Oceans teeming with 10 million kinds of microbe

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The diversity of microbes living in the world's oceans may be more than 100 times greater than previously estimated, according to a survey of marine life.

Scientists working in marine sites around the world, including several North Atlantic sites between Greenland and Iceland, were astonished to find that they had massively underestimated the diversity of single-cell organisms that, despite being invisible to the naked eye, make up 98 per cent of all life in the oceans.

An international team of marine biologists carried out the study with the help of DNA probes which can quickly distinguish between thousands of life forms in a single glass of seawater.

Only 5,000 marine microbes have been named and formally described by scientists, but the true number of bacterial species living in the ocean could be between five and ten million, said Mitchell Sogin, director of the Marine Biological Laboratory in Massachusetts.

"This study shows we have barely scratched the surface. Over the last 10 to 20 years molecular studies have shown there to be more than 500,000 kinds of micro-organisms," he said.

"In our new study, we discovered more than 20,000 in a single litre of seawater, having expected just 1,000 to 3,000. These observations blow away all previous estimates of bacterial diversity in the ocean," Dr Sogin said.

"Just as scientists have discovered through ever more powerful telescopes that stars number in the billions, we are learning through DNA technologies that the number of marine organisms invisible to the eye exceeds all expectations, and their diversity is much greater than we could have imagined."

Marine microbes are living descendants of the most ancient forms of life on Earth, and without them life in the sea and on land would not be possible, which is why scientists want to know more about their diverse roles. "Microbes constitute the vast majority of marine biomass and are the primary engines of the Earth's biosphere. They are the oldest life forms, the primary catalysts of energy transformation, and fundamental to the biogeochemical cycles that shape our planetary atmosphere and environment," Dr Sogin said. The latest research, published in the Proceedings of the National Academy of Sciences, was carried out by researchers working on the International Census of Marine Microbes, part of the Census of Marine Life, a 10-year initiative to catalogue the rich diversity of the oceans before it is depleted.

Scientists took seawater samples from several sites around the world, including a hydrothermal vent on an underwater volcano in the Pacific Ocean.

The researchers analysed the seawater with a DNA technique known as 454 tag sequencing, which quickly tells the investigators how many different kinds of microbe are present in the sample. Microbes still play a pivotal role in the chemical cycles of growth and decay that are vital for all life on Earth.

"They can live without us, but we are totally dependent upon them for our continued survival," Dr Sogin said. "We know we're going through a global change and micro organisms are vital to our survival."

Dr Jesse Ausubel, of the Alfred P Sloan Foundation in New York, said: "A swimmer taking just a swallow of seawater may be consuming an entire zoo of 1,000 different forms of bacteria. That is how it seems the mysterious microbial world operates."