Question

If the number of photosynthetic organisms on the earth decreased drastically, which would happen as a result?

Answer 1

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What would happen is that we would lose our only conversion source for oxygen. To survive, a small number of us would acquire artificial and chemical converters, but those would need to be confined to small spaces, which would be leaving us no chance for general life in the world, like animals and the rest of the human population.

Another thing is that those who use artificially created oxygen, would also eventually die, because of the collapse of the food chain and other "food" we eat. Let's think about it, there would be no food for cows, pigs, chickens, etc.. no meat for us and other carnivores/omnivores. We would all eventually die off!