Sea stars. (Or, picoplankton from the Pacific.) Photo from Daniel Vaulot on Wikimedia Commons.
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.