Photosynthesis occurs in vastly different forms, for e.g. some prokaryotes perform anoxygenic photosynthesis, and on the other hand, cyanobacteria, algae and land plants use oxygenic photosynthesis. Likewise, in land plants, most organisms rely on so-called C3 photosynthesis, but several tropical species as maize or sugarcane use a variant called C4 photosynthesis in which the first photosynthetic product is malate, a 4 carbon compound, rather than phosphoglyceric acid the more classical 3 carbon compound. Another example of the variation of the photosynthetic mode is found in so-called CAM (crassulacean acid metabolism) plants where CO2 fixation takes place at night rather than during the light, enabling these plants to resist extreme climatic conditions.