Most of Phytoplankton are microorganisms that are autotrophs. This means they are involved in "Primary production" These microorganisms are using photosysnthesis to make Organic compounds from CO2 and water. There are some organisms, part of phytoplankton, that are heterotrophs. They obtain their carbon source by ingesting other organisms. A good example of that would be some Dinoflaggelates.
Some Algae are components of Phytoplankton. Cyanobacteria, is also called Blue-Green algae, is responsible for an event called "Algal bloom", which is a colarations of water due to high concentration of algae.