The continual input of energy, mostly from sunlight, sustains the process of life. Sunlight allows plants, algae and cyanobacteria to use photosynthesis to convert carbon dioxide and water into organic compounds like carbohydrates. This process is the fundamental source of organic material in the biosphere. There are a few exceptions to this, such as ecosystems living around hydrothermal vents on the ocean floor, which derive their energy from the chemical compounds such as methane and hydrogen sulfide. In either case, the overall productivity of an ecosystem is controlled by the total energy available.