Dissolved Oxygen: When Photosynthesis Poisons Itself
In a bright, dense culture, photosynthetic oxygen can supersaturate to levels that inhibit the very growth that produced it.
- Dense, lit cultures can supersaturate O2 far above 100% air saturation
- High O2 drives photorespiration and reactive-oxygen stress, cutting productivity
- Aeration and mixing strip excess O2; MAIN's model penalizes growth above ~300% sat
Oxygen is the product of photosynthesis, and it is easy to assume more of it means a healthier culture. In a photobioreactor, the opposite can be true. Photosynthesis splits water and releases O2 inside the vessel; a dense, brightly lit culture generates O2 faster than it can escape, so dissolved oxygen can climb well above air saturation — 200%, 300%, or more is routinely reported in productive closed reactors.
That excess oxygen harms growth two ways. It fuels photorespiration — Rubisco, which fixes CO2, also reacts with O2, wasting energy when the O2-to-CO2 ratio rises. And high O2 with strong light promotes reactive oxygen species that damage the photosynthetic apparatus, compounding photoinhibition. The culture is poisoned by its own success.
MAIN’s digital twin encodes this: its dissolved-oxygen term leaves growth unaffected up to a threshold — configured near 300% saturation in the Spirulina preset — then applies a smoothly increasing penalty. The practical remedy is physical: mixing and aeration strip excess O2 (and bring carbon to the cells and keep filaments suspended). Read this way, sustained supersaturation says the culture is over-lit or under-mixed relative to its density — and the fix is more turbulence, not more light.