Why Doesn't a Super-Bacterium Take Over the Ocean?

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To the naked eye, great expanses of ocean can seem nearly devoid of life. But examine just a teaspoon of seawater and you'd reach a drastically different conclusion. Not only is there life, there is gobs of it – anywhere from one million to five million bacteria. In that same sample, you'd also find up to fifty million viruses, which aren't technically alive, but buzz with activity. How these viruses and bacteria interact in this hidden world shapes the fate of the entire ocean.

As many as a billion species of bacteria inhabit the oceans, and if it were up to any one of them, they would probably rule uncontested. Such a monopoly would prove devastating for most other ocean life, however. When bacterial species explosively bloom unchecked, they can produce toxins in large concentrations, block out sunlight for photosynthesizing organisms below the surface, and deplete oxygen in the water. Sometimes this can happen over hundreds of miles. Imagine a bloom that covered an entire ocean!

Thankfully, the ocean's viruses, which are mostly bacteriophages (attacking only bacteria), prevent any single bacterial species from achieving unfettered control. When one bacterial species does bloom out of proportion, it becomes a tantalizing target for viruses. Stephen Palumbi, a Professor of Biology and Director of the Hopkins Marine Station at Stanford University described what happens next in his book The Extreme Life of the Sea, co-authored with his son, science writer Anthony Palumbi.

"Viruses mutate, and some of those mutations allow them to attack the huge hoard of bacterial prey, infiltrating the bloom. Fresh viruses exploded from a dead bacterium need only travel a few microns to the next bacterium to start the next killing cycle."

Simply called "kill the winner," this process can quickly quell a bacterial uprising, returning the ecosystem to balance. The remaining viruses, specialized to kill the now abated bacteria, simply float around, harmless to plants and animals.

"Kill the winner is a natural safety valve for success – a blind mechanism keeping any single microbe from taking over the environment," Palumbi further explained.

"Single-celled organisms and viruses are the fastest evolvers on our planet, and 'kill the winner' is Earth's single most high-stakes game. It's been played in every drop of ocean, every day, for probably the past 3 billion years. Because it keeps any one microbial species from becoming too successful, too dominant, it may be partly responsible for the balance of diversity in the sea."

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