As mentioned earlier,the ultimate source of all the chemical energy in cells comes from the sun. Because this source is directly available only to the cells of photosynthesizers, most organisms are either directly or indirectly dependent on photosynthesis. A few chemoautotrophs derive their energy and nutrients solely from inorganic substrates. The other major products of photosynthesis are organic carbon compounds, which are produced from carbon dioxide through a process called carbon fixation. On land, green plants are the primary photosynthesizers; and aquatic ecosystems, where 80% to 90% of all photosynthesis occurs, algae, green and purple bacteria, and cyanobateria fill this role. Other photosynthetic prokaryotes are green sulfur, purple sulfur, and purple nonsulfur bacteria.