by Rick MacPherson
 

The Galapagos Islands, giant tortoises, and finches may provide the popular backdrop for Charles Darwin, whose 200th birthday the world celebrates on February 12, 2009. But as a coral reef ecologist and conservationist, I’m drawn to one of Darwin’s less familiar contributions to our understanding of the natural world. In addition to his fondness for beetles, birds, and barnacles, Charles Darwin was also fascinated by coral reefs. So much so, that he spent the better part of his life attempting to understand their origins.

In March of 1832, just three months into what would be his five-year voyage aboard the HMS Beagle, Darwin recorded some of his first observations of living corals. While the Beagle crew was busy mapping the hazardous waters surrounding the Abrolhos Islands between Salvador and Rio de Janeiro, Brazil, Darwin fixed his eyes beneath the surface of the shallow tropical seas.

“The bottom of the adjoining sea is thickly covered by enormous brain stones; many of them could not be less than a yard in diameter,” records Darwin in his zoology notes.  “Without being in the immediate presence of limestones, how extraordinary it is that these polipi should be able to obtain such an enormous stock of Carb. of Lime. This is an instance (perhaps not a strongly marked one) where there is a great formation of corals, the lime obtained without the neighborhood of volcanic action.”

We might be tempted to chuckle at the quaint, Victorian use of brain stones to refer to what we now know are brain coral colonies. Darwin recognized that coral is indeed a living organism, though it’s easy to see how they would be mistaken for stones.  The living portion of a stony coral colony is just a delicate film of tissue growing atop what appears to be rock. What truly puzzled Darwin at the time, however, was how such massive stony coral constructions could have formed in waters that seemingly lacked an important chemical building block for limestone, which was known at that time as carbonate of lime.

Fijian coral reef: Coral reefs are composed of more than just coral: they are home, protection, and life support systems to millions of species, including humans. Photo by Jeff Yonover.

The origins of limestone from the remains of marine organisms deposited in ancient seas was commonly understood by 1832 (for instance, the chalky-white Cliffs of Dover are composed mainly from fossilized, microscopic marine plankton). But in addition to carbonate of lime, Darwin was likely unaware of the high concentration of calcium in seawater as well. Darwin believed at that time that the growth of corals required that only a steady supply of carbonate of lime be brought up to the seabed by volcanic action in the vicinity. In reality, the coral animal collects the necessary building blocks for its stony skeleton from its surrounding waters.  But Darwin lacked a complete picture of coral metabolism and, at the time, was unaware of a remarkable symbiotic partnership playing out within every coral colony.
 

Consider for a moment the individual coral animal known as a polyp. Coral polyps are soft-bodied, delicate invertebrates that look a bit like minute sea anemones. In a world of biting, grazing, and nibbling neighbors, a soft and relatively defenseless body is not an asset. In the case of reef-building corals, individual polyps have developed the ability to live in colonies and to build sometimes massive and intricate communal skeletons of calcium carbonate. They do this by extracting calcium and carbonate ions from seawater and converting it into a limestone skeleton.

Over time, aggregations of a variety of coral colonies and other species can form biological structures known as coral reefs. Coral reefs become more than the sum of their individual parts, creating a home, habitat, and life-support system for millions of species. You can’t help but feel dwarfed and impressed by the sheer scale of production required to produce the impressive biological architecture of a modern coral reef. In fact, the enormous Great Barrier Reef in Australia is the only biological structure that can be seen from space. Given that reef-building corals inhabit only a narrow band of tropical waters on the planet, and that these clear waters are typically nutrient poor, it’s easy to see how the source of coral productivity could have perplexed Darwin.

But there’s more to the coral story than meets the eye. Somewhere along the history of life, reef-building corals began playing host to a symbiotic, photosynthetic algae called zooxanthellae.  Existing in a mutualistic relationship with coral, microscopic zooxanthellae live in a coral’s tissue, giving it its unique and beautiful color.  During Darwin’s time, no one was aware of the existence or important symbiotic function of zooxanthellae in the life of reef-building corals. In overly simplified terms, it’s like a landlord-tenant relationship. The coral landlord provides the algae with a home—a protected environment in which to live. In return, the algae pays rent to the coral animal in the form of oxygen and waste removal. Most importantly, zooxanthellae supply the coral with the metabolic building blocks necessary to produce its calcium-carbonate skeleton at a much accelerated rate.

It has been estimated that as much as 90 percent of the organic material produced photosynthetically by zooxanthellae is transferred to the host coral tissue. This is the driving force behind the growth and productivity of coral reefs. Sometimes when corals become physically stressed, the polyps expel their algal cells and the colony takes on a stark, white appearance. This is commonly described as “coral bleaching.”  If the polyps live for too long without their zooxanthellae, coral bleaching can result in the coral's death. One of the best understood triggers for coral stress that results in mass coral bleaching events is that of sea surface temperatures.
 

As a young marine biology student, I learned that corals stretch back in the fossil record to approximately 500 million years ago. The first close relatives of modern reefs as we know them evolved around 250 million years ago.  The Great Barrier Reef, largest reef system on the planet, is a relative youngster.  It began building about 500,000 years ago and the living Barrier Reef you dive today is only about 8,000 years old.  You might imagine my surprise to discover that despite their deep pedigree in the history of life, corals are quite delicate and sensitive life forms.  In fact, reef-building corals are the Goldilocks of the animal kingdom. When their surrounding water temperature is either too hot or too cold, the algal tenants within the coral may be unable to carry out their cellular metabolism and, as a result, be unable to pay “rent” to their coral landlord.  The coral polyp host runs a tight ship and will evict any deadbeat algal tenants unable to hold up their end of the arrangement.  Devoid of their algal symbionts, the bleached, stressed coral polyp will await new, more thermally-fit free floating algal cells in need of lodging.

So, what’s the “just right” zone for corals? That depends on the coral species, but reef-building corals have evolved to survive only a narrow temperature range: plus or minus as little as 2º F from ambient conditions can create a stressful situation.

Over the past several decades, steadily increasing levels of carbon dioxide and other greenhouse gasses have been correlated to steadily increasing global temperatures, both in the atmosphere and in the oceans.  Incidents of increased sea-surface temperatures as a result of human-accelerated climate change have taken their toll on coral reefs worldwide through mass bleaching events.

Coral bleaching: When stressed, reef-building coral hosts can evict their symbiotic algal tenants. The algae provides the coral with its distinctive color. If bleaching persists, the coral animal will eventually die. Photo by Wolcott Henry.

But is it really possible that temperature alone is what’s behind our current coral crisis?  After all, the Earth has warmed and cooled before in the history of life and coral reefs have kept pace.  And if the exciting work from the University of Miami’s Dr. Andrew Baker proves successful, we may be able to develop techniques to enhance the thermal tolerance of corals by “inoculating” at-risk coral colonies with more heat-resistant strains of zooxanthellae and help them survive dangerously warming oceans around the world. 

If it were simply the threat from global warming, coral reefs might be able to cope with temperature fluctuations.  But coral reefs are suffering a death by a thousand cuts.  When you consider coastal development, unsustainable fishing, water pollution, unsustainable tourism, rising sea levels, and the effects of increased human demand on a sensitive ecosystem, it’s no surprise that nearly 50 percent of our planet’s coral reefs have been functionally destroyed.  And what is left is at considerable risk.  A National Oceanographic and Atmospheric Administration report released this past summer indicated that half of all U.S. coral reefs are in poor to fair condition.  The first-ever ecological report card for the Mesoamerican Barrier Reef—second largest reef system on the planet stretching from Mexico’s Yucatan Peninsula, through the entire Belize Barrier Reef complex, along the coast of Guatemala and out to the Bay Islands of Honduras—released in November 2008 showed failing grades and painted an overall picture of a reef in danger and in need of immediate protection.
 

But another human-induced threat to coral health has been silently growing, and may represent the greatest challenge yet to maintaining vibrant, healthy coral reefs into the future. It’s a particularly insidious threat to coral and brings us back to the story of a young Charles Darwin aboard the Beagle pondering how such enormous coral formations can be constructed from seawater.  To understand, we need to consider some basic chemistry.

	Tourism is the world's largest industry, and impacts from unsustainable development, as seen here in Cancun, Mexico, have taken their toll on coral reefs. Photo by Wolcott Henry.

Carbon dioxide (CO2)—a byproduct of burning trees, grasslands, and fossil fuels like oil, coal, and gas—naturally dissolves in seawater to form carbonic acid, a few other compounds, and, importantly for this discussion, carbonate. Carbonate combines with calcium in seawater to form calcium carbonate, which is in turn used by marine organisms to build shells, or in the case of coral, the stony, calcium carbonate (limestone) part of the reef.

So far, so good.  Problems arise, however, when atmospheric CO2 concentrations increase, leading to the greenhouse effect.  The additional CO2 in the ocean means more carbonic acid, which makes seawater more acidic and reduces the availability of carbonate. With less available carbonate, the water becomes less saturated with important compounds used in constructing coral reefs.  This increase in seawater acidity as a result of excess CO2 has been labeled ocean acidification [see recent Terrain.org article] and was first brought to the attention of the scientific community by Ken Caldeira, a chemical oceanographer now at the Carnegie Institution for Science at Stanford University.

Ocean acidification is a challenging situation for a coral polyp. As carbon dioxide concentrations increase and oceans become more acidic, the polyp’s rate of construction (calcification) and the quality of its “building” (the polyp’s own tiny portion of the reef and the reef as a whole) will go down. Even with coral animal and algal symbionts operating at full efficiency, a zero-sum game can be reached where rates of calcification are negated by rates of dissolution.  Obviously, this is not so great for the individual coral polyp, but it is also not so great for the polyp’s coral neighborhood, which in this case, is an entire coral reef ecosystem. As the problem becomes too severe, the reef may begin to erode faster than it can be built.

Lack of education and awareness on the part of tourists can result in unnecessary damage to coral, as seen here in Maui, Hawaii. Photo by Liz Foote.

In 2004 at the 10th International Coral Reef Symposium (ICRS) in Okinawa, Japan, the global threat from acidification was perceived to be quite low among attendees. What ranked at the top of concern? Coral bleaching—which makes sense when you consider that a bleaching event is perhaps the most visible and bleak calling card for the effects of climate change on our oceans. But in just four years since the Okinawa meeting, scientific opinions have shifted considerably.   This past summer, the overwhelming consensus at the 11th ICRS in Fort Lauderdale, Florida, was that ocean acidification now represents the greatest threat facing global reefs.

Where coral bleaching is an acute threat, acidification is a creeping one that builds slowly over time. Where bleaching is visible, acidification is invisible and hard to measure. And although bleaching can often kill coral colonies, acidification does not (at least in the short to medium term). Perhaps the most important difference between these two threats, however, is that adaptation by coral colonies to bleaching events is possible because as mentioned earlier, coral polyps can take up new, more heat-tolerant strains of symbiotic algae. But when coral skeletons dissolve due to lowered seawater pH, little adaptive options remain.

How do we as a biodiversity conservation community combat a threat that not only stands to decimate coral reefs but potentially erase any vestige that they existed in the first place? Which reefs should we prioritize for protection: the strong or the weak? Do we focus our energies on the most “valuable” reefs (however you wish to characterize that word), or do we throw our energies at those reefs most likely to succeed? These are far from academic questions as they get to the heart of the issue of how limited conservation capacity should best be spent.
 

	Healthy coral reefs provide food, storm protection, and cultural identity to coastal communities, such as those in Papua New Guinea. Photo by Rick MacPherson.

Even in the face of some very grim possibilities, there is still reason for hope.  Promising research has demonstrated that some stony coral skeletons can undergo complete dissolution in acidic conditions, yet individual coral polyps can survive, sometimes up to a year. While the coral skeletons were completely gone, reproductive ability was maintained. 

The upshot is that perhaps some stony corals could potentially wait out acidification scenarios until pH can return to normal levels again. Of course, this notion of coral waiting out acidification misses the point that it’s the complex carbonate reef architecture itself that supports the ecosystem. Losing coral skeletons means loss of niche space and habitat, loss of substrate for encrusting or attached life, and an overall weakened frame for future re-calcified corals to settle on.

While the ability of some corals to survive decalcification can offer some hope in worst-case scenarios of ocean acidification, our responses need to consider the larger reef complex. And of course, the best response is to not let ocean acidification get to the point of decalcified reefs, which is admittedly easier said than done.

We need to make our messages resonate not just among us “coral converted,” but outside symposium walls and into the public consciousness and policymaker’s awareness. We need to encourage and empower our leaders and citizens to make smart choices, make meaningful reduction in CO2 emissions, and think beyond their own lifetimes. It’s not too late, but time is not on our side.

What a truly incredible testament it would be to the human species if we could say we faced this global challenge of ocean acidification—and managed to avert it. That we recognized the value of ancient, living, symbiotically-driven coral reefs as a source of food, storm protection, potential medicines, recreation, cultural traditions, and for their aesthetic value and acted as a global community to protect them for future generations.
 

Coral reefs have survived for millions of years. Human activities over the past century now threaten reefs with extinction. Photo by Jeff Yonover.

Darwin’s fascination with coral reefs would last a lifetime.  In his 1842 work, The Structure and Distribution of Coral Reefs, seventeen years before publication of On the Origin of Species, Darwin formulated an explanation for the formation of coral atolls in the South Pacific.  In his autobiography, Darwin gives us a very clear account of the way in which the leading idea for his theory of coral reefs originated in his mind from his first encounter on the Beagle with the enormous brain stones off the coast of Brazil.  He writes, “No other work of mine was begun in so deductive a spirit as this, for the whole theory was thought out on the west coast of South America, before I had seen a true coral reef. I had therefore only to verify and extend my views by a careful examination of living reefs.”

Though lacking all the observations needed at first, Darwin was able to correctly deduce a general explanation and understanding of coral reef formation that would allow generations of future coral reef scientists to build upon.  The responsibility before us is to protect Earth’s fragile, threatened reefs.  In so doing, we not only safeguard an ecosystem that in turn supports us, but we also maintain an unbroken living lineage of discovery of our natural world, stretching back to a young Englishman on his first sea voyage to what will hopefully be many generations to come of young explorers who become, as Darwin describes in his introduction to The Structure and Distribution of Coral Reefs, “struck with astonishment, when first beholds one of these vast rings of coral.”