The oceans of our planet are so immense that it's often difficult to accurately determine what their occupants are up to.
A recent paper in Science focuses on picoplankton, the tiniest ocean occupants but also the most widespread. It's a segment of life driven by photosynthesis (in the photo above, all the small orange dots are photosynthetic cyanobacteria). The new study found that the day-light cycle has an effect on more than just the dominant photosynthetic species, however. Non-photosynthetic, heterotrophic microbes also appeared to exhibit differences as a result of available sunlight.
This summary offers a more detailed breakdown.
