Microalgal-bacterial consortia: From interspecies interactions to biotechnological applications

Microalgae have emerged as a renewable and sustainable candidate for bioenergy production coupled with pollutant removal from wastewater. However, costly biomass harvesting, insufficient biomass productivity, and energy-intensive extraction methods are major bottlenecks restricting their large-scale development. To break through such limitations, researchers have focused on the technologies towards consolidating microalgal-bacterial consortia, which exhibit numerous advantages related to economy, energy, and environment, due to the cooperative interactions between microalgae and bacteria. This paper recapitulates the most recent work on microalgal-bacterial interaction mechanisms, and describes the diverse biotechnological applications of microalgal-bacterial consortia. Based on this review, the interaction mechanisms cover substrate exchange, cell-to-cell signaling, and horizontal gene transfer. Nutrient availability, growth phase, and cultivation conditions are major factors affecting their interactions. In terms of wastewater treatment, attached microalgal-bacterial consortia are economically feasible and technically superior compared to suspended microalgal-bacterial consortia. Appropriate carrier, bioreactor type, operation mode, operational factor, and further perspectives for engineering attached microalgal-bacterial consortia are critically assessed. Bacteria play an important role in promoting microalgal growth, enhancing bio-flocculation and facilitating cell wall disruption, and thus expanding the application potential of microalgal biofuel production. The current state of other promising biotechnological applications of microalgal-bacterial consortia (particularly mitigation of CO2 emissions, microalgal bloom control, and electricity generation) and the appropriate strategies to enhance their practical applications are discussed. The major challenges to scale up microalgal-bacterial consortia and corresponding recommendations for further research are also addressed.