Carbon cycling takes place through atmosphere, ocean and through living and dead organisms. Fossil fuel also represents a reservoir of carbon. During photosynthesis comparatively large amount of carbon is fixed annually in the biosphere. And again a considerable amount of carbon is returned back to the atmosphere as ${\mathrm{C}\mathrm{O}}_2$ through the respiration of different flora and fauna. Decomposers also contribute substantially to the ${\mathrm{C}\mathrm{O}}_2$ pool by their processing of waste materials and dead organic matter of land or oceans. It is estimated that 4 × 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.