Question

How did early heterotrophs contribute to higher levels of oxygen in Earth’s atmosphere?

Answer 1

Following the evolution of photosynthesis in prokaryotic cyanobacteria, the oxygen levels in the ocean began to rise, causing iron to precipitate out as iron oxide. Once all the iron was precipitated, oxygen levels in the atmosphere began to increase from 2.7 BYA to 2.2 BYA.

After this, oxygen continued to increase - but did so much more rapidly. Here's where your heterotrophs probably come in. Scientists have hypothesized that this second "surge" in oxygen levels came as a result of the evolution of eukaryotic cells containing chloroplasts --> This process is also known as endosymbiosis.

Early heterotroph engulfs another heterotrophic prokaryote (forming mitochondrion) and cyanobacterium (forming chloroplasts), resulting in a primitive eukaryotic cell which undergoes photosynthesis. So this would be my guess to answer your question - heterotrophs contributed, in a sort of roundabout manner.