The most abundant organisms in the ocean are unicellular bacteria. Two species that occur in all oceans are Prochlorococcus marina – a photosynthetic cyanobacterium which produces ~25% of the oxygen on the planet each year – and Pelagibacter ubique – a heterotrophic bacterium that is the most abundant bacterial species in the sea. These tiny (<0.5 μm diameter), ubiquitous, free-living organisms have extremely streamlined genomes. They are also nonmotile. How do they acquire the nutrients that they need for growth if they are unable to swim towards nutrient sources?
Ian Joint (MBA), with Jonathan Zehr (University of California, Santa Cruz) and Joshua Weitz (Georgia Institute of Technology) have now shown that molecular diffusion is so rapid that it supplies daily requirements, even in the extremely oligotrophic oceanic gyres 
Bacteria are present in the surface ocean at cell densities of more than 108 cells L-1. This sounds a large number but cells are very small and many hundreds of cell diameter apart; the oceans are mostly empty space. In the oligotrophic ocean, ammonium concentration (the preferred nitrogen source) is often less than 10 nanomoles L-1. This means that a volume of seawater equal to the volume of a Prochlorococcus cell (~0.5 μm3) contains only a few molecules – 100 million times less than the nitrogen content of a cell. Ammonium ions have to be accessed from a very large volume of seawater, relative to the cell size. Our anthropocentric solution to this problem would be to move towards a nutrient source. However, Zehr et al. show that, even if Prochlorococcus cells were motile, they could not access sufficient NH4+ molecules by swimming at the speeds that are typical of bacteria. However, molecular diffusion is a very rapid process, which alone could provide more than 4 times the daily nitrogen requirement of a Prochlorococcus cell.
Therefore, the solution to the problem of how to get enough nutrients is to sit and wait; diffusion will bring all the nutrients that you need if you are a small bacterial cell. Rapid diffusion rates also explain why these ubiquitous oceanic bacteria lost, or never had, the ability to swim. Swimming would not have improved their ability to access nutrients and loss of motility genes did not compromise their ecological fitness.
 Zehr , J.P., Weitz, J.S., Joint, I. (2017). How microbes survive in the open ocean: ubiquitous nonmotile microorganisms access dilute nutrients via diffusion. Science 357 (6352), 646-647 doi: 10.1126/science.aan5764.