What would happen if there were no photosynthesis?

It would lead to a swift, massive and deadly chain effect. Firstly, the plants will die as they cannot respire (since it needs oxygen from photosynthesis) and also from the lack of food. This will cause the food cycle to be disrupted as all organisms depend directly or indirectly on plants for food. There will be a rise in CO2 (Carbon Dioxide) and a decrease in oxygen, in which most organisms will die as we need oxygen to survive. Also, the acidic levels of oceans will increase as Dissolving CO2 in seawater increases the hydrogen ion (H+) concentration in the ocean, and thus decreases ocean pH(increasing acidity).