New York, Nov 7 (IANS): At a time when the Earth was a forbidding place for most modern life forms due to low oxygen levels, sulfur-breathing bacteria thrived in ocean water, analysis of 2.5 billion years old rock samples shows.

The Brazilian rocks the researchers sampled show only trace amounts of oxygen, a sign they were formed before oxygen became abundant.

"These rocks are telling us the bacteria were there 2.5 billion years ago, and they were doing something significant enough that we can see them today," said study co-author
James Farquhar from University of Maryland.

Bacteria dependent on sulfate were plentiful even though sea water typically contained about 1,000 times less sulfate than it does today, the findings showed.

For the study, the University of Maryland geology doctoral student Iadviga Zhelezinskaia analysed the biochemical signals of sulfur compounds found in 2.5 billion-year-old carbonate rocks from Brazil.

The rocks were formed on the ocean floor in a geologic time known as the Neoarchaean Eon.

The Brazilian carbonate rocks' isotopic ratios showed they formed in ancient seabed containing sulfate from atmospheric sources, not continental rock.

And the isotopic ratios also showed that Neoarchaean bacteria were plentiful in the sediment, respiring sulfate and emitted hydrogen sulfide -- the same process that goes on today as bacteria recycle decaying organic matter into minerals and gases.

How could the sulfur-dependent bacteria have thrived during a geologic time when sulfur levels were so low?

"It seems that they were in shallow water, where evaporation may have been high enough to concentrate the sulfate, and that would make it abundant enough to support the bacteria," Zhelezinskaia added.

The study appeared in the journal Science.