Bioreactors that employ a synthetic microbial community hold potential to overcome limitations of those based on a single species, which embrace a higher level of complexity due to the inter-species interactions. In this work, a number of generic system structures involving two cross-feeding species and various types of inhibition have been studied, together with two three-species cases where a third species is introduced to fulfil a specific function. These cases are represented by mathematical models and inspected through bifurcation analysis and numerical simulation to reveal how the system structure and parametrization affect stability and productivity of the bioreactor. The results show that inhibitions generally lead to reduction in both productivity and stability, and that the presence of a negative feedback loop and a positive feedback loop may give rise to oscillation and bi-stability, respectively, depending on the strength of the inhibitions involved. The intended gains by the introduction of a third species may be achieved when its negative side-effect is sufficiently moderate, and at the cost of reduced stability. As observed in several cases, the changes in stability and productivity do not always follow the same trend, implying trade-off between the two objectives in the engineering of such bioreactors.

Several recent experiments of microalgal polyculture have observed an increased yield compared to monocultures, leading to the suggestion that improved use of the available light resource due to complementary light absorption spectra may be among the underlying reasons. Using numerical simulation, this work explores the impact of conditions (including both operation settings and species traits) on the possibility of biomass overyielding by a co-culture of two algal species. From the study of a system with simplified light absorption spectra, it was found that both operation settings such as incident light intensity and loss rate of algal biomass and species traits such as light use efficiency and maximum specific growth rate can affect the possibility and degree of overyielding. Co-culture of two actual species with light absorption spectra that exhibit complementarity was also simulated, which further confirmed the importance of some of these factors in a realistic case. This work thus suggests that complementary light absorption alone may lead to overyielding in a polyculture of microalgae, only when a number of biological and operational conditions are met.