The top predators¹ of the Southern Ocean, far-ranging seabirds, are tied both to the health of the ocean ecosystem² and to global climate regulation through a mutual³ relationship with phytoplankton（浮游植物）, according to newly published work from the University of California, Davis. When phytoplankton are eaten by grazing crustaceans⁵（甲壳纲动物） called krill（磷虾）, they release a chemical signal that calls in krill-eating birds. At the same time, this chemical signal -- dimethyl sulfide, or DMS -- forms sulfur⁶ compounds in the atmosphere that promote cloud formation and help cool the planet. Seabirds consume the grazers, and fertilize⁷ the phytoplankton with iron, which is scarce in the vast Southern Ocean. The work was published March 3 in the Proceedings⁸ of the National Academy of Sciences.

 

"The data are really striking," said Gabrielle Nevitt, professor of neurobiology, physiology⁹ and behavior at UC Davis and co-author on the paper with graduate student Matthew Savoca. This suggests that marine¹⁰ top predators are important in climate regulation, although they are mostly left out of climate models, Nevitt said.

 

"In addition to studying how these marine top predators are responding to climate change, our data suggest that more attention should be focused on how ecological¹¹ systems, themselves, impact climate. Studying DMS as a signal molecule¹² makes the connection," she said.

 

Nevitt has studied the sense of smell in ocean-going birds for about 25 years, and was the first to demonstrate that marine top predators use climate-regulating chemicals for foraging¹³ and navigation over the featureless ocean. DMS is now known to be an important signal for petrels（海燕） and albatrosses（信天翁）, and the idea has been extended to various species of penguins¹⁴, seals, sharks, sea turtles, coral reef fishes and possibly baleen¹⁵ whales, she said.

 

Phytoplankton are the plants of the open ocean, absorbing carbon dioxide and sunlight to grow. When these plankton⁴ die, they release an enzyme¹⁶ that generates DMS.

 

A role for DMS in regulating climate was proposed by Robert Charlson, James Lovelock, Meinrat Andreae and Stephen Warren in the 1980s. According to the CLAW hypothesis, warming oceans lead to more growth of green phytoplankton, which in turn release a precursor¹⁷ to DMS when they die. Rising levels of DMS in the atmosphere cause cloud formation, and clouds reflect sunlight, helping¹⁸ to cool the planet. It's a negative feedback loop to control the planet's temperature.

 

Savoca and Nevitt looked at 50 years of records on seabirds' stomach contents, combined with Nevitt's experimental results of which species use DMS to forage¹⁹. They found that species that respond to DMS overwhelmingly fed on krill, which graze on phytoplankton.

 

On land, there are several known examples of plants that respond to attack by insects by producing chemicals that attract predators that eat those insects. Nevitt and Savoca propose the same thing happens in the open ocean: when phytoplankton come under attack by krill, the DMS released as they die attracts predators that eat the krill.

 

The birds have something else to contribute. The Southern Ocean lacks large land masses, and is relatively²⁰ poor in iron, which has to be washed out from land by rivers. Krill are rich in iron, but birds can absorb relatively little of it -- so the rest is excreted back into the ocean, promoting plankton growth.

 

The work suggests that by linking predatory seabirds and phytoplankton -- the top and bottom levels of the food chain -- DMS plays an important role in the ocean ecosystem, which affects climate by taking up carbon, as well as a physical role in generating clouds, Nevitt said.

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