Last year, Dr. Tim Shank led an expedition into the deep sea of the Coral Triangle, finding dozens of new species. The diversity of species the team is describing may be evidence for a deep-sea Wallace line. Read more at the Economist.
“Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. 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.”
- Roger Revelle and Hans Suess, 1957
There are five major extinction events known; these events had profound effects on the ecosystems of the Earth and influenced evolutionary processes. Coral reefs, for instance, show ‘reef gaps’ in the geologic record, which is taken to mean that reefs have taken many millions of years to fully recover after such events. Shallow, calcifying corals are particularly useful in exploring these, and other, smaller, background extinctions due to their nature as an ecosystem that actively produces and archives its geologic present and past. In 2008, J.E.N. Veron published a report in Coral Reefs that profiled these events, and their causes, finding that these events are closely tied to the carbon cycle. So when were these five major extinction events (ME) and what do we know about them?
End Ordovician ME When: 434 million years ago (Ma). Why?: This co-occurred with a period of high global temperatures and potentially high levels of atmospheric carbon dioxide. Sea level changes, shifts in ocean chemistry and other causes have also been implicated. Biotic Effects: 60% of all genera of land and sea life were obliterated. Although some corals survived, reefs disappeared for a few millions years, forming the first true ‘reef gap’ known.
Late Devonian ME When: 360 Ma. Why?: Extraterrestrial impact (considered unlikely by most) may have triggered global changes. Atmospheric carbon dioxide dropping (uptake by plants), low temperatures, and shifting sea levels have also been suggested. Biotic Effects: Mostly marine life effected. No real recovery of coral-sponge reefs (the early Devonian hosted extensive reefs globally).
End Permian ME When: 251 Ma. Why? Shifting ocean chemistry, atmospheric carbon dioxide flux, acid rain, deoxygenated surface waters, and massive volcanic venting are all possible (and largely interrelated) causes. Biotic Effects: Big. Really big. 82% of all genera and up to 95% of all marine species went extinct. It is thought that most corals and most other marine calcifiers were among the missing. Reefs did not appear again for another 10 millions years and when they emerged, it was in the form of the Scleractinia, rather than their ancient selves.
End Triassic ME When: 205 Ma. Why? The usual. Sea-level flux, ocean chemistry changes, and high temperatures are implicated, but clear evidence is scant. Biotic Effects: About half of marine invertebrates and up to 80% of land qaudapeds (four-legged things). A third of scleractinian families and 75% of scleractinian genera were included in this (remember your biological classification scheme from junior high: King Philip Came Over For Good Sex—kingdom, phylum, class, order, family, genus, species). Again, a reef gap came after this event (6-8 Ma in duration).
End Cretaceous ME (K/T) When: 65 Ma. Why?: We have wide agreement on a single cause for this one! A bolide (meteoroid) hit near the Gulf of Mexico, causing tsunamis and widespread volcanic activity. Then came the resulting dust clouds that thrust cold darkness onto the world, and the subsequent greenhouse warming from impact-related methane and carbon dioxide release, among other things. Biotic Effects: Everyone knows about this one….the dinosaurs met their end. Very sad. But what about the invertebrates! Some species of corals did survive (70% of all scleractinian genera did not), but reefs did not appear for at least 10 million years. Bivalves, gastropods, forams, and many other taxa were near extinction or went completely extinct (i.e. ammonites). Land animals were also nearly completely decimated.
It is easy to think of extinctions events as just that—single point, acute events. But they are usually an accumulation of processes that have built up over time. For example, the Cretaceous, even before the bolide impact, was extremely volatile in terms of sea level and global temperatures. However, some of these processes that lead to extinctions may be more instrumental in the demise of organisms that others.
Following environmental prerequisites for reef development, Veron rules out causes of mass extinctions not attributable to the carbon cycle and finds common threads in these extinction events in the form of shifting ocean chemistry and reduced pH (acidification). The ocean is a carbon sink, uptaking up to a third of atmospheric carbon. As more carbon dioxide enters the ocean, pH is lowered, and the carbonate chemistry of the oceans shifts. This can reduce calcification in marine organisms, which is one of the defining characteristics of a coral reef. Calcifying algae, which play a major part in reef consolidation (think of them as biotic cement), accrete even more soluble skeletons. Acidification, and other carbon cycle disturbances, is implicated as a cause in these mass extinctions mainly due to the process of elimination.
So in light of its potential role in extinctions, ocean acidification—happening today-- is not something taken lightly. Experimental studies widely show reduced calcification under shifting chemical conditions. It’s true that carbon dioxide levels have been higher in the geologic past, but no evidence exists for the current (human-driven) rate of increase. Organisms may not be able to adapt quickly enough. Even in terms of absolutes, by 2100, atmospheric carbon dioxide is predicted, conservatively, to reach 500 ppm—a value not seen in the at least the past 740,000 years. Oceanic pH is also predicted to drop another 0.3-0.4 units by 2100. We’re running a geophysical experiment without a control and as a result, we’re already looking at a certain amount of committed warming and acidification. Anthropogenic change is beginning to have severe impacts on global ecosystems, and if continued unabated, these impacts will be exacerbated through space and time.
Image: Fossil colonial coral from Anne Burgess on Wikimedia Commons.
This mainly serves as a quick review of Veron’s 2008 article. References (and those therein) drawn upon for this post are below, as well as some suggestions for further reading:
Doney SC, Balch WM, Fabry VJ, Feely R (2009a) Ocean acidification: a critical emerging problem for the ocean sciences. Oceanography 22:16-25
Hoegh-Guldberg O et al. (2007) Coral reefs under rapid climate change and ocean acidification.Science 318:1737-1742
Jackson JBC (2008) Ecological extinction and evolution in the brave new ocean. PNAS 105:11458-11465
Kleypas JA, Buddemeier BW, Archer D, Gattuso JP, Langdon C, Opdyke BN (1999) Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284:118-120
Royal Society (2005) Ocean acidification due to increasing atmospheric carbon dioxide. The Royal Society: London
Revelle R, Suess H (1957) Carbon dioxide exchange between atmosphere and ocean and the question of an increase in atmospheric CO2 during the past decades. Tellus IX, 1
Sabine CL et al. (2004) The oceanic sink for anthropogenic CO2. Science 305: 367-371
Veron, J. (2008). Mass extinctions and ocean acidification: biological constraints on geological dilemmas Coral Reefs, 27 (3), 459-472 DOI: 10.1007/s00338-008-0381-8
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.
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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
I have not forsaken you, dear reader(s)!
The new semester has been quite busy and Anthozoa was unfortunately neglected.
However, I’ll be writing about how glaciers and iceskates are similar in the coming days. Get excited about ice!
In the meantime, whet your appetite with this article on the evolution of corals in the deep-sea and their subsequent diversification into the shallows. This idea has some really interesting implications regarding how corals have managed to survive as a taxa for the past tens of millions of years, through varying environmental conditions. Enjoy!
Copyright (c) 2009 Richard Ling
So there’s another consequence for our fossil fuel addiction…it’s called ocean acidification. Ocean acidification is actually a bit of a misnomer (the oceans are actually a bit on the basic side, so during ‘ocean acidification’ they are actually becoming less basic, rather than acidic, for now), but I digress. During what some have begun to call the Anthropocene (the period of time in which humans have begun to have major environmental and climatic effects—think the Industrial Revolution to well, now), human-produced carbon emissions have risen substantially…about 40% or so from preindustrial levels of about 280 parts per million. This really shouldn’t be news to you, especially with the widespread coverage of carbon dioxide’s role in climate change. However, ocean acidification is an issue that really hasn’t been disseminated very well, and it has some potentially dire consequences. Here’s the deal: the ocean acts as our planet’s only true carbon sink, about a third or so of atmospheric carbon dissolves into the ocean. As all of this carbon (usually in the form carbon dioxide) dissolves into the sea, it almost immediately reacts with water and undergoes a series of reactions. I’ll spare you the chemical details for now, but the bottom line is that through these carbonate chemical changes, the ocean’s pH is reduced (becoming less basic), and the ratio of dissolved inorganic carbon has been changed. What does that mean? Well, organisms in the ocean that calcify, usually to make skeletons, tests, or shells (e.g. some species of corals, certain types of plankton, etc.), make use of the carbonate ion in seawater. Marine calcifying organisms precipitate calcium carbonate (limestone); calcium is not limiting in the ocean, but carbonate is thought to be. As the oceans acidify, the concentration of carbonate falls, potentially making it more difficult for organisms to form calcium carbonate (in a broad sense).
Let’s take the case of corals. Scleractinian corals are those that can precipitate a calcium carbonate skeleton, and are sometimes referred to as reef-building corals. The particular type of calcium carbonate that these organisms precipitate is called aragonite (some other organisms use calcite, the other mineralogical species, but let’s just consider corals for now). Under acidified conditions, the oceans become less saturated in respect to aragonite, meaning that aragonite does not precipitate from seawater as readily, potentially bad news for marine organisms with aragonite skeletons such as scleractinian corals. The pH of the ocean has already dropped about 0.1 below pre-anthrocene levels and is expected to drop another 0.3-0.4 units by the end of this century. This may not seem like a lot, but for organisms that have evolved to occupy very narrowly defined environmental parameters (e.g. temperature, sediment load, light, etc.), this would be a tremendous physiological test. Corals in many locales are already under thermal stress due to rising sea surface temperature and face the threat of bleaching (due to dissociation from their symbiotic dinoflagaellate algal cells–but that’s another story).
Quite a few studies have sought to quantify ocean acidification’s effects on scleractinian corals, and the overall theme is that calcification appears to be reduced with decreasing pH. This trend seems to be quite robust and linear in some cases. However, as with most things, the devil’s in the details. Earlier studies seeking to simulate ocean acidification in the laboratory do so with acid additions in seawater. Does this lower the pH? Sure. Does it decrease the aragonite saturation state? Absolutely. But how acid additions accomplish this is not really comparable with what is happening in the ocean. Adding acid to seawater not only decreases carbonate concentrations, but also bicarbonate (another major ion in seawater). When carbon dioxide dissolves into the ocean, carbonate decreases, but bicarbonate actually goes up (carbonate ions are increasingly converted to bicarbonate). A recent study (Jury et al. 2009) actually took another approach, in that they adjusted seawater chemistry with a combination of bubbling in carbon dioxide and manipulating total alkalinity. They found that calcification rates of corals (they used Madracis mirabilis sensu in their laboratory experiments) did not respond consistently to pH or aragonite saturation state. Ocean acidification in respect to scleractinian corals appears to be a much more complicated issue than previously thought; and it seems that the responses of these organisms are not determined by aragonite saturation state alone. However, this is a complex issue in which multiple parameters are involved and different coral species may be sensitive to different facets of seawater chemistry. A more holistic view of calcification and marine chemistry will have to be adapted by researchers seeking to elucidate these tenets of global change.
Ocean acidification isn’t limited to corals. All marine calcifying organisms are thought to be potentially affected, especially as carbon emissions escalate. Effects on less charismatic calcifying creatures, such as coccolithophores (a type of plankton) and pteropods (a gastopod), may have profound effects on oceanic geochemical dynamics, climate change, and even food webs. In addition, the lesser-known brethren of shallow corals, deep-sea corals, may be affected as well. More to come on these residents of the deep later.
Note (24 March 2011): It has come to my attention that this essay could be misconstrued to mean that the effects of ocean acidification have been overstated. This is not the case. This essay highlighted a specific paper that reported interesting results that may indicate that this phenomenon is not as simple as previously thought. Unknowns abound, particularly at the ecosystem level. However, the evidence for ocean acidification’s effects on marine communities is extensive and I believe it to be one of the most important anthropogenic impacts—large-scale disturbances in the carbon cycle are nothing to be trifled with and what we are currently doing is geologically unprecedented. See my further treatment of OA here, at The Urban Times, and here.
The following references (and the works cited therein) provide a nice review of the topic, and information provided by them was drawn upon for this post.
Image: Richard Ling on Flickr (cc 2.0)
Jury, C., Whitehead, R., & Szmant, A. (2009). Effects of variations in carbonate chemistry on the calcification rates of Madracis auretenra (=Madracis mirabilis sensu Wells, 1973): bicarbonate concentrations best predict calcification rates
Global Change Biology DOI: 10.1111/j.1365-2486.2009.02057.x
Orr, J., Fabry, V., Aumont, O., Bopp, L., Doney, S., Feely, R., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R., Plattner, G., Rodgers, K., Sabine, C., Sarmiento, J., Schlitzer, R., Slater, R., Totterdell, I., Weirig, M., Yamanaka, Y., & Yool, A. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms Nature, 437 (7059), 681-686 DOI: 10.1038/nature04095
