this is the most important cycle if you talk about any existence of life. carbon-oxygen cycle occurs between autotrophic organisms AO (plants and algae) and heterotrophic organisms HO (animals). AO takes the carbon dioxide (CO2) released by HO as a by-product of cellular respiration. this will become their source of carbon and will use photosynthesis to manufacture glucose (AO food) from it. as this happens, the AO releases oxygen (O2) as a by-product of photosynthesis which then used by HO to breathe. in this way, both organisms live.