What role organisms play in:
(i) Water cycle    (ii) Oxygen cycle    (iii) Nitrogen cycle.

The roles organism play in different cycles are
(i) Water cycle: Organisms like plants transpire that is the major source of water evaporation in the atmosphere. The water vapour condenses into droplets and precipitates as rain and snow. this is how water is returned to the atmosphere. 
(ii) Oxygen cycle: Organisms use molecular oxygen in respiration and release CO₂. These organisms also use oxygen in compound form, e.g., H₂O. Green plants release O₂ in the atmosphere as by-product of photosynthesis. Thus organisms play major role in balancing oxygen and carbon dioxide cycles in nature.
(iii) Nitrogen cycle: The molecular nitrogen present in the atmosphere cannot be utilised by living organisms until it converted into usable form, e.g., nitrates. Micro-organisms carry out the fixation and assimilation of free atmosphere nitrogen. In this way, molecular nitrogen enters ion the living component of the biosphere. De-nitrification of nitrates by denitrifying bacteria (Pseudomonas) causes escape of free nitrogen in the atmosphere by decaying dead bodies of plants and animals.