Photosynthesis supplies all oxygen for life, and it began in the sea. Amazingly, it all relies on one basic kind of chemical reaction that was achieved over two billion years ago by cyanobacteria, and cyanobacteria living as endosymbionts in plants and phytoplankton still produce almost all our oxygen. This programme tells the story of the cyanobacteria, how they changed our planet and are involved in supporting its many life forms. We look at the four major groups that photosynthesise in the sea: cyanobacteria, eucaryote phytoplankton (microalgae), seaweeds (macroalgae), and seagrasses. These groups are responsible for supplying around half the oxygen we breathe—so in terms of understanding the processes that keep us alive, this is a very important story.