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Spring 1993

Research Magazine > ARCHIVE > Spring 93 > Article

Mending Holes in the Ocean's Food Web
by Cara D. Runsick

Lawrence Pomeroy didn't set out to undermine his own theory.

But in the near-freezing waters off the Newfoundland coast, the ecologist found bacteria behaving in a way that challenged his landmark model of how energy and food circulate through the ocean.

"My point now is, `Whoa, wait a minute; it doesn't do everything,'" Pomeroy said of his microbial food web theory, which is now standard fare in marine ecology textbooks. "There are times when it doesn't work, and there are things it doesn't explain."

What has given the zoologist cause to pause is the odd behavior of some microorganisms near the Grand Banks of the North Atlantic.

In early spring, when an immense bloom of algae releases a buffet of nutrients into the water, nearby bacteria don't do what they're expected to do: They don't eat.

The microbial food web theory suggests that they should. In fact, it suggests they should eat heartily.

Pomeroy is determined to find out why they don't. Along the way, what he is learning about the ocean's bacteria -- from frozen Arctic ice caps to the balmy Gulf Stream -- is not only challenging his original theory, but it may change the way scientists view the basic biology of bacteria.

"I'm not interested in painting by the numbers," said Pomeroy, an Emeritus Alumni Foundation Distinguished Professor of Zoology at the University of Georgia. "I'm interested in sketching out the big picture."

A Loop in the Chain

Before Pomeroy entered the picture, the traditional scientific model of the ocean food web was pretty straightforward: Energy from the sun gave life to tiny marine algae, which were eaten by slightly bigger grazing animals, which were consumed by small fish or shrimp, which in turn were meals for larger fish, and so on up the line through larger marine animals like seals or sharks.

Under that scientific model, microorganisms like bacteria were thought of as the ocean's sanitation system, deriving their living from feces and other excretions, but not contributing much to the ocean's overall productivity.

But Pomeroy challenged the accepted model with his notion of a microbial "loop" in the food web. His landmark 1974 article, The Ocean's Food Web: A Changing Paradigm, proposed a more complex system. The microbial food web theory retained the traditional pathways, but also suggested that bacteria siphon off much -- even as much as half -- of the ocean's energy.

Because bacteria reproduce very quickly, the microbial food web could be one of the most productive portions of the marine environment.

Bacteria That Don't Behave

That should also be true of bacteria in the Newfoundland spring. As the March sun begins to warm the swells of the North Atlantic, algae start their spring bloom, reproducing at high rates and releasing nutrients into the chilly water around them.

According to Pomeroy's original model, hungry bacteria encountering such a feast would start eating immediately and reproduce quickly. They would process algal waste and return nutrients to the algae in altered forms, which in turn would accelerate the algal bloom.

But in the Labrador Current, they don't. There the bacteria eat very little and reproduce very slowly. Instead, they just drift along. Dying algae simply fall to the bottom where they are consumed by bottom-feeding shrimp and fish -- creatures that are several steps up the food web.

"All of the stuff the bacteria would have been consuming was falling to the bottom and was available for the animals there," Pomeroy said. "It was probably contributing to the very resilient and productive fisheries that they have on the Grand Banks."

Temperature vs. Temptation

Pomeroy and Don Deibel, a former UGA graduate student and now a scientist at Memorial University of Newfoundland, set about studying the eating habits of the bacteria. They described the situation as "a two-factor problem."

Different types of microbes will grow fastest at different temperatures and with specific amounts of food. The optimum amount of food for growth also will vary with the temperature. In particular, if their surroundings are very cold, bacteria need more food to grow at the same rate.

The bacteria Pomeroy studied near Newfoundland took about a month to double their number, even when the blooming algae provided abundant food. In the luxurious conditions of a lab, similar bacteria could double in as little as 20 minutes.

When the spring bloom begins, water temperatures in this subarctic area of the Atlantic still hover around freezing, near the lower end of the range at which the bacteria can grow. At these frigid temperatures, the bacteria need even higher concentrations of nutrients than the blooming algae provide.

"We're now showing that there are times when the microbial food web is virtually turned off. These are winter or early spring times, when the water is still cold," Pomeroy said.

If all this sounds interesting only to bacteria and biologists, consider what it could mean if scientists are right about the much-heralded trend toward global warming.

Even slightly warmer water at the right time of year could stimulate bacterial feeding and "have significant ramifications all the way through the rest of the food web," Pomeroy said.

Because current spring temperatures are marginal -- allowing only a few bacteria to reproduce -- an increase of only a few degrees could allow the rest to "turn on" when the algal bloom peaks. Then they might consume all of the nutrients they currently ignore in the spring, leaving considerably less food for other ocean creatures.

A Global Condition

If the bacteria's behavior were isolated to the Grand Banks of Newfoundland, perhaps fewer people would take notice.

But Pomeroy's research is truly global, including sites in Antarctica, Australia, Japan, Canada's Northwest Territories and the remote Eniwetok Atoll in the Marshall Islands.

It was much closer to home -- just about 75 miles off the Georgia coast -- that Pomeroy and UGA colleague Bill Wiebe recently sampled and tested ocean bacteria to see how they behave in warmer water.

"Lo and behold, the same two-factor problem applies; it's just that you shift the temperature range," Pomeroy said. "At winter temperatures, the bacteria off Georgia are as inactive as the microbes of the Newfoundland spring."

These results come despite the fact that the winter ocean temperatures off Georgia are higher than those of the warmest summer water near Newfoundland.

"This means that a global temperature change could have a profound effect on food webs all over the world, not just in very cold regions," Pomeroy said.

Wiebe and Pomeroy carried their work a step further, to the bacterium, E. coli, a resident in the human digestive tract. E. coli grows best at body temperature, which is much warmer than the ocean at any time.

"The E. coli have the same problem, it's just in a different ballpark," Pomeroy said. "You have to shift both the temperature scale and the food concentrations." This implies that the bacteria's shift in food requirements may not be a glitch in an isolated system, but a condition present in many types of microbes.

In some scientific circles, Pomeroy's conclusions about these exceptions to the food web theory may be as startling as his initial theory was in 1974.

"There is always skepticism, there is always debate, and there are those who think we have overspeculated," Deibel said.

But Pomeroy and colleagues consider the new findings an opportunity to expand the older theory into specific situations.

"I see this as an evolution of the 1974 theory," Deibel said. "That theory said that bacteria are important [in the food web]. The more recent work presents one major set of circumstances under which they are not important. The '74 paper did not address this. It didn't go into this level of detail."

The level of detail is an essential factor in expanding on Pomeroy's research. Much more information is needed, Pomeroy said, before specific predictions can be made about how bacteria will behave in varying climates. It may be that the microbial loop of the food web will turn off in winter, but remain active in summer. In that case, the winter food web may operate much the way the pre-1974 theory described the overall food web.

"Right now we could guess," Pomeroy said. "I could give you a scenario, but I can't really quantify it well enough to glorify it with a model. We're trying to develop the kinds of data that will permit us to do that."

Where Do We Go From Here?

For researchers like Pomeroy, these findings are opportunities rather than obstacles. Pomeroy, in particular, is excited about the new results.

"I can't walk away from it right now," he said. "I'm going to stay with it, push it at least far enough that I've convinced the scientific community that this is a real problem."

As an elder statesman of ecology, Pomeroy knows that he is posing questions others will be investigating far into the future.

"The generation before me really posed most of the questions that we're still trying to answer," Pomeroy said. "By degrees, we're getting more powerful technology to put it all together.

"People always did think about all of the parts of a system, but they didn't formally write them down as a model, and they certainly didn't put them into a computer and grind it out," he said.

Pomeroy said he also sees a shift in scientific concerns. Global issues are moving to the forefront and are being modeled in increasingly complex ways.

"I think the importance of looking at global systems is going to drive a lot of work in the next decade or two," he said.

Once Pomeroy makes such an observation, many scientists would agree they might as well get started, because, as Deibel said, "Pomeroy has an amazing ability to synthesize information from different sources, different people, and papers -- to absorb detail and distill it into pioneering visions of where it is we ought to be going."

Cara Runsick, who has earned a master's degree in microbiology from the Georgia Institute of Technology, is a graduate student in journalism at the University of Georgia.

Oceanography By Snowmobile
by Cara D. Runsick

Sixteen-gauge shotguns aren't exactly standard equipment in most scientific laboratories. But then, most labs aren't frequented by polar bears.

Such are the hazards of oceanography in the Arctic.

"I said, `What is the gun for?' and he said, `The polar bears,'" recalled Dr. Bill Wiebe of his first trip with geochemist Steve Macko and UGA zoologist Lawrence Pomeroy to study the activity of bacteria locked in ocean ice.

"We're on the ice drilling a hole so we can get samples," said Wiebe, a UGA microbiology professor. "I've got gloves on; he has gloves on. The gun, in order to keep it warm, is in a tent, in a scabbard. A polar bear could come up and tap you on the shoulder and not ever have a problem."

The problems, instead, are for the scientists, who must bear bitter temperatures in their search for answers to why bacteria sometimes don't behave the way the microbial food web theory suggests they should.

On expeditions within 600 miles of the North Pole, Pomeroy and his colleagues have uncovered evidence that supports a revised version of the theory -- that both temperature and availability of food affect whether the bacteria become active in the ocean's food web.

It was by fishing through six feet of ice that Pomeroy and Macko, a professor at the University of Virginia and chief scientist and organizer of the Arctic trips, found that bacteria in Arctic sea ice- unlike those floating in the ocean near Newfoundland- indeed are active in algal blooms that take place in the ice.

Working in such conditions is something of an adventure, said Pomeroy and Macko. "They fly you in and you live on an iceberg. You go out in a snowmobile and drill a hole in the sea ice," Pomeroy said.

When the spring sun shines through the sea ice, algae and other organisms actually grow in brine pockets inside the ice, producing amino acids that the bacteria can consume. Under these conditions, the ice-bound bacteria thrive.

The amino acid levels, in fact, were above the concentration required to grow Artic Ocean bacteria in the laboratory at the temperature of sea ice.

"These findings also support the revised version of the microbial food web," Pomeroy said. "Because when extremely large amounts of food were available, the bacteria were able to reproduce rapidly in spite of the cold."

And in spite of the cold, Pomeroy and his colleagues still have fond feelings for their Arctic laboratory -- polar bears notwithstanding.

"You're walking around a place no one has ever seen. There is actual quiet -- no wind, no human sounds, no animal sounds," Macko said.

The complete quiet, combined with the 24-hour daylight, can have strange effects on workers there, he said.

"You'll be working and decide that maybe it's time to go to bed," Macko said. "You'll look down at your watch and it's 3 a.m."

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