NEWS OF THE WEEK BIOLOGICAL
CHEMISTRY
OXYGEN'S GIFT Without 0 2 , life would be a lot simpler
A
OXYGEN BOOST This pruned version of a much larger network depicting 6,836 metabolic reactions indicates relationships and differences among oxygen-involved reactions and metabolites (red) and anoxic ones (blue), whose origins predate the arrival of oxygen into the atmosphere a few billion years ago.
GRAND PLOT POINT IN THE
history of life was the center-stage entry of oxygen into the atmosphere roughly 2.2 billion years ago. At the time, oxygen was a waste gas that emanated from the bacterial innovators of photosynthesis and posed mortal hazards to the anaerobic life that had been evolving for more than a billion years. With oxygen's ar-
rival, the choices for the planet's microbial masses were to die, hide from oxygen, or evolve ways to live with it. Using a bioinformatics approach involving data from the genomes of 70 of today's aerobic and anaerobic microbes, as well as thousands of enzymatic reactions, Jason Raymond of Lawrence Livermore National Laboratory and Daniel Segre of both LLNL and Boston University have attempted to infer "how oxygen availability changed the architecture of metabolic networks" {Science 2006,311,1764). It's the type of insight that researchers can use to reconstruct how early microbial life evolved into complex life forms. The researchers used a simula-
B I O G E O C H E M I S T R Y
ANCIENT GAS Fluid bubbles inside this 3.5 billion-year-old sample of quartz (viewed in crosspolarized light) may contain biologically produced methane.
10
METHANE FROM ANCIENT MICROBES Evidence suggests the organisms lived much earlier than previously thought
A
C & E N / MARCH 2 7 , 2006
PROVOCATIVE NEW ANALY-
sis of 3.5 billion-year-old rocks has turned up evidence for the early existence of microbes that produce methane. T h e study, conducted by geoscientist Yuichiro Ueno and colleagues at Tokyo Institute of Technology and other institutions, not only may push back the earliest known date of such organisms by 700,000 years but also suggests that they could have influenced climate during the early Earth age known as the Archean era. "The results are entirely con-
sistent with models we've published for the Archean atmosphere," says James F. Kasting, a geosciences professor at Pennsylvania State University. Ueno's group examined fluid bubbles trapped inside quartz from an ancient, much-studied rock formation in Western Australia (Nature 2006,440,516). They found methane containing carbon with a low 13C/12C ratio, which is characteristic of biological origin. The authors did not find large hydrocarbons that would have suggested the methane could have been produced by thermal processes.
tion technique called metabolic network expansion in which a set of seed compounds, such as ammonia, pyruvate, coenzyme A, carbon dioxide, and oxygen, is fed into an existing database of more than 6,800 enzymatic reactions that collectively occur in both aerobic and anaerobic microbes. As these compounds react, their products join the seed set, and the procedure repeats until no new products can be formed. By comparing runs of seed sets that include oxygen with those that don't, oxygen's effect on the overall metabolic system becomes apparent. "The arrival of oxygen had a massive effect on metabolic innovation," says molecular ecologist Paul Falkowski of Rutgers University, noting that the gas appears to facilitate more than 1,000 metabolic reactions unseen among anaerobes. Without an oxygeninstigated spate of biochemical evolution like this, Raymond suggests, life on Earth might still be single-celled.—IVAN AMATO
They "have probably uncovered the oldest known samples of biologically produced gas," writes Don E. Canfield, director of the Nordic Center for Earth Evolution at the University of Southern Denmark, in a commentary accompanying the Ueno report. Methane-producing microbes belong to the biological domain Archaea, whose members frequently thrive under extreme conditions and are among the earliest forms of life. Methane is one of the greenhouse gases posited to have kept Earth warm several billion years ago when the nascent sun was relatively cold and faint. Conclusions drawn about early Earth from rare ancient rocks can be controversial, notes David Valentine, assistant geosciences professor at the University of California, Santa Barbara. The carbon isotope signatures, he believes, may not be "the smoking gun" but are nonetheless "very interesting."— ELIZABETH WILSON
WWW.CEN-0NLINE.ORG
