The stunted growth of coral reefs.

 
New York City shelters its 8.3 million residents within a complex, three-dimensional matrix of concrete and steel. Construction crews must work on the integrity of the city’s structural skeleton constantly just to keep up with growth and age. Like a living organism, the city is expanding up, down and outwards while repairing and replacing decrepit trusses and frames with new, high quality materials.

It is a common big-screen fantasy to imagine the physical decay of New York City if these construction efforts were to suddenly cease in some kind of post-apocalyptic madness. However, the structural collapse that would bring New York City back to nature is actually occurring within nature, and you don’t need CGI graphics to imagine it. Just last week a team of scientists shared new findings that warming oceans are reducing the size and strength of coral reefs in two different parts of the world.

A coral reef is not unlike a city. Reefs are massive underwater edifices that support a stunning diversity and density of organisms. But instead of using building materials like concrete and steel, coral reefs house their residents within and upon a skeleton of calcium carbonate. This calcium carbonate is continually laid down by the reef's construction crew – growing colonies of coral polyps.

Individual coral polyps are actually small, soft-bodied animals that grow affixed to a hard surface. For protection they secrete calcium carbonate near their base. This calcium carbonate accumulates as the polyps grow, clone themselves, and multiply into large colonies. Over many generations, the calcium carbonate left behind becomes the skeleton of the reef, which still consists of a living colony of coral polyps on the surface.

The ability of coral polyps to produce calcium carbonate, called calcification, depends on a host of environmental factors. For example, seasonal differences in water temperature cause calcification to increase in the summer and decrease in the winter, resulting in alternating layers of high and low calcium carbonate density. It creates a pattern similar to the annual growth rings of a tree. The scientists used this pattern to measure the extent of growth and the density of calcium carbonate produced every year by coral colonies. They focused on two genera, Porites and Montastraea, from the Great Barrier Reef and Mesoamerican Barrier Reef. They compared these annual growth data to existing records of warming sea surface temperatures spanning at least a decade.

One of the coral genera, Porites, showed the greatest sensitivity to warming temperatures regardless of location. In both the Great Barrier Reef and the Mesoamerican Barrier Reef, Porites experienced sharp declines in calcification rate as sea surface temperatures increased. If climate change models are correct, at the given rate of decline, Porites in the Great Barrier Reef will stop laying down calcium carbonate entirely by 2100. In the warmer Mesoamerican Barrier Reef, calcification by Porites will cease in 2060. The genus Montastraea has also experienced reduced rates of calcification, although not as extreme. Montastraea in the Mesoamerican Barrier Reef will experience a 40% reduction in calcification by 2100.

The consequences of reduced calcification will manifest differently in the two genera. As temperatures rise, Porites produces calcium carbonate at the same density, but compensates for reduced calcification by not extending as far. Therefore, Porites reefs will grow more slowly and could be outcompeted for space by other organisms. Montastraea continues to extend in warmer temperatures, but it does so at the expense of calcium carbonate density. Much like osteoporotic bones, a Montastraea reef with reduced calcium carbonate density is more susceptible to physical and biological damage. Warming water temperatures will compromise both types of reefs in their ability to support biodiversity, either in terms of space or strength.

As grim as they seem, these predictions are likely conservative. The scientists mention that they don’t consider other factors that affect calcification such as coral mortality, coral bleaching, disease, and the negative consequences of pollution, erosion and other environmental concerns. The slow decline of coral reefs may not be fodder for a disaster flick, but piles of stunted, brittle coral reef will be utterly disastrous for the world’s oceans in a time that is already considered a biodiversity crisis.

Carricart-Ganivet JP, Cabanillas-Tera´n N, Cruz-Ortega I, Blanchon P (2012) Sensitivity of Calcification to Thermal Stress Varies among Genera of Massive Reef-Building Corals. PLoS ONE 7(3): e32859. doi:10.1371/journal.pone.0032859 

Photo: A view of the Mesoamerican Barrier Reef, Belize en.mesoamericanreef.org

The ocean's most exclusive community.

The great thing about science is that questions lead to answers. The bad thing is that through this process, a subject that was once novel and strange slowly loses its mystique. When the thrill is gone and the mysterious becomes mundane, the jaded biologist longing for that delicious scientific buzz need only look down - way way down. And, oh my GOODNESS, a really thrilling bit of science was just pulled up from the uncharted ocean depths and published in PLoS Biology.

Deep-sea hydrothermal vents! Is there anything more amazing? They are remote like outerspace but with thriving communities of freaky biota. And as a team of researchers, led by Dr. Alex Rogers of Oxford University, recently found – if you’ve seen one you have NOT seen them all.

These ecosystems are so mystifying because they are fundamentally different from the ecosystems we are used to seeing. Whether you are in tropical rainforest, arctic tundra, or open ocean, nearly all food webs are built upon the plants and algae that harness sunlight to transform carbon dioxide into organic molecules. Photosynthesis is incredible, but also pedestrian. Things get really strange when you look into the darkness and find bizarre organisms that have capitalized on a different energy source – poisonous, smelly hydrogen sulfide gas.

Deep-sea hydrothermal vents, found at an average depth of 2100 meters, spew plumes of hot water from the earth’s crust. This water can be as hot as 400˚C and contains high concentrations of hydrogen sulfide. The surrounding water is nearly freezing and dark as night with pressures so great it keeps the hot plumes from boiling. Still some organisms have managed to thrive in this oppressive environment. Not surprisingly, it all comes down to the microbes. Bacteria and archaea living in and around the vent utilize the energy stored in the bonds of hydrogen sulfide to fix carbon dioxide into organic molecules. This process, known as chemosynthesis, was only a theory until it was observed in action at the hydrothermal vents of the Galapagos Ridge in 1977.

The unusual properties at the base of the vent food web radiate up through all the animals it supports. The giant tube worm that hosts chemosynthetic bacteria within its body is the most familiar image. In many ways it has assumed the role of the community’s iconic species. That is until Dr. Rogers and his team restored the mystique of the hydrothermal vent ecosystem.

Departing from the vents of the tropics and subtropics that are relatively easy to access, the team examined the communities on the East Scotia Ridge (ESR), 500 km to the east of Cape Horn between South America and Antarctica. At a depth of more than 3000 meters in the Southern Ocean, the ESR has two ridge segments with hydrothermal activity, E2 and E9. A deep-sea drive by the remotely operated vehicle Isis revealed that these areas are completely devoid of the tubeworms, polychaetes, clams, and shrimp that we’ve come to expect in hydrothermal vent communities. Rather, they host a complex community of endemic organisms – organisms that haven’t been seen anywhere else – notably a new species of crab, stalked barnacles, limpets, snails, sea anemones, and a seven-armed starfish.

The biological diversity of these areas is built upon the diverse landscape. In some spots chimneys as tall as 15 meters release concentrated plumes of mineral-rich water from the Earth’s crust. This water emerges at temperatures exceeding 300˚ C, and when it hits the near-freezing water of the ocean floor, the minerals fall out of solution and create that black smoker appearance. In other areas there is more diffuse vent flow with temperatures closer to the surroundings. Even between the two sites there is variation in the chemical composition of the vented liquid. These differences could affect the microorganism populations at the two sites which would have cascading affects up the food web.

The truly thrilling thing about the ESR discovery is not the strange biota, because, let’s be honest – finding new species in a remote habitat is old hat. The amazing thing is WHY the species are so strange and why the ESR community is different from the ones we see in similar ecosystems. While they seem inhospitable to us, hydrothermal vents are the only suitable habitat for these organisms. In that way they are just like islands out at sea or parks in an urban landscape. Biogeography is the study of species distributions across space – the traits of an organism that lead it to new areas and the barriers that stand in its way. And remarkably, when you consider all the geologic, hydrologic, and biologic pieces of the puzzle, it appears that hydrothermal vent communities suggest the same patterns of biogeography that govern terrestrial communities.

Deep ocean organisms face unimaginable hurdles to dispersal. Larvae might catch a ride on an ocean current, but many of them won’t last long before passing by another hydrothermal vent. These vents are found only at the boundaries of tectonic plates, which would serve as a great dispersal corridor if they corresponded with the currents. They don’t. Even more daunting is the surface to sea-bed Polar Front, which encloses the Southern Ocean and effectively blocks the entry of outside organisms. At the Polar Front water temperatures and salinity levels change abruptly, creating an insurmountable physiological challenge to most organisms attempting to cross. Knowing this it’s really no shock that the ESR has so many endemic species and so few of the usual suspects. With these barriers preventing migration, the populations of the ESR have been held in reproductive isolation for millions of years with the forces of evolution at work.

However, over geologic time scales ocean currents and plate movements are not even constant, which adds a whole new twist to the story! The hydrothermal vents appeared when the ESR began to spread – around 15 million years ago. That period corresponded with climatic conditions that made the Polar Front less intense, meaning that organisms dispersing from other vent communities actually had a chance to colonize this brand new environment. But the gates closed around 13.8 million years ago when the climate changed and the Polar Front strengthened.

Even more interesting is the phylogenetic history of one of the ESR endemics, which seems to corroborate the geologic and climatic stories. A new species of Kiwa crab, found in the vents of the ESR, is closely related to K. hirsuta of the nearby Pacific Antarctic Ridge. By looking at differences in their genetic markers, researchers loosely estimated that the two species diverged around 12.2 million years ago. Other ESR animals show similarity to species found in hydrothermal vents in the lower latitudes of both the Atlantic and Pacific. The dispersal of organisms from two oceans was likely aided by the Antarctic Circumpolar Current, which circulates around Antarctica, linking the Atlantic, Pacific, and Indian Oceans.

Dr. Rogers' team’s research adds another layer of complexity to the biogeography of vent ecosystems, even suggesting that the Antarctic vents comprise a new biogeographic province. For scientists and non-scientists alike it represents a whole new world of mysteries to be revealed, recharging our hope for big, exciting discoveries.

Rogers AD, Tyler PA, Connelly DP, Copley JT, James R, et al. (2012) The Discovery of New Deep-Sea Hydrothermal Vent Communities in the Southern Ocean and Implications for Biogeography. PLoS Biol 10(1): e1001234. doi:10.1371/journal.pbio.1001234

She's baaaack...

Recent reports by NOAA revealed that la Nina has returned to the tropical Pacific and strengthened over the month of August. The sister of el Nino, La Nina is the cool phase of the ocean warming phenomenon, during which surface temperatures of the equatorial east-central Pacific change by at least 0.5 degrees Celsius. Last month, temperatures dropped by 1.3 - 1.8 degrees. It seems like this chilly little girl is back, and she may be sticking around into 2011.

By some oceanic and climatic mystery that remains unsolved, el Nino and la Nina have a powerful influence over the weather conditions in many parts of the world. These events, which tend to alternate in cycles of 3-6 years, can alter seasons, upset fisheries, and increase the occurrence of extreme weather such as floods, droughts, hurricanes, and cyclones. Over the past two years in California, el Nino played a role in everything from nerve-wracking drought to vanishing Chinook salmon. As a result, it was easy to blame el Nino for anything that was at least slightly annoying. Rain on my birthday? Hot temperatures on the day that I decided to wear lined wool pants? Flight delays at SFO? Damn you el Nino.

Will la Nina be as good a scapegoat as her brother? Nature News has some answers.