HARVARD GAZETTE ARCHIVES
Oceans key to global warming:
How much man-made carbon dioxide can the Earth handle?
By Elizabeth Gehrman
Special to the Gazette
According to the Environmental Protection Agency, these are a few of the things we know about global warming: The average land-surface temperature of the Earth has risen by 1 degree Fahrenheit in the past century. Precipitation has increased by about 1 percent, and the sea level has risen 6 to 8 inches, in part due to the melting of mountain glaciers.
What we don't know is what these numbers - seemingly tiny increments - mean to the ecology and to human society. How will global warming affect plant growth? What about animal populations? Will entire cities be buried by seawater? Will we have to give up our cars?
One of the largest unknowns about global warming is, How much of an overload of man-made carbon dioxide can the Earth take? And the answer to that question probably lies in large part in the deep salt waters that cover approximately 71 percent of the planet. The oceans are the largest global-storage reservoirs of carbon on Earth besides rocks, and our first line of defense in any short-term process affected by human interference.
'Wait a minute . . . there are archaea'
"We have so little idea at this point of what goes on in 90 percent of the depth of the ocean," says Ann Pearson, an assistant professor of biogeochemistry in Harvard's Earth and Planetary Sciences Department. "We're trying to get ... baseline knowledge of what kinds of organisms live there and what they do if we're going to have any prayer of really being able to say what's going to happen in the short term."
To that end, Pearson is studying archaea, part of the domain of single-celled, nucleus-free microscopic organisms called prokaryotes that, despite having been discovered as late as 1977, have been elevated to the status of one of the three branches of life on Earth, along with eubacteria and eukaryotes.
"For a very long time when looking at ocean life people only thought about macroscopic organisms," says Pearson. "They thought about phytoplankton, they thought about zooplankton. Then the perspective broadened, and bacteria were recognized as a huge part of the food chain - they exude a certain amount of carbon into the water column . . . "
So, bacteria were taken into account, and for a time that was considered sufficient, until, says Pearson, "somebody else came along and said, 'Wait a minute. In the prokaryotic world there are bacteria and there are archaea.' "
For years, scientists thought archaea contributed little to the ecology because they were considered extremophiles, living only in places such as geysers and radically salty or acid water. As such, they were believed to be, much like the first snowboarders, simply a fringe element in ecological society. But a decade or so ago, scientists began noticing archaea turning up in their surveys of ocean life at moderate temperatures. "Actually there are all kinds of archaea free-floating in the water column," says Pearson, "which is about the least extreme environment you can get on Earth. In fact, it's probably the most stable."
Cousins of bacteria
And the archaea were not just visiting. "It turns out that Group I archaea" - called Crenarchaeota - "make up as much as 50 percent of the prokaryotic life in the deep ocean, beneath 1,000 meters," Pearson says. They've also been found in the shallow and midwaters of polar, temperate, and tropical latitudes, in the sediment on the ocean floor, even in the gut of a certain kind of sea cucumber. "The majority of the volume of the world's oceans is half bacteria and half Group I archaea," Pearson says, "and nobody knew that until just a couple of years ago."
But now that we know they're there, the question becomes why. What are they doing there? And how do these minute creatures relate to global warming? Well, for one thing, even though each individual cell is minuscule, together they make up half the microbial biomass of the sea. For another, according to Pearson, "It's very hard to say that the models of carbon dynamics that we're using right now, trying to figure out what's going to happen in terms of global climate change, are actually accurate" - partly because they don't include the contributions of such a populous lifeform.
It's possible, she adds, that the archaea are, like their cousins the bacteria, simply heterotrophic organisms - that is, those that obtain food from organic material - consuming the last bits of carbon that float down from the photic zone, where sunlight penetrates enough to allow photosynthesis. But it's unlikely. Pearson believes many archaea are actually autotrophic: Just as plants make their own food through photosynthesis, archaea may make their own food through chemosynthesis, producing new biomass by converting carbon dioxide into organic carbon in the absence of sunlight.
To find out, she plans to use a technology she and Wood's Hole scientist Tim Eglinton developed while working at Wood's Hole Oceanographic Institution's accelerator mass spectrometry facility in 1996. While most radiocarbon dating works from a single source, such as a wood chip or a shell, Pearson developed a method of dating individual molecules. She can take a heterogeneous sample, such as marine sediment, extract it chemically into the different classes of molecules it's composed of, and figure out where each of those individual molecules came from. "It's very important in terms of global carbon distribution," she says, "to know whether the majority of marine sediment comes from terrestrial or biological production, such as trees, soil, and erosion, or whether it comes from marine surface production or plankton."
Bringing the deep ocean to Harvard
Unlike bacteria, archaea cannot be cultured in the lab; because so little is known about them, efforts to propagate them have failed. They must be studied in their natural environment. Since one can't bring a microscope down into the deep ocean, Pearson is doing the next best thing: She's bringing the deep ocean to Harvard. In February, she spent two weeks at the National Energy Laboratory in Hawaii, where surface, midwater, and deep water pipelines pump 27,000 gallons a minute. Once she recovers enough archaeal compound from the filters, she will do isotopic work on those and, as comparative samples, on the bacteria collected as well.
The third phase of her research moves away from the water collected in Hawaii to sponge samples gathered in the California lab of Edward DeLong, who is considered the foremost expert on the abundance of archaea in nonextreme circumstances. The sponge, a host organism to a species of symbiotic archaea, is the closest thing to a culture scientists have. Pearson intends to take samples from it and look for proteins and genes that would point to specific metabolic pathways indicated by the isotopic analyses. This will allow her to say, she explains, "this species in that environment does that. There are very few people who have tried to actually figure out what archaea do, because the tools simply aren't there. So one of the things that I do is try to develop those tools that link the biological question with the geochemical approaches to answer these questions. That's why I'm in the geology department."
Regarding industrialized nations' emissions of carbon dioxide into the atmosphere, Pearson says, "We're talking messing up the entire global ecosystem down to the microbial level. If you affect the climate the entire ecology will be altered. There is no separation between the two. Anybody who thinks there is, is missing the point."