Biomass recycling and species competition in continuous cultures

A chemostat with cell feedback is analyzed for three kinds of limiting nutrient: a substrate dissolved in the inflow, a gas bubbled directly into the reactor, and light. The effects of recycle are distinct in each case, because the relationships between hydraulic detention time and nutrient inflow are different for each type of nutrient, Effluent recycle, in which the recycle stream is more dilute than the reactor, is discussed in terms of cell detention time and nutrient limitation. Results from chemostat cultures of the blue‐green alga, Spirulina geitleri, demonsrtat cell feedback under light limitation. Maximum Productivity is fixed by the incident light intensity. At a particular dilution rate recycling increases or decreases productivity by taking cell density closer or further from the optimum density. Cell recycle with heterogeneous populations can change the outcome of species competition. Selective recycling of one species can reverse this outcome or stabilize coexistence by its selective effect on cell detention time. Experimental results from light‐limited mixed cultures of S. geitleri and a Chlorella sp. verify this.