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CORVALLIS - Using new techniques, microbiologists at Oregon State University have shown that one of the smallest known bacteria is also one of the most abundant organisms on earth. These tiny ocean microbes are so small that billions will fit into a teasp oon; and so abundant that, at 200 million metric tons, they have about the same mass as all of the fish in the oceans.

The discovery, reported today in the journal Nature, was made by scientists at OSU's Laboratory for the Isolation Of Novel Species (LIONS), and focuses on a group of marine bacteria known as SAR11.

The authors reveal that SAR11 is even more abundant than previously suspected, making up more than one-third of all bacterial cells in the ocean surface waters, and almost one-fifth of cells in deeper water.

"Vast populations of SAR11 increase during the summer and decrease during the winter, in a cycle that correlates with the buildup and decline of dissolved organic carbon in the ocean surface," said Robert Morris, an OSU doctoral candidate and lead author of the study. This suggests that SAR11 has an active role in the oceanic carbon cycle, which affects concentrations of carbon dioxide in the Earth's atmosphere.

The study is the latest in a series that began more than a decade ago with the discovery of SAR11 by OSU microbiologist Stephen Giovannoni, the lab's director and one of the authors of the article in the Dec. 19 issue of Nature.

Giovannoni helped pioneer the use of ribosomal RNA to detect unknown microbes, which led to the discovery of SAR11 in water samples first collected in the Sargasso Sea. Last year the OSU team of scientists successfully cultured this elusive bacterial grou p.

This latest article measures the abundance of SAR11 using a different approach to quantifying the tiny microbes. The scientists targeted the bacterial group with a probe designed to connect only to a molecular sequence unique to SAR11. Morris added a fluo rescent tag so when the probes combined with their target sequences, SAR11 cells would light up like fireflies.

Using a fluorescent microscope, Morris could see the cells containing the SAR11 gene sequence and counted the number of SAR11 cells in a mixture of other marine bacteria. The results provide the first direct quantitative measurement of the abundance of SA R11 in seawater.

"The microbial world is an unexplored frontier populated with species that have never been identified, named or studied," said Giovannoni. "SAR11 was the first major group of uncultured bacteria to be discovered by gene cloning and sequencing techniques, and it has since played a leading role in the development of new microbe hunting techniques."

The significance of this research goes beyond marine science, according to Giovannoni. Bacteria and other microorganisms dominate Earth's biomass and are major players in biogeochemical cycles.

Understanding that significance, the National Science Foundation Microbial Observatories Program, Oregon Sea Grant and the Murdock Charitable Trust support ongoing research at LIONS to explore how these tiny, ubiquitous microbes connect to the Earth's lar gest ecosystem, the oceans.