The term "algae" covers many different organisms capable of producing oxygen through photosynthesis (the process of harvesting light energy from the sun to generate carbohydrates). These organisms are not necessarily closely related. They range from Picoplankton between 0.2 to 2 micrometers in diameter to fronds of giant kelp are as large as 60 meters in length. Many algae are unicellular.
Closterium
Synura