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Wednesday, July 29, 2009

Michael Cunliffe, Peter Liss, Oliver Wurl: Sea-surface microlayer habitant is distinct phytoplankton ecosystem, ocean-atmosphere interface

Scientists Find a Microbe Haven at Ocean’s Surface

A LITTLE OFF THE TOP Little Kilo Moana skims thin layers from the ocean’s surface near its namesake, a University of Hawaii research vessel. Scientists have learned that the top hundredth-inch of the ocean is an ecosystem all its own. (Photo: Masaya Shinki)

The world’s oceans are like an alien world. The National Oceanic and Atmospheric Administration estimates that 95% of them remain unexplored. But the mysteries do not start a mile below the surface of the sea. They start with the surface itself.

Scientists are now discovering that the top hundredth-inch of the ocean is somewhat like a sheet of jelly. And this odd habitat, thinner than a human hair, is home to an unusual menagerie of microbes. “It’s really a distinct ecosystem of its own,” said Oliver Wurl, of Canada’s Institute of Ocean Sciences.

This so-called sea-surface microlayer is important, scientists say, in part because it influences the chemistry of the ocean and the atmosphere. “One of the most significant things that happens on our planet is the transport of gases in and out of the ocean,” said Michael Cunliffe, a marine biologist at the University of Warwick in England. The ocean stores a large fraction of the global-warming gases we produce; at the microlayer, the gases are pulled down.

“It’s the ocean breathing through its skin,” Dr. Cunliffe said.

Sailors have long known that the surface can be covered with oily slicks (hence the phrase “pouring oil on troubled waters”). But when scientists began studying the surface in the mid-20th century they found it vexing. A scientist cannot just dunk a bucket into the ocean without dredging up deeper water as well. “Even defining the surface is hard, since it’s moving up and down,” said Peter Liss, a professor of environmental sciences at the University of East Anglia in England.

So scientists had to invent some tools to skim the surface. Dr. Liss and his colleagues, for example, chill a piece of glass with liquid nitrogen and lower it into the sea, freezing water it contacts.

These tools have allowed scientists to discover that the top hundredth of an inch is chemically distinct. It is loaded with molecules carried up by air bubbles and concentrated at the surface.

Recent surveys carried out by Dr. Wurl and his colleagues have revealed that the microlayer has a rich supply of sticky clumps of carbohydrates. These carbohydrates are made by single-cell organisms called phytoplankton that live lower in the ocean to stick together in colonies. Eventually the carbohydrates break off the phytoplankton and clump together. Dr. Wurl’s studies indicate that many of them rise to the microlayer, forming a film.

“I really imagine it as tiny pieces of jelly floating on the ocean,” Dr. Wurl said.

It may be hard to imagine such a fine coat of slime holding together for long on top of the heaving ocean. But Dr. Wurl has found that it is quite durable. “We have collected microlayer samples with wind conditions of 16 to 18 knots,” he said. “It’s not pleasant to be in a small boat at that wind speed. That tells us the microlayer is pretty stable.”

Dr. Wurl and his colleagues report the findings in a paper to be published in the journal Marine Chemistry. He suspects that when waves disrupt the jellylike microlayer, air bubbles deliver sticky material back to the surface.

Dr. Cunliffe, who has replicated Dr. Wurl’s results, argues that these studies mean that the microlayer is a special kind of habitat for microbes. The gelatinous film calms the turbulence in the microlayer, which may make it easier for bacteria to attach to the particles and feed on the molecules flowing past.

To document the sort of microbes that live in the microlayer, Dr. Cunliffe and other researchers are collecting surface water, breaking open the cells it contains, and sequencing the genes they hold. They compare the microlayer residents to the microbes that live a few inches deeper.

“We’re finding consistently different communities,” Dr. Cunliffe said. The microlayer communities are dominated by groups of microbes well known for forming biofilms on more familiar surfaces, like rocks in streams, our teeth and the insides of sewer pipes.

“They’re always the usual suspects,” Dr. Cunliffe went on. “If our hypothesis is correct, it makes complete sense.”

Dr. Liss called the finding “a really interesting result, because it shows that the microlayer is a really different environment.”

Scientists say it is important to become better acquainted with this mysterious ocean skin, because it may play a critical role in the environmental well-being of the planet. Studies have shown, for example, that pollutants like pesticides and flame retardants can be trapped in the microlayer.

Dr. Cunliffe and his colleagues have identified bacteria in the microlayer that devour important chemicals like methane and carbon monoxide. The microlayer is also crucial to the ocean’s ability to absorb carbon dioxide, a potent greenhouse gas.

“It’s actually sucking the carbon dioxide down into the water column,” Dr. Cunliffe said.

Dr. Liss said the microlayer was “clearly important, because it’s where the ocean and the atmosphere interact.”

“But it’s difficult to study,” he added, “so it hasn’t received as much attention as it ought to.”


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