Life as we don't know it

Biologists are getting a new perspective on microbial life by studying it indirectly. Detailed examination of an unpromising habitat can uncover an unexpected hospitable environment. Comprehensive analysis of residues of life-related chemicals can reveal unsuspected diversity. Such studies tend to produce incremental knowledge rather than startling revelations. Yet, taken together, they are building a science of indirect exploration that will greatly aid the search for life on other worlds.

Heidy Mader at Britain's University of Bristol found this out when she took a careful look at the living space provided by glacial ice. Biologists know bacteria can sometimes scrape out a living inside ice. But Dr. Mader's research - published in the current issue of the Geological Society of America's journal Geology - is a comprehensive exploration of the liquid water that forms around ice crystals.

The GSA says her results "show for the first time that a viable habitat for bacterial life exists within glacial ice." Bacteria line up along tiny water veins, which are surprisingly rich in nutrients. This isn't the way biologists had expected bacteria to live. "The results suggest that ice is a viable habitat for life on Earth and other icy planets," the GSA says.

Deep-sea sediments are another intriguing habitat. Microbes living there may represent 10 percent of our planet's living biomass. Studies of DNA residues in some sediment samples suggest most of these microbes are forms of bacteria. But it's hard to know if the DNA comes from living organisms or fossils. Christopher House at Pennsylvania State University and his team are zeroing in on living organisms.

They are focusing on chemicals called lipids and on a form of the DNA-like molecule called RNA. These are indications of living cells. They are finding organisms called Archaea, not bacteria. Archaea live in extreme habitats such as deep-sea hot vents or inside cow stomachs. Their research, published online last month, establishes Archaea as important members of the deep-sediment community. They survive in this food-short habitat by slowing down their metabolism. A population of these Archaea can take as long as a couple of thousand years to replace itself - making them some of the longest-lived organisms on Earth, perhaps.

The larger lesson is that scientists can uncover more and more data about life forms they cannot directly study as they refine their ways to indirectly explore. "We used methods that identify only active cells and found Archaea," Professor House says.

Such exploration also is useful in understanding microbial life that can be studied directly, such as organisms that live throughout the sea. Ed DeLong at the Massachusetts Institute of Technology and colleagues are taking what he calls "a shortcut" by analyzing DNA from whole microbe communities.

Last January in the journal Science, they reported what they're finding at seven depths, from the surface to 13,000 feet deep in the tropical Pacific. They trace microbes' life-cycles at these depths from indirect exploration. This approach "is providing new insight into what makes microbes tick in the real world," Professor DeLong says.