Written in Stone

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One of the most important chemical traces left by ancient bacteria is a group of compounds called 2-methyl-BHPs. In 1999, Roger Summons, an MIT professor of geobiology, and colleagues found these compounds in 2.5-billion-year-old rocks from the ­Hamersley Basin in western Australia. These rocks, from an iron mine, are similar to the polished one in Newman's office. Today, oxygen-producing photosynthesizers called cyanobacteria are the primary producers of these BHPs. For this reason and many others, including certain characteristics of the Hamersley site, Summons and others have interpreted the find as evidence that cyanobacteria were carrying out modern photosynthesis 2.5 billion years ago. "The logic was that these compounds are made by cyanobacteria; cyanobacteria do oxygenic photosynthesis; therefore oxygenic photosynthesis was going on at that time," says Newman.

Newman thinks that her own research casts doubt on this conclusion. She has been studying another strain of bacteria that produce BHPs: so-called purple bacteria, which cannot use water to produce oxygen. Instead, they oxidize iron, hydrogen, or various organic compounds. "We're trying to figure out the function of [BHPs] in the cells that make them today," she says. "Our preliminary findings indicate that BHPs have no direct connection with photosynthesis." ­Summons, who collaborates with Newman on some of her research, doesn't take her skepticism personally; he's confident that her work will lead to important insights into these compounds and, especially, why and how bacteria make them. However, he also points out that her findings don't disprove the theory that chemical traces left by cyanobacteria are preserved at Hamersley.

Meanwhile, Newman's work with bacterial compounds known as phenazines is illuminating a problem more immediate than the mystery of how our oxygen-rich air came to be. By changing the way scientists understand these organic molecules, her research could lead to new treatments for chronic bacterial infections.

Phenazines have long been classified as "secondary metabolites," by-products of the processes that produce more critical metabolic compounds. They've also long been known to act as antibiotics. But Newman has demonstrated that phenazines also have profound effects on microbial survival and development.

Newman got the idea for this research while studying bacteria that, strange as it sounds, use iron-containing rocks to "breathe." Humans use oxygen to burn the carbon in, say, a tuna sandwich, creating energy; the oxygen's role is to accept electrons from the carbon. Iron plays a similar role for the rock-breathing bacteria, which get their energy when they transfer the electrons in carbon-containing compounds like glucose to the iron in rocks. It's not breathing in the human sense--the iron itself does not enter the cells, as oxygen enters our lungs. Rather, rock-breathing bacteria pass an electrical current to the iron using molecules that act as electron shuttles. These molecules transport electrons from one bacterial cell to the next and ultimately to the surface of a ferrous rock, like the hands of an audience ferrying a crowd-surfing rock star. Newman and her colleagues hypothesize that phenazines might act as electron shuttles in other bacteria.