Photosynthetic microscopic plankton. Coloured transmission electron micrograph (TEM) of a section through some Prochlorococcus cyanobacteria cells. These single-celled marine micro-organisms are thought to be the most abundant photosynthetic organisms on Earth, playing an important role in regulating levels of carbon dioxide and oxygen in the atmosphere. They are found in the tropical and subtropical oceans of the world, reaching levels of 100,000 cells per millilitre of sea water. They are among the smallest organisms in the ocean. The section reveals bands of membranes (green), which contain the chlorophyll used in photosynthesis. Magnification: x23,000 at 6x7cm size.