Question

what would happen to the planet if all oxygen breathing organisms became extinct and only plants remained on the earth

Answer 1

Many plants depend on animals primarily insects for pollination and sexual reproduction. If the animals (insects that pollinate) most flowering plants would be unable to reproduce and would go extinct.

Both plants and animals undergo cellular respiration producing Carbon Dioxide. Animals produce only Carbon Dioxide while plants produce both Carbon Dioxide and Oxygen. If animals went extinct there would be less Carbon Dioxide to support photosynthesis and more complex plants would have a difficult time adapting to the reduced levels of Carbon Dioxide.

Plants that survived the extinction of animals would be much simpler than presently complex plants.

Perhaps some plants would become predators of other plants to absorb the nitrogen content of other plants because of the lack of animals to engage in the recycling of nitrogen content.