Mysterious microbes trade genes

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Bacteria with a jaw-breaking name are forcing scientists to rethink the biology of microscopic ocean life. These microbes eluded the explorations of marine biologists for centuries. Now a growing understanding of their ability to adapt to changing environments is rewriting the rules of how marine microbes interact and evolve.

Sally Chisholm at the Massachusetts Institute of Technology and Rob Olson at the Woods Hole (Mass.) Oceanographic Institution discovered this minute wonder - called Prochlorococcus - in 1985. Since then, Chisholm's lab has taken the lead in exploring the microbe's lifestyles and genome. Lab members' latest findings show that different strains of the bacterium swap genes as readily as college students swap songs over the Internet. Viruses play the role of computer file-swapping programs.

In her lab's announcement, Professor Chisholm explains that the new findings are beginning to paint "a picture of gene diversity and gene flow in the most abundant photosynthetic cell on the planet." Papers published last month in Science magazine present the technical details. From being totally unknown to science, this photosynthetic marvel has revealed itself to be so abundant that it accounts for up to 48 percent of the ocean's net primary biological production. Chisholm says it forms "an important part of the food chain in the oceans." It even supplies "some of the oxygen we breathe."

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The studies show that the genomes in all the strains of the bacterium have distinctive regions in their DNA - so-called genetic "islands" - that have a variety of potentially useful instructions. They represent contingency plans for meeting the challenges of marine life.

One or more of those genes can kick in and give the organism the capabilities it needs to survive and thrive when environmental changes call for it. The studies also show that the bacteria live in waters swarming with viruses. These readily pick up a genetic "island" from one bacterium and transfer it to another. Biologists have seen this happen with pathogenic bacteria. The new studies suggest it is the standard way of microbial life in the sea.

MIT Prof. Edward DeLong and various colleagues are using a particularly tricky kind of DNA forensic study to trace the bacterium's habits. They have developed a computer program that takes the data from DNA fragments in seawater and assembles it into sequences long enough for the team to identify which microbes are present and even what they have been up to. The researchers found genes from surface-water bacteria that code for an ability to move around to find food. Other genes from deeper-water microbes reflect an ability to cling to descending food particles.

In spite of centuries of research, microbial life is one of the biggest unknowns in marine biology.

Chisholm explains that scientists are not just looking at various species. They are trying to understand a total life system. It's a system in which microbes co-evolved with each other and with the chemistry and physics of the sea. Chisholm notes that "chemistry, physics, and biology are tightly linked as a system, meaning you can't change one without altering the other two."

Her team hopes that Prochlorococcus will be a key organism in revealing how that system of sealife works.

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