Posted on: Thursday, August 9, 2001

Bacteria fertilizing change in oceans

By Jan TenBruggencate
Advertiser Science Writer

A group of single-celled organisms is fertilizing the oceans in ways science has not previously understood, and they may be increasing their numbers around the Hawaiian Islands.

The discovery, made 60 miles north of O'ahu, is driving major changes in the understanding of how life in the oceans works.

For Hawai'i, researchers are able to show there are large-scale changes occurring in local waters, but they can't yet say what that will mean to those who live on land.

One key change is the increasing realization that the tiniest organisms have a dramatic effect on the oceans — much greater than anyone thought, said Dave Karl, professor of oceanography at the University of Hawai'i and co-author of an article yesterday in the journal Nature that describes the finding.

Researchers led by University of California at Santa Cruz scientist Jonathan Zehr found that some single-celled cyanobacteria — organisms once known as blue-green algae — are actively processing nitrogen.

They convert it from the form in which it appears in the atmosphere and dissolved in the ocean to a compound that marine plants can use as a kind of fertilizer.

The process, Karl said, is similar to that used by bacteria in legumes and certain other land plants to convert atmospheric nitrogen into usable nitrogen. It is called "fixing" nitrogen.

The cyanobacteria produce roughly twice as much nitrogen as they need, making large amounts available to other organisms.

Scientists collected bacteria from the waters at a research station north of O'ahu. Since 1988 oceanographers have collected samples and data monthly by ship and using a mooring at the site. The continuing series of experiments is called the Hawai'i Ocean Time-series.

Recently, scientists from California, Hawai'i, the Woods Hole Oceanographic Institute in Massachusetts and the Georgia Institute of Technology isolated the cyanobacteria out of water samples collected at the site.

These bacteria are so small that it takes a microscope with 1,000-power magnification to see them, and even then, Karl said, they all look alike.

"They're just little balls of organic matter," he said. "But we're finding that they are fundamentally different from one another."

Using new genetic analysis techniques, the researchers were able to show that some of the cyanobacteria are able to perform two key transformations. Like other cyanobacteria, they conduct photosynthesis, using sunlight to process carbon in the water. But they can also take nitrogen molecules, made up of two nitrogen atoms, and convert them into a form of ammonia, which Karl said is "the biogenetic precursor to all nitrogen compounds in organisms."

The researchers found that these nitrogen fixers were fairly common. There are as many as 10 million of them in a quart of sea water.

Karl said there is evidence the numbers of these cyanobacteria may be increasing around Hawai'i as the result of global warming. One of the results of oceanic and atmospheric warming is less mixing between the warm surface ocean and the deeper waters, a process oceanographers call stratification.

"These organisms tend to live better under stratified conditions," he said.

What isn't clear is how that will affect the oceans from the perspective of humans. More nitrogen-fixing cyanobacteria could mean more fertilizer for oceanic plantlife, which could then process more carbon and grow faster, resulting in more food for fish, and eventually more fish.

But maybe not.

"We know that the ocean is processing more carbon, making more food. We don't know yet whether it is making more fish," Karl said.

Much of that carbon could simply be sinking to the ocean depths. That's potentially good in terms of pulling carbon dioxide out of the atmosphere and removing one of the main factors implicated in global warming. But it could also mean the carbon isn't going into increased fishery production.

Furthermore, cyanobacteria themselves can have negative effects, and more of them might be a bad thing. Many contain toxins that make them unpalatable to other species, and some are problems for humans directly. The organism responsible for "swimmer's itch," an itching sensation all over one's body, is a cyanobacterium, for instance, he said.

For science, the discoveries open up new realms for research. "The closer we look at the oceans, the more important the tiniest organisms appear to be," wrote Nature writers Jed Fuhrman and Douglas Capone.