2. Principles of cropping system design

2.1 Limiting factors

The limiting factors are, in descending order: light, carbon source, mineral nutrients (assuming temperature and pH maintained within acceptable ranges, and a bacteriologically sound culture). Mineral and carbon limitations can be avoided, but the contribution of light will very often appear limiting (it always controls the kinetics and energetics of the process). This leads to the development of reactors with specific geometries, where light capture and exploitation become key points. This will have to be reconciled with operating constraints (level of control, target production, available water resources, CO ₂ ...), strain requirements and behavior (sedimentation, biofilms).