Acidification may halve coral class of 2050

Experiments suggest big drop in egg fertilization and larval settlement

By midcentury, growing acidification of the world’s oceans may undermine sexual reproduction in elkhorn coral badly enough to halve the supply of youngsters settling down to build reef.

NIGHT SPAWN On one night after an autumn full moon, elkhorn corals across much of the Caribbean simultaneously release sperm-and-egg bundles that mingle and then break apart as sperm finds eggs from different parents. Coral fertilization may decline as increasing carbon dioxide pushes seawater closer to acidity. Evan D”Alessandro/RSMAS, University of Miami

Two juvenile elkhorn corals only 26 days old have survived a sojourn in the open water but have successfully settled onto a surface to begin their life’s work of building a colony. Rebecca Albright/RSMAS, University of Miami

Acidification, which happens as increasing levels of atmospheric carbon dioxide dissolve in the ocean and form acid, is expected to threaten established coral reefs worldwide in coming decades. In tests with seawater modified to reflect conditions expected later this century, sperm of the coral Acropora palmata successfully fertilized eggs 13 percent less often on average compared with sperm in today’s seawater, says Rebecca Albright of the University of Miami. In some of these tests using low sperm concentrations, which Albright suspects are more realistic, fertilization success dropped by as much as 64 percent.

Making that decline even more worrisome were tests indicating trouble with the next step in successful coral reproduction. With the same modified seawater, larvae from fertilizations that did succeed had more trouble settling successfully on a reef, Albright and her colleagues report in a paper to be posted online the week of November 8 in the Proceedings of the National Academy of Sciences. Settlements dropped by 45 percent.

“It’s important to remember there are compounding effects,” Albright says, referring to studies that have focused on life stages separately instead of combining consequences.

Since the beginning of the industrial revolution, global seawater has dropped from about 8.2 on the pH scale to between about 8.05 and 8.1, not low enough to push seawater from a base to an acid but enough to have biological effects. Each unit on the scale reflects a shift in acid concentration by a power of 10, so the effect to date has been about a 30 percent increase in acidity.

For the tests, Albright and her colleagues bubbled carbon dioxide gas into natural seawater to mimic the predicted ocean chemistry for a world where emissions have driven atmospheric carbon dioxide from the current concentration of about 387 parts per million up to 560 ppm, an increase many climate scientists expect by midcentury.

She and her colleagues also tested a case of atmospheric carbon dioxide levels reaching about 800 ppm by 2100. Elkhorn coral fertilization and settlement suffered even more at those concentrations. The pH change could reduce the supply of youngsters getting down to work by 73 percent, the researchers say.

Sexual reproduction is not the only way corals expand. Individuals can clone themselves, but sex maintains the genetic diversity that researchers hope will help corals cope with a disrupted environment.

“It is a big deal if you lose sexual reproduction even in species with very effective means of reproducing asexually,” says ecologist Steve Gaines of the University of California, Santa Barbara. In corals and most other invertebrates, the sexually produced offspring are the ones that colonize new places.

“One of the limits with this kind of study is that it doesn’t tell you whether there is any potential for evolutionary changes to deal with the new stress,” Gaines says. In the real world, organisms might be able to adapt to changes. He notes, however, that the environment now is changing unnaturally fast.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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