Water at the start, and later
“Liquid acquisition” (SN: 1/15/11, p. 26) discusses two new models about how Earth got its water. But the two models are not mutually exclusive. Indeed, I wonder if perhaps two (or more) sources of water may be the only way to match all of the observed isotopic abundances. Is anybody working along these lines?
Stanley Friesen, Frederick, Md.

A combination of the two proposed scenarios—some of Earth’s water coming from the planet’s immediate surroundings at its formation, some delivered later from space—is definitely possible. Some researchers believe that this may in fact be the best solution to the puzzle. —Ron Cowen

Arsenic-eating microbes
I was glad to see the article (“Microbe swaps arsenic for key ingredient of life,” SN: 1/1/11, p. 5) on arsenic incorporation by microbes. The news media implied that the arsenic incorporation was natural.

In the 1970s, Richard L. Raymond Sr. started an industry using bacteria present well below the ground surface (where there’s an absence of light) to degrade hydrocarbon contamination. The hydrocarbons are the electron donors and energy source for the microbes. Over the years, we have learned to use chlorinated solvents as the electron acceptors. Oxygen, nitrate and sulfate all play roles in the process. The use of microbes for soil and groundwater remediation is now a large industry.
Bob Norris, Longmont, Colo.

When tales of the arsenic-consuming bacteria from Mono Lake first appeared in the news, it was indeed a big deal, oversetting all our experience with biochemistry. I was just itching to know how the organism GFAJ-1 managed to do the metabolism. But no such luck; the news coverage just degenerated into shrill accusations, devoid of scientific details. Thank you for reporting the science behind the discovery. You have kept alive my faith in Science News.
Tom Kimmel, Tucson, Ariz.