It is decomposed by decomposers and brought back into the food chain later. Decomposers like bacteria and fungi break down any amount of living and non-living organic matter that is left unconsumed in the ecosystem.
Matter that is not used by consumers in the food chain become "detritus". Detritus, meaning debris or waste, are materials that are missed or uneaten by consumers. Because they do not participate in higher trophic levels, these materials are left to decompose. Detritus is often considered the "missing mass" when measuring the total mass in the food chain.
Explanation: Photosynthesis removes CO2 from the atmosphere and replaces it with O2. Respiration takes O2 from the atmosphere and replaces it with CO2.
