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Microbial Life in the Oceans More Diverse than Previously Believed

By Pippa Wysong, Access Excellence

WOODS HOLE, MA (08/30/06) - Microbial life in the ocean is far more diverse than ever imaged with upwards of 20,000 different kinds of bacteria living in a single liter of seawater. A study of samples taken from several sites in the North Atlantic shows that the number of different kinds of bacteria is larger, by orders of magnitude, than what was found in previous estimates.

altResearchers did their count of microbe types by using an improved version of a DNA sequencing device. Results were published in the August 8, 2006 edition of the Proceedings of the National Academy of Sciences (PNAS), and was part of the International Census of Marine Microbes.

"What we did in our PNAS paper is try to speed up the process, make it more efficient, for getting molecular evidence of microbial diversity," said the study's lead author Mitchell Sogin, PhD, director of the Josephine Bay Paul Center at the Marine Biological Laboratory in Woods Hole. Only short fragments called sequence tags were sequenced from ribosomal RNAs rather than complete ribosomal RNA genomes, he said.

Samples were obtained from three different locations in the North Atlantic that were several hundred miles apart and in different climatic regions. Samples were taken at varying depths, with some being from 500 to 600 meters deep, and others between 1,700 to 4,000 meters deep. Additional samples were taken at a fourth site, the Axial Sea Mount not far from the Juan de Fuca Ridge where there is an underwater sea volcano.

Once results came in, researchers were surprised at a number of findings. One was the sheer number of bacterial types seen. "We thought we might see 20% to 50% more diversity," Dr. Sogin said. The number of types of bacteria is at least 100 times greater than previous estimates, numbering about 25,000 different kinds of microbes in a liter of sea water. Researchers predict that the total biodiversity of bacteria in the oceans is probably greater than five million different kinds of micro-organisms.

In the PNAS paper, the authors explain the reason why there is such diversity: "Given the enormous number of microbes and their vast metabolic diversity, the accumulation of mutations during the past 3.5 billion years should have led to very high levels of genetic and phenotypic diversity."

Another surprise was that much of the diversity was seen in organisms that were not previously known and appeared in very low quantities; what biologists dub 'low abundance organisms'. It's hard to say what low abundance organisms being present means – whether these types of bacteria are simply rare, or perhaps occur in higher levels in other parts of the ocean, Dr. Sogin said.

Researchers also noted that samples from any single region showed different types of bacteria that were highly unlike each other. For instance, in samples from the Axial Sea Mount "we expected them to be different from each other, but not wildly," he said.

Microbes constitute from 50% to 95% of the entire biomass of the Earth's oceans, and are already known to play vital roles in ocean health, and the sustaining and development of life on the planet.

Microbial life first appeared somewhere between 3.5 to 3.9 million years ago. "Life was exclusively microbial until 560 million years, or so, ago. At least 80% of our history was exclusively microbial," Dr. Sogin said. During those millions of years, microbes slowly changed the composition of the atmosphere as well as the composition of chemicals in the ocean. They are often described as the engines of the Earth's biosphere.

"They're responsible for all the main carbon transformations that occur on Earth. They harvest the energy from the sun...They catalyze all the major biogeochemical transformations. They are completely responsible for all nitrogen cycling. No other organisms except bacteria carry out nitrogen cycling," he said. In fact, microbes are essential for sustaining all life on the planet.

"Our existence is totally dependent on the microbial world properly functioning to maintain habitability. They can live without us but we cannot possibly survive without microbes," Dr. Sogin said. Most of the bacteria living in the oceans are not dangerous to people, some are symbiots to other life forms living in the ocean, plus some metabolise sulphur contributing to another part of the chemical processes on the planet.

Bacteria are not the only microbial life in the ocean, but are believed to make up close to two thirds of the oceans' biomass. Other microbes include Archaea (which make up most of the remainder of the biomass), protists and unicellular fungi, but due to technical limitations these were not tested for.

There are several steps researchers would like to take in future research. One is to take samples from many more sites (preferably thousands) in the world's oceans. Another is to figure out how to cultivate some of the microbes in the lab since many kinds of microbes don’t survive out of their natural environment. Once cultivated, they can be studied and researchers can start figuring out what specific functions microbes play in the environment.

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