section_path: Ecology/Primary producers
section_name: Primary producers
paragraph_text: Primary producers
  
  The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis, at first by cyanobacteria and later by photosynthetic eukaryotes, radically changed the composition of the early Earth's anoxic atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems. Plants form about 80% of the world biomass at about 450 gigatonnes (4.4 × 10 long tons; 5.0 × 10 short tons) of carbon.