Answer: Oxygen
Step-by-step explanation:
Producer organisms include plants and algae and refer to organisms that are able to produce their own food by synthesizing it from inorganic matter. This is what plants and algae are able to do through the process of photosynthesis.
During photosynthesis, these organisms produce food for themselves and in doing so produce oxygen as a by-product. This is what contributes mostly to the concentration of oxygen in the atmosphere.