The world's oceans contain many more types of microorganisms than had been thought, scientists report.
"Microbiologists have formally described 5,000 microbial species. This study shows we have barely scratched the surface," said Mitchell Sogin of the Marine Biological Laboratory at Woods Hole, Mass.
"In our new study, we discovered more than 20,000 in a single liter of seawater, having expected just 1,000 to 3,000. The number of different kinds of bacteria in the oceans could eclipse 5 to 10 million," he added.
Sogin and colleagues used an experimental technique to sample bacteria by comparing small variable regions of DNA, known as sequence tags, rather than amplifying full genes.
Their findings are reported in Monday's online edition of Proceedings of the National Academy of Sciences.
Their work is part of the International Census of Marine Microbes, part of a 10-year project launched in 2000 involving researchers in more than 70 countries studying the diversity of life in the oceans.
One-liter water samples were collected from various parts of the oceans and analyzed for their content of microorganisms.
While a small number of species dominated, there were a large number of rare species, what Sogin said amounts to a rare biosphere.
The public generally thinks about microbes as something that might make them sick, Sogin said in a telephone interview, "but for 80 percent of the history of Earth, microbes were the only forms of life, they totally shaped the habitability of Earth."
Microbes that are in low abundance in one area may be more abundant elsewhere, he said.
"This implies there is a whole world of unexplored habitat that oceanographers are only just becoming aware of," he added.
And at a time of climate change, currently common microbes could be damaged and rare species might become more abundant, sort of like understudies in a theater taking the stage when the star falls ill, he said.