Vance Oyama's Gas Exchange Experiment on Viking found oxygen evolution within the dark chamber of the experiment. This was ascribed to chemistry such as oxidants or superoxides because at that time the only way microbes were known to evolve oxygen was by photosynthesis which requires light.
However, now it is known there are microbes that can evolve oxygen in the dark and interestingly they can do so by metabolizing chlorates or perchlorates:

Possibilities for life on Mars - a surprising new microbe.
RGClark
22-January-2005, 05:40 PM
http://www.bautforum.com/life-space/15752-possibilities-life-mars-surprising-new-microbe.html

The Atacama Desert in Chile has the largest naturally occurring deposits of perchlorate on Earth. I wrote to some researchers who had done life tests in Atacama to see if the perchlorate-metabolizing microbes had been detected there. I was told they weren't, but the MSNBC article suggests they have been found there:

Scientists: Martian soil similar to Chile’s desert.
Perchlorate find still needs confirmation; it wouldn’t rule out habitability.
By Alan Boyle
Science editor
MSNBC
updated 8:11 p.m. ET, Tues., Aug. 5, 2008
"However, some organisms actually thrive on perchlorates and have been enlisted for cleaning up chemical spills. Perchlorate-loving microbes have been found in Chile's Atacama Desert and Antarctica — two of the places that have been compared to the Red Planet's cold, dry environment."
http://www.msnbc.msn.com/id/26043028/

Then we may use Atacama samples containing these microbes to see if they can emulate the results seen in the Viking's Gas Exchange Experiment.

Bob Clark