What is the key to success on the oceanic micro-scale?

The existence of the worlds presumably most abundant organism has been unknown to science until its appearance in DNA samples from the Sargasso Sea in 1990, which gave this organism the somewhat obscure name SAR11. This is a small bacteria who's number has been estimated to make up one quarter of all marine bacteria, implying that there are probably more than 10^28 SAR11 bacteria on Earth (for comparison, the estimated number of stars in the universe is roughly 10^23, i.e. there are 100 000 SAR11 bacteria for each star). The question of what lies behind this success has fascinated microbiologists, not the least because SAR11 is a simple organism with a small genome and hence few genes. Why is such a minimalistic strategy apparently key to success? The debate has first and foremost been guided by the ideas that SAR11 either is so successful due to its ability to defend itself against viruses and predators, or due to its ability to compete with other bacteria for limiting resources. In the last number of the prestigious american science journal PNAS (Proceedings of the National Academy of Sciences), scientists from the Institute of Biology at the University of Bergen analyze the following trade-off: If the cost of viral defense is high in terms  of reduced growth and reproduction ability (and vice versa), then those individuals investing too much into competition will be eaten or get infected, and those who invest too much into defense will starve. A logic consequence is that the key to success appears not to lay in either of the two extreme strategies, but rather in using defense mechanisms where the price in terms of reduced competitive ability is minimal. The scientists found that the success of SAR11 can be explained when taking a new perspective on SAR11. Instead of considering SAR11 as a species composed of similar individuals, the scientists show how an evolutionary arms-race between SAR11 bacteria and their viruses leads to a number of different clones within the SAR11 bacteria, where some clones are strong competitors for limiting resources and others have strong  defense against viruses, while most of the clones lie between these two extremes.
 
The work is part of the recently completed doctorate thesis by Selina Våge and was conducted as part of the ERC Advanced Grant project MINOS to professor Frede Thingstad. The work gives a theoretical framework that can link genetic information from molecular analyses to marine ecosystem properties.
 
Read more in the original article, which was published in PNAS online on May 13, 2014.