Without plants and algae, specifically autotrophs, the atmosphere of Earth would change dramatically. First, carbon dioxide that is normally used during photosynthesis by producers would cease and glucose would not be produced affecting the food chains everywhere. When carbon dioxide is converted to glucose sugar, this is called carbon fixation. Second, the carbon dioxide which would remain in the atmosphere would build to dangerously high levels as all the living things carrying out respiration would be adding to the amount. Carbon dioxide is a waste product formed after respiration occurs in living things. Carbon dioxide is also a greenhouse gas. The excess would lead to accelerated global warming. It would allow the sun's rays to enter the atmosphere but would trap the heat, raising the Earth's temperatures. During photosynthesis, water taken in by plants is split into hydrogen and oxygen gas. This is the oxygen released by producers into the atmosphere. If producers were gone, oxygen would not be added to the atmosphere and eventually, all the aerobic organisms would use up the supply. Eventually, these organisms would die off. The atmosphere of Earth contains 20.95% oxygen and 0.039% carbon dioxide normally. This would change drastically without producers.
Photosynthesis, specifically the calvin cycle, involves taking light energy and carbon dioxide from the atmosphere and using it to synthesize sugars. In this process, there is a second byproduct - oxygen - that is produced with the sugar. By eliminating these autotrophs, you will have a significant increase in carbon dioxide in the atmosphere and a limited supply of oxygen that would continue to decrease because there would be no way to replenish it.