In late 2007, hundreds of dead seabirds washed up on the shores of the Monterey Bay, just 45 minutes south of San Francisco. Their feathers were covered in grease, much like oil spill victims. Yet rescue workers were left scratching their heads when no petroleum could be found in local waters. "There were no confirmed reports of oil. That was the first thing that the bird people looked for," said Raphael Kudela, an oceanographer at the University of California, Santa Cruz.

To investigate the strange incident, Kudela teamed up with researchers from the California Department of Fish and Game (CDFG), the Monterey Bay Aquarium Research Institute (MBARI), and the Moss Landing Marine Laboratories (MLML). Last Friday, the team published the first report of the incident online in the academic journal Public Library of Science One.

Mystery oil acts like soap

Eighty miles north of where the birds were found, the Cosco Busan oil spill had just dumped 50,000 gallons of oil into the San Francisco Bay. The two species that kept popping up on the shores of Monterey were loons and grebes—the very birds most commonly affected by winter oil spills in the region. Yet birds usually wash up in the immediate vicinity of a spill, said Kudela, and no patches of petroleum could be located in the Monterey Bay.

To make matters more confusing, researchers soon discovered the yellow-green slime that coated the birds' feathers was not petroleum-based. Like oil, the mystery substance stripped the natural waterproofing from the feathers, killing 207 birds and leaving another 350 soggy, hypothermic, and unable to hunt for food. Further tests confirmed the slime was protein-based—it had come from a living creature, not an oil well.

The researchers now say the mystery slime is a soap-like protein made by the infamous Red Tide algae, which happened to be in full bloom along the central coast in late 2007. "We knew that generally, all [algae in Red Tide's class] are capable of producing proteins like this (including this particular Red Tide), but we had not previously seen this type of an impact."

Usually, soap-like substances, called surfactants, consist of circular clusters of lipids with opposite charges on either end. While Kudela and his colleagues were not able to identify this specific compound's structure, the protein they isolated behaves much like a surfactant. "It's highly polar, and forms foam," says Kudela, who believes the structure consists of two proteins linked together.

Studies with the algae made the protein look even more suspicious. When samples were swished around in beakers, a sticky, slimy foam formed. "We grew the algae in the lab, and when we shook it up it produced foam," says Kudela. Reports of a dense sea froth had accompanied the incident, and researchers speculate the waves in the ocean acted like a blender. Because the water was churning hard when the bloom peaked, the algae were crushed open, spilling the proteinaceous soap into the water.

(Top row) clean feathers dipped into Akashiwo sanguinea culture grown in the lab, but with no foam. (Bottom) the same feathers dipped into the culture after shaking vigorously to produce foam, showing the matting/wetting of the feathers.

Credit: Raphael Kudela.

The frequency, size and duration of Red Tides have increased substantially around the world since 2004, and researchers link the rise to climate change and the impact on surface water temperatures

The final test involved dipping clean feathers into clean and foamy water samples. It turns out the Red Tide by itself didn't damage the feathers—the foam had to be present. "We were able to demonstrate that normal bird feathers dipped in the foam became water soaked (this didn't happen in normal sea water)," says Dave Jessup, of the California Department of Fish and Game, Office of Spill Prevention and Response. Jessup and Kudela worked collaboratively on the project.

"We tested deionized water, laboratory cultures of algae with no foam, and foam we 'made' in the laboratory with [Red Tide] algae. We also tried mixing in beach sand, to make sure it wasn't something else. In each case, it was the algal foam that caused the problem," says Kudela.

Researchers speculate the foam concentrates the soap-like proteins, making it possible for them to coat the feathers. Without this froth, the same proteins in seawater simply wash off.

Warming oceans may mean more slime

"This is an interesting paper because it brings to light a previously unreported mechanism whereby harmful algal blooms (HABs) can have deleterious impacts on the health of marine animals," says David Caron, a biologist at the University of Southern California who did not contribute to the research findings. "We are presently witnessing an increase in the number and severity of HAB events, and it is essential to document and fully understanding any linkages between harmful algal blooms and ecosystem health," he says.

The frequency, size and duration of Red Tides have increased substantially around the world since 2004, and researchers link the rise to climate change and the impact on surface water temperatures. Red Tide is a dinoflagellate, and goes by Akashiwo sanguinea in scientific circles. It turns the water a rusty-brown color, and often produces toxins that harm humans and wildlife.

Red tide, or Akashiwo sanguinea, turns water a rusty-brown color.

Credit: Raphael Kudela.

The winter 2007 bloom proved to be benign, with the exception of the foam. Even the sticky slime proved not to be directly toxic. "It is, however, very harmful to birds," adds Kudela. Birds may not be the only victims, though. A study published in 1990 linked Red Tide to a mysterious slime that smothers coral reefs in Costa Rica and Panama.

Kudela believes the findings may help answer questions about other mysterious bird-stranding events. Sometimes a few dozen birds wash up coated in oil, but no spill can be found. In other news reports, a stream of petroleum appears on the surface of the water, even when there has been no recent leak. As shipwrecks sometimes take decades to begin leaking oil, mystery bird strandings are often blamed on unidentified sunken ships, even when there is no conclusive evidence of petroleum in the water or any reason to believe the oily birds actually floated above the site of a wreck.

Kudela believes Red Tide may play a role in at least some of these cases. “There was another 'mystery spill' in Monterey about 10 years ago, during a red tide, that caused bird strandings,” says Kudela “so we think this has occurred in the past, but has never been associated with Red Tides.”

After testing samples taken from 283 dead birds, Kudela was able to rule out the presence of any trace petroleum, as well as a long list of other harmful algal byproducts. Water samples were similarly clean, save for the Red Tide and mystery protein.

Of the 350 rescued animals, about 40 percent recovered and returned to life at sea.

PLoS ONE, 2009. DOI: 10.1371/journal.pone.0004550