SAR11-clade organisms (aka Pelagibacter) and MGI archaea are about tied for most abundant cells on earth. Each has a estimated global population of ca. 10^28 cells. Prochlorococcus' distribution is fairly limited to warmer climates. SAR11 & MGI are found just about everywhere. The claims are based on the quality of the data used to make them and are probably pretty accurate. The bigger question is "why are they the most abundant?"

SAR11-clade organisms (aka Pelagibacter) and MGI archaea are about tied for most abundant cells on earth. Each has a estimated global population of ca. 10^28 cells. Prochlorococcus' distribution is fairly limited to warmer climates. SAR11 & MGI are found just about everywhere. The claims are based on the quality of the data used to make them and are probably pretty accurate. The bigger question is "why are they the most abundant?"

Perhaps, Pelagibacter ubique is the most abundant bacteria in the world.

The abundance and spread of marine bacteria species depend on several conditions and scientific temporal data available and thus it's obvious to consider that there's one kind of abundant marine bacteria but abundant marine communities of bacteria

Also don’t forget the phototrophic sulphur bacteria’s such as Chromatium sp., Chlorobium sp. and Thiocapsa sp. (Chromatiaceae and Chlorobiaceae) from anoxic deep waters. Especially from the biggest anoxic basin Black Sea.!

A tough question when considering how difficult it is to define a bacterial species... Whether SAR11 clade seems to be the most abundant bacterial group on Earth, Prochlorococcus is a genus, so not really comparable because of different taxonomic levels. Whereas estimations propose 10^28 cells of SAR11, A recent paper published in PNAS (Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus, Flombaum et al. 2013, doi: 10.1073/pnas.1307701110) estimate an abundance of about 3x10^27 Prochlorococcus cells, and about 7x10^26 Synechococcus cells.