SEQUENCE of organisms through which nutrients and energy pass as one organism eats another.In a food chain, each organism occupies a different trophic level, defined by how many energy transfers separate it from the basic input of the chain.Food webs consist of many interconnected food chains and are a more realistic representation of consumption relationships in ecosystems.Energy transfer between trophic levels is inefficient (with a TYPICAL efficiency of around 10% ).This inefficiency limits the length of food chains.In ecology, a food chain is a series of organisms that eat one another.PHOTOAUTOTROPHS, such as plants, use energy from SUNLIGHT to make organic compounds (sugars) out of carbon dioxide in photosynthesis. Other examples of photoautotrophs include algae and cyanobacteria.Chemoautotrophs use energy from chemicals to build organic compounds out of carbon dioxide (or similar molecules). This is called chemosynthesis.For instance, there are hydrogen sulfide-oxidizing chemoautotrophic bacteria found in undersea vent communities (where no light can reach).Autotrophs are the foundation of every ecosystem on the planet.Autotrophs form the base of food chains and food webs, and the energy they capture from light or chemicals sustains all the other organisms in the community.