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This, unfortunately, is incorrect.

I can see where the answerer is coming from, but the answer is still incorrect.

SAR11 (Pelagibacter clade [group of closely related organisms] of the Synechoccus types of bacteria) is the most common organism on earth, accounting for roughly 25% of all the bacteria in the ocean. Prochlorococcus and Synechoccus species dominate the microbial ecology of the ocean. They account for 25% of global photosynthesis. Yet, they were mostly unknown until Sallie W. Chisholm, a professor at the Massachusetts Institute of Technology, first discovered Prochloroccus in 1988.

The marine bacterium SAR11, the most common organism on earth. It was discovered by Stephen Giovannoni of Oregon State.

[text taken from http://oceanworld.tamu.edu/resources/oceanography-book/microbialweb.htm]

Hope this is a satisfactory answer.