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Marine Microbes
Genomic tools reveal unexpected diversity of ocean life
By Birgit Reinert

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Scientists have identified new and diverse groups of microscopic marine organisms in the surface waters of the northern Pacific Ocean. The photosynthetic bacteria, which derive their energy from sunlight, are unrelated to any previously known types. The discovery may lead researchers to revise current notions about the oceanic food web and the carbon and energy cycle in marine ecosystems.

Sample of oceanic bacteria as seen when using epifluorescence microscopy.

A team led by microbiologist Edward F. DeLong, of the Monterey Bay Aquarium Research Institute (MBARI) in California, analyzed bacterial DNA from marine plankton. Using genomic tools, they identified a variety of bacterial photosynthetic genes never seen before and concluded that the ocean has far more light-absorbing microbes than previously thought.

"These new analytical approaches are letting us see for the first time some of the basic properties and characteristics of some of the most abundant organisms on the planet—planktonic microbes," DeLong is quoted as saying. His team used an analytical method called polymerase chain reaction after reverse transcription of RNA.

The newly discovered organisms—which belong to the group of AAP (aerobic, anoxygenic, phototrophic) bacteria—may be able to use light and organic matter simultaneously, according to David M. Karl, of the University of Hawaii in Honolulu, who wrote a News and Views article accompanying the findings in Nature.

The vast majority of these bacteria have gone undetected in part due to the difficulty of growing them in the laboratory; traditional cultivation techniques do not work with these microbes.

"In the past, the discrepancy between the number of bacteria that are easily cultured and the abundance of microorganisms found in ocean samples has been a source of frustration and uncertainty," writes Karl, "so genomic studies of this sort are likely to become increasingly important in the future."

Abundant life in the ocean: plankton bloom in the Bering Sea.

Nearly all ocean life depends on photosynthetic organisms such as plants, algae and some bacteria. Their ability to transform sunlight into energy and nutrients makes them the foundation of the marine food web.

"The discovery of new marine AAP bacteria through culture-independent genomic analyses emphasizes the complementary nature of culture-based and cultivation-independent approaches," the researchers conclude. MBARI microbiologists collaborated on the study with researchers of The Institute for Genomic Research in Rockville, Maryland.

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Béjà, O. et al. Unsuspected diversity among marine aerobic anoxygenic phototrophs. Nature 415, 630-633 (February 7, 2002).
Karl, D.M. Microbiological oceanography: Hidden in a sea of microbes. Nature 415, 590-591 (February 7, 2002).

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