Prochlorococcus marinus, a tiny globular cyanobacterium.
A couple of articles about ocean bacterial diversity.
First a report on the 2006 Science paper that suggested marine microbial diversity could be 10 to 100 times greater than previously thought.
"From an evolutionary perspective, they are of pivotal importance. They were the only kinds of life on Earth for approximately 80% of the planet's history. All multi-cellular life depends upon microbial processes. The microbes can live without us but we are totally dependent upon them for our continued survival. Exploration of this newly discovered rare biosphere' could become a major field of marine biology."
The second is more recent and describes the discovery a few months ago of a species of cyanobacteria that appears to have abandoned photosynthesis for a life of Nitrogen fixation.
"For it to have such an unusual metabolism is very exciting. We're trying to understand how something like this can live and grow with so many missing parts."