experiment becomes a reality10 (which would mean a drop of 0.6 pH units). If current CO2 emission levels persist, it will take about 150 years to reach this point (this cannot be seen in the graph of fig.2, because the line halts at the year 2100). Although these are all just estimates, it is clear that now is the time to act.
In the light of current climate changes, we can ask ourselves; what will our world look like 100, or even a 1000 years from now? Will human populations stabilize, and will the economical, political and societal actions be enough to turn the tide?
It is clear that life on earth has seen dramatic changes. What is less clear is if life is able to keep up, as changes today occur at an alarming rate. The late Carl Sagan once compared our planet's atmosphere to the layer of varnish on a globe, and this comparison makes it easy to understand that 6 billion people can alter its composition quickly. During the next centuries our oceans will suffer more acidification than they have over the past 300 million years9 (excluding dramatic events such as meteors and large-scale volcanic eruptions1,10), if anthropogenic emissions persist.
"We are conducting a great geochemical experiment, unlike anything in human history and unlikely to be repeated again on Earth. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years" (Revelle and Suess, 1957; Houghton, 2005).
Even if many species would be able to adapt such as the Mediterranean ones, the consequences would be disastrous. With the collapse of the reefs many ecological niches will disappear; places where many reef invertebrates and fish find their living space. Natural barriers of over a 100 countries will no longer exist, leading to dangerous floods during storms. Ecotourism will suffer a terrible blow, which will have a great impact in countries such as Australia, Indonesia and the Philippines.
The political actions taken worldwide, such as the Kyoto protocol, will hopefully be a start to a new future. Scientific reports such as those from the IPCC (Intergovernmental Panel on Climate Change) will continue to stimulate both governments and society to keep the environment in mind. Optimistic prognoses show that if we start limiting burning of fossil fuels the tide can be turned. And with this, life in the oceans will have a future again.