Shielding from the Storm

Courtesy of Russ Hopcroft

Scientists search for genes that help calcifying organisms weather the effects of climate change.

By Alla Katsnelson
The pteropod Limacina helicina, sometimes referred to as a sea butterfly, has a translucent shell and a pair of swimming wings called parapodia, and is plentiful in polar seas.

This month, two California scientists will spend hours wading waist-deep through -2°C water on an Antarctic beach near McMurdo Research Station. Using scoops constructed from plastic beakers secured to the ends of broom handles, Victoria Fabry of the California State University, San Marcos, and Gretchen Hofmann of the University of California, Santa Barbara, will collect thousands of tiny, shelled planktonic...

As the ocean absorbs more carbon dioxide, its acidity rises. Fabry has shown that when the ocean becomes more acidic, the translucent calcium carbonate shells of pteropods, a type of sea snail, grow thinner; at ocean carbon dioxide levels that the Intergovernmental Panel on Climate Change (IPCC) predicts by 2100, the shells will simply start to dissolve. One of the shells' functions is to protect the creatures from predators, Fabry says. Whether and how well they will be able to survive without their shells is unclear.

"Gene expression studies can start to forecast the impact." --Gretchen Hofmann

Other researchers have observed similar effects in calcifying creatures such as corals, mussels, and shelled phytoplankton called cocolithophores. Now, armed with a recent National Science Foundation grant, Fabry and Hofmann plan to go one step beyond observational studies to sequence the pteropod genome. Rather than adding pteropods to the list of species to be tackled by the Joint Genome Institute's (JGI) Community Sequencing Program, a process that can take years, the duo will work with 454 Life Sciences, whose technology can sequence several million base pairs per hour. The platform lacks the accuracy and precision of JGI's sequencing, but it has its benefits. "You can sequence the entire genome in an afternoon," says Hofmann. The aim: To identify genes and molecular processes that calcifying organisms might use to buffer themselves against the effects of climate change. "Molecular tools are one of the few ways to start quantifying adaptation," says Fabry.

Most marine organisms can adapt easily to sudden increases in alkalinity, which occur naturally under conditions such as algae blooms, notes David Hutchins at University of Southern California, who studies the effects of ocean carbon dioxide levels on nitrogen fixation in phytoplankton. "The problem is, there is really no precedent for acidification, so we don't know how much they can tolerate." So far, most studies have examined physiological responses to rising acidity under short-term laboratory conditions, but the change will happen gradually, over decades. Gene expression studies can start to "forecast the impact," says Hofmann, by identifying mechanisms that regulate the plasticity of organisms to environmental effects. Such studies will also help determine a "tipping point," an acidity level above which adaptation won't be possible, says Fabry.

Pteropods are a staple menu item for an enormous range of organisms, including other plankton, salmon, and whales, so negative effects on their survival are likely to reverberate across the food chain. Moreover, shell building in calcifying marine organisms plays a major role in Earth's "biologic pump" - the movement of elements such as carbon and nitrogen through the air and ocean.

For now, Hofmann is working on the California sea urchin, whose sequence was completed in 2006, and includes classifications of genes by function. In a study recently submitted for publication, her lab looked at a handful of biomineralization and stress-response genes in sea urchin embryos exposed to acidification levels under IPCC's predicted best-case and worst-case scenarios and to different temperatures. Using quantitative PCR, they found that under conditions of high carbon dioxide levels, biomineralization genes remained unaffected, but HSP70, a key stress-response gene, appeared less able to respond to increases in temperature. So, even if biomineralization can proceed when acidity rises, Hofmann says, there is still "a cost of the altered environment to the embryo." Her lab is using a custom-made microarray to test the expression of 1,200 sea urchin genes involved in biomineralization, cellular stress response, and cell signaling under IPCC-predicted conditions. "That's a tool that would be really neat to use with the pteropod, once we have the sequence," she says.

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