By: Pam Frost Gorder
COLUMBUS, Ohio—The future health of the world’s coral reefs and the animals that depend on them relies in part on the ability of one tiny symbiotic sea creature to get fat—and to be flexible about the type of algae it cooperates with.
In the first study of its kind, scientists at The Ohio State University discovered that corals—tiny reef-forming animals that live symbiotically with algae—are better able to recover from yearly bouts of heat stress, called “bleaching,” when they keep large energy reserves—mostly as fat—socked away in their cells.
“We found that some coral are able to acclimatize to annual bleaching, while others actually become more susceptible to it over time,” said Andréa Grottoli, professor in the School of Earth Sciences at Ohio State. “We concluded that annual coral bleaching could cause a decline in coral diversity, and an overall decline of coral reefs worldwide.”
The study, which appears in the July 9 online edition of Global Change Biology, indicates that some coral species will almost certainly decline with global climate change, while others that exhibit large fat storage and flexibility in the types of algae they partner with will stand a better chance of enduring repeated rounds of stress as oceans get hotter.
It also suggests that the most adaptable species would make good targets for conservation efforts because they are most likely to survive.
“If we conserve reefs that contain coral species with these survival traits, then we’re hedging our bets that we might be able to preserve those reefs for an extra decade or two, buying them enough time to acclimatize to climate change,” Grottoli said.
Corals are essentially colorless; the brilliant browns, yellows, and greens that we associate with them are actually the colors of algae living inside the corals’ animal cells. That’s why, when stressed coral dump most of the algae from their cells, their bodies appear pale, or “bleached.”
Bleached corals can recover by growing more algae or acquiring new algae once water temperatures return to normal. This research shows that corals’ ability to switch the type of algae that they internally grow has a large effect on their recovery.
But if corals don’t recover and reefs die, thousands of fish species and other sea creatures lose their habitat.
Normally, bleaching is a rare event. But by 2025, Caribbean waters are expected to be hot enough that the coral living there will be stressed to the point of bleaching once a year. The rest of the tropics are expected to experience annual bleaching by 2050.
Previous studies have only followed coral through one bleaching event, or through two events several years apart. So Grottoli and her team tested what would happen if they subjected some common Caribbean corals to bleaching for two years in a row.
Corals can supplement their diet by eating plankton, but they get most of their energy from their symbiotic relationship with algae. The algae get nutrients from the coral, and the corals get to siphon off sugars that the algae produce in photosynthesis. Like humans, corals can store excess energy as fat.
Two key survival strategies emerged in this study: the most resilient corals stored up fat reserves in times of plenty, and were willing to switch to a new dominant algal type in order to gather food in times of stress. Corals that didn’t store fat or were stuck with their algal partner didn’t fare as well.
And species that bounced back from one round of bleaching didn’t necessarily bounce back a second time.
“We found that the research on single bleaching events is misleading,” Grottoli said. “Species that we think are resilient to temperature stress are actually susceptible and vice versa when stressed annually.”
Grottoli and her colleagues tested three corals from Puerto Morelos Reef National Park, off the coast of Mexico. Two years in a row, they plucked samples of Porites divaricata, Porites astreoides, andOrbicella faveolata—more commonly known as finger coral, mustard hill coral, and boulder coral—from the ocean floor, and placed them in warm water tanks in an outdoor lab until the corals bleached. Both times, the researchers returned the corals to the ocean to let them recover. They measured several indicators of how well the different species recovered, including the number and type of algae present in the corals’ cells and remaining energy reserve.
The mustard hill coral kept lower fat reserves, and partnered with only one algal species. It recovered from the first round of bleaching but not the second. The boulder coral kept moderate fat reserves, but partnered with six different algae and changed between dominant algal types following each bleaching. It recovered from both rounds of bleaching, though it’s growth slowed.
The real winner was the finger coral, which switched completely from one algal partner type to another over the course of the study, and had the largest fat reserves—47 percent higher than the boulder coral or mustard hill coral. The finger coral was barely even affected by the second bleaching and maintained a healthy growth rate.
The bottom line: as some species adapt to climate change and others don’t, there will be less diversity in reefs, where all the different sizes and shapes of coral provide specialized habitats for fish and other creatures. Interactions among hosts, symbionts, predators and prey would all change in a domino effect, Grottoli said. Reefs would be more vulnerable to storms and disease in general.
It sounds like a bleak picture.
“We’re actually a bit optimistic, because we showed that there’s acclimation in a one-year window, that it’s possible,” she said. “In two of our three coral species, we have recovery in six weeks. The paths they took to recovery are different, but they both got there.”
Coauthors on the study included Grottoli’s former graduate students Stephen Levas, Verena Schoepf, and Justin Baumann; Ohio State research associate Yohei Matsui; and Mark Warner of the University of Delaware and his graduate students Matthew Aschaffenburgand Michael McGinley.
This research was funded by the National Science Foundation.
Coral Gardens: A school of surgeonfish cruise coral reefs near Palmyra Atoll.
If current climate trends follow historical precedent, ocean ecosystems will be in state of flux for next 10,000 years, according to Scripps Oceanography researchers.
If history’s closest analog is any indication, the look of the oceans will change drastically in the future as the coming greenhouse world alters marine food webs and gives certain species advantages over others.
Scripps Institution of Oceanography, UC San Diego, paleobiologist Richard Norris and colleagues show that the ancient greenhouse world had few large reefs, a poorly oxygenated ocean, tropical surface waters like a hot tub, and food webs that did not sustain the abundance of large sharks, whales, seabirds, and seals of the modern ocean. Aspects of this greenhouse ocean could reappear in the future if greenhouse gases continue to rise at current accelerating rates.
The researchers base their projections on what is known about the “greenhouse world” of 50 million years ago when levels of greenhouse gases in the atmosphere were much higher than those that have been present during human history. Their review article appears in an Aug. 2 special edition of the journal Science titled “Natural Systems in Changing Climates.”
For the past million years, atmospheric CO2 concentrations have never exceeded 280 parts per million, but industrialization, forest clearing, agriculture, and other human activities have rapidly increased concentrations of CO2 and other gases known to create a “greenhouse” effect that traps heat in the atmosphere. For several days in May 2013, CO2 levels exceeded 400 parts per million for the first time in human history and that milestone could be left well behind in the next decades. At its current pace, Earth could recreate the CO2 content of the atmosphere in the greenhouse world in just 80 years.
In the greenhouse world, fossils indicate that CO2 concentrations reached 800-1,000 parts per million. Tropical ocean temperatures reached 35º C (95º F), and the polar oceans reached 12°C (53°F)—similar to current ocean temperatures offshore San Francisco. There were no polar ice sheets. Scientists have identified a “reef gap” between 42 and 57 million years ago in which complex coral reefs largely disappeared and the seabed was dominated by piles of pebble-like single-celled organisms called foraminifera.
“The ‘rainforests-of-the-sea’ reefs were replaced by the ‘gravel parking lots’ of the greenhouse world,” said Norris.
Changing marine life characteristics: Comparison of present, past, and future ocean ecosystemstates. Click on image for larger view. Image courtesy of Science
The greenhouse world was also marked by differences in the ocean food web with large parts of the tropical and subtropical ocean ecosystems supported by minute picoplankton instead of the larger diatoms typically found in highly productive ecosystems today. Indeed, large marine animals—sharks, tunas, whales, seals, even seabirds—mostly became abundant when algae became large enough to support top predators in the cold oceans of recent geologic times.
“The tiny algae of the greenhouse world were just too small to support big animals,” said Norris. “It’s like trying to keep lions happy on mice instead of antelope; lions can’t get by on only tiny snacks.”
Within the greenhouse world, there were rapid warming events that resemble our projected future. One well-studied event is known as the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago, which serves as a guide to predicting what may happen under current climate trends.
That event lasted about 200,000 years and warmed the earth by 5-9° C (9-16° F) with massive migrations of animals and plants and shifts in climate zones. Notably, despite the disruption to Earth’s ecosystems, the extinction of species was remarkably light, other than a mass extinction in the rapidly warming deep ocean.
“In many respects the PETM warmed the world more than we project for future climate change, so it should come as some comfort that extinctions were mostly limited to the deep sea,” said Norris. “Unfortunately, the PETM also shows that ecological disruption can last tens of thousands of years.”
Indeed, Norris added that continuing the fossil fuel economy even for decades magnifies the period of climate instability. An abrupt halt to fossil fuel use at current levels would limit the period of future climate instability to less than 1,000 years before climate largely returns to pre-industrial norms. But, if fossil fuel use stays on its current trajectory until the end of this century, then the climate effects begin to resemble those of the PETM, with major ecological changes lasting for 20,000 years or more and a recognizable human “fingerprint” on Earth’s climate lasting for 100,000 years.
Co-authors of the review are Sandra Kirtland-Turner of Scripps Oceanography, Pincelli Hull of Yale University, and Andy Ridgwell of the University of Bristol in the United Kingdom.
In a future shaped by climate change, only the strong -- or heat-resistant -- will survive. A study published in the Proceedings of the National Academy of Sciences opens a window into a genetic process that allows some corals to withstand unusually high temperatures and may hold a key to species survival for organisms around the world.
"If we can find populations most likely to resist climate change and map them, then we can protect them," said study co-author Stephen Palumbi, a senior fellow at the Stanford Woods Institute for the Environment and director of Stanford's Hopkins Marine Station. "It's of paramount importance because climate change is coming."
Coral reefs are crucial sources of fisheries, aquaculture and storm protection for about 1 billion people worldwide. These highly productive ecosystems are constructed by reef-building corals, but overfishing and pollution plus rising temperatures and acidity have destroyed half of the world's reef-building corals during the past 20 years. The onslaught of climate change makes it imperative to understand how corals respond to extreme temperatures and other environmental stresses.
Although researchers have observed that certain corals withstand stresses better than others, the molecular mechanisms behind this enhanced resilience remain unclear. For their study, Palumbi, lead author Daniel Barshis, a Stanford postdoctoral scholar, and other researchers looked at shallow-reef corals off Ofu Island in American Samoa to determine how they survive waters that often get hotter than 32 degrees Celsius / 90 degrees Fahrenheit during summer-time low tides.
Utilizing cutting edge DNA sequencing technology, the scientists examined the corals' gene expression when subjected to water temperatures up to 35 degrees Celsius / 95 degrees Fahrenheit. "These technologies are usually applied to human genome screens and medical diagnoses, but we're now able to apply them to the most pressing questions in coral biology, like which genes might help corals survive extreme heat," said Barshis.
Heat-resistant and heat-sensitive corals had a similar reaction to experimental heat: hundreds of genes "changed expression" or turned on to reduce and repair damage. However, the heat-resistant corals showed an unexpected pattern: 60 heat stress genes were already turned on even before the experiment began. These genes are "frontloaded" by heat resistant corals -- already turned on and ready to work even before the eat stress began. "It's like already having your driver's license and boarding pass out when you get close to the TSA screener at the airport, rather than starting to fumble through your wallet once you get to the front of the line," Palumbi said.
The findings show that DNA sequencing can offer broad insights into the differences that may allow some organisms to persist longer amid future changes to global climate. "We're going to put a lot of effort into protecting coral reefs, but what happens if we wake up in 30 years and all our efforts are in vain because those corals have succumbed to climate change," Palumbi said.
As with strong corals, finding species most likely to endure climate change -- "resilience mapping" -- is the first step toward protecting them, Palumbi said. "The solutions that we're looking for must, at least partially, be out there in the world."
Everyone wants good news about coral, but we shouldn’t misinterpret the latest findings.
By: Merinda Nash
As Doha disappoints on delivering any real progress on reducing global CO2emissions, new research demonstrates that a key component of coral reef structures may be more resilient in the face of increasing CO2 levels, and consequent declining seawater pH, than was previously thought.
However, while this is a good news story for the reefs, it does not mean that the entire reef is going to survive the negative impacts of ocean acidification associated with rising atmospheric CO2.
While investigating the mineral structure of a common coral reef alga, Porolithon onkodes, we found that there was an extra mineral, dolomite, present in many of the algae collected from the high-energy reef front environment.
These algae are known as coralline algae because, similarly to corals, they produce a carbonate skeleton. Corals produce a carbonate mineral CaCO3 called aragonite. The coralline algae form magnesium calcite, a mineral that is mostly CaCO3 but with 10-20% substitution by magnesium for calcium.
In many reefs the upper-most reef front structure is predominantly built by coralline algae, as delicate branching corals cannot develop in this wave break zone. Therefore, discovering that this algae looks to be a survivor under higher CO2 scenarios is most definitely good news, particularly for tropical island communities that are protected from high energy waves by these algal ridges.
The discovery that these algae produce dolomite, which is 50% magnesium instead of calcium and chemically very stable, was in itself an exciting discovery. Dolomite is most familiar to people as the mineral that gives the Dolomite Alps of Italy their name: dolomitized fossil coral reefs dominate the alps.
This dolomitization process was thought to be a chemical alteration of the reef limestone that took place long after a coral reef had died. Discovering that living algae in modern coral reefs can form this dolomite prolifically meant reconsidering what we thought we knew about the chemical stability of these algae.
Our experiments showed that dolomite corallines had 6-10 times less dissolution of the skeleton compared to the coralline algae without dolomite. Although dissolution increased in the high CO2 water, the total rate was still minimal. We found that dolomite was common in algae from many tropical reefs, but it seems to be restricted to the shallow, highest energy parts of the reef.
What does this finding mean for the Great Barrier Reef and other tropical reefs? First, I must make clear that our research related to coralline algae, not corals.
The Canberra Times and The Age ran the story with the headings “Coral may be climate change’s silver lining” and “Resistant algae good news for coral”. Considering our research was actually on algae, not coral, the Age has the most accurate headline. We are sadly accustomed to the critical role of coralline algae being overlooked in favour of the more visually appealing corals.
Coralline algae are not the same as the symbiotic zooxanthellae algae that live within the coral branch. Coralline algae are a pink encrusting algae that grow over dead and living corals and other reef substrate. The reef front below the exposed coralline algae surface is typically built of overlapping layers of coral and coralline algae. Our results demonstrate that the structural role provided by this coralline algae looks set to continue under higher CO2 levels than we previously thought.
The Australian reported our research with two other positive coral news stories with the headline “Forget the doom: coral reefs will bloom”. The coral stories related to corals thriving in conditions thought to be inhospitable to their survival.
On the face of it this all points to a positive future for the coral reefs. However, the key point that is often missed is that a coral and a coral reef are not the same thing. Just because a coral grows, it does not necessarily follow that a reef will form, just as a few trees growing in harsh conditions does not indicate a forest can form.
The recent CSIRO marine report card for ocean acidification identified the need for research at an ecosystem scale to understand the complex and sometimes inter-related responses of various reef organisms to ocean acidification.
It is possible to have a reef made of coralline algae without corals, as happened ~16 million years ago during the Mid-Miocene climactic optimum when CO2 and temperatures were higher than today. However, a reef made only of coralline algae will not support the biodiversity presently found on our tropical reefs.
With the Doha talks failing, maybe world leaders are counting on our world being more resilient to climate changes than we thought. While this research shows that coralline algae may be more resilient than we thought, unfortunately, we still can’t rely on this to save our reefs.
By: Mario Aguilera
University of California - San Diego
Novel excavation technique attributes prior damage to land clearing and overfishing
The decline of Caribbean coral reefs has been linked to the recent effects of human-induced climate change. However, new research led by scientists at Scripps Institution of Oceanography at UC San Diego suggests an even earlier cause. The bad news – humans are still to blame. The good news – relatively simple policy changes can hinder further coral reef decline.
Employing a novel excavation technique to reconstruct the timeline of historical change in coral reefs located on the Caribbean side of Panama, a team of scientists led by Scripps alumna Katie Cramer and current Scripps Professor of Oceanography and Smithsonian Tropical Research Institute (STRI) Emeritus Staff Scientist Jeremy Jackson has determined that damage to coral reefs from land clearing and overfishing pre-dates damage caused by anthropogenic climate change by at least decades.
"This study is the first to quantitatively show that the cumulative effects of deforestation and possibly overfishing were degrading Caribbean coral and molluscan communities long before climate change impacts began to really devastate reefs," said lead author Cramer, currently based at the Global Coral Reef Monitoring Network at the International Union for Conservation of Nature.
Coral reefs have suffered alarmingly since the 1980s due to coral bleaching and coral disease, thought to stem from the warming of the oceans due to anthropogenic, or human-induced, climate change. However, until recently, the impact of prior human activities on Caribbean coral reefs had not been studied with experimental techniques.
Historical records and qualitative surveys provide hints that declines in corals in some parts of the Caribbean occurred as far back as the early 1900s after coastal lands began to be cleared to make way for plantations. However, the current study is the first to quantify the changes that reef corals and mollusks have undergone as a result of long-term stress caused by the deposition of silt, nutrients, and pollution onto coral reefs from land clearing and the depletion of reef fish that prevent algae from overtaking reefs.
"Because researchers did not really begin to study Caribbean reefs in detail until the late 1970s, we don't have a clear understanding of why these reefs have changed so dramatically since this time," said Cramer. "So, we set out to reconstruct an older timeline of change on reefs by looking at the remains of past reefs – coral skeletons and mollusk shells."
To reconstruct this timeline, the team dug below modern reefs in incremental layers and, using radiocarbon dating of the coral skeletons they found, linked fluctuations in the types and numbers of coral and mollusks over time to historical records of land clearing. Changes in the relative numbers of these various species represent clear indicators of the overall health of the coral reef.
The team also improved upon the standard technique of taking long, narrow core samples of coral fossils that cannot track fluctuations in the numbers of larger species of coral.
"We wanted to look at the whole complement of the coral community," said Cramer.
To catalog the relative numbers of dozens of coral and molluscan species, the researchers dug two-foot-wide by three-foot-deep pits into reefs at several coastal lagoon and offshore sites near Bocas del Toro, Panama, that were heavily affected and less affected by land runoff, respectively. At each of these sites they also conducted surveys and recorded the composition of living corals.
"We dug up over a ton of coral rubble and tens of thousands of shells," said Cramer, who led the fieldwork at STRI and likened the laborious experience to doing underwater construction.
Systematically sifting through the coral and shell fossils, the scientists noted several indicators of environmental stress, including a decrease in the overall size of bivalves such as oysters, clams, and scallops, a transition from branching to non-branching species of coral, and large declines in the staghorn coral and the tree oyster, which were once the dominant coral and bivalve on these reefs.
These indicators were observed in layers of the excavated pits at coastal lagoon sites that were dated before 1960 and as far back as the 1800s, corresponding to a period of extensive deforestation in the Bocas del Toro region. Similar evidence of environmental stress at offshore sites was dated after 1960, indicating that the negative impacts of land clearing have more recently begun to affect reefs further offshore.
With the decline of the branching coral species, the reefs now have fewer nooks and crannies that are used as habitat for reef fish and other organisms. Also, the non-branching species that have taken their place grow at a much slower rate. "Consequently, there is less of a chance that the reefs will be able to keep up with sea level rise from climate change," said Cramer.
"Because the governments of the world have yet to undertake any meaningful efforts to mitigate climate change, it is of the utmost importance that locally caused stressors to reefs such as overfishing and deforestation are minimized," said Cramer. "Advocating for more intelligent use of land as well as implementing sustainable fisheries management, those are things that can be done right now."
###The research team, which also includes Jill Leonard-Pingel of Scripps, Thomas Guilderson of the Lawrence Livermore National Laboratory and the Institute of Marine Sciences at the University of California at Santa Cruz, and Christopher Angioletti, will publish its findings in the April issue of Ecology Letters. An early online version has been released today.
This research was funded by the National Science Foundation, the Smithsonian Institution, the Center for Marine Biodiversity and Conservation at Scripps, the UC San Diego Academic Senate, and the PADI Foundation's Project AWARE.