A review on microalgal growth stress parameters for sustainable biofuel production

Rapid growth and high carbon-fixation properties make microalgal biomass a preferred source of energy-production technologies, particularly biodiesel and bioethanol, as well as nutritional supplements and pharmaceuticals. Today, microalgal biofuel technology, which is continuing to be developed as a surrogate for fossil-fuel technology, is the focus of constant research and development, especially regarding biomass yield and lipid production. Single stressors on microalgae have been commonly examined for efficient biofuel production; however, the synergetic effects of stressors on the lipid and biomass productivity of microalgae might lead to enhanced lipid properties. In addition, considering more than one parameter might be time-consuming; thus, many models, such as the surface-response methodology, can enable significant optimization of microalgal productivity economically. As well as multiple stressors, adaptive stressors considered in "adaptive laboratory evolution" could provide robust strains that alter low lipid yields. Although the field has progressed substantially in recent years, much about microalgae, their biological potential, how to efficiently utilize them, and their metabolic responses to various stressors remain unknown. Using an extensive review of recent developments in single-stress strategies, combined-stress strategies, and adaptive stress strategies, this paper gives an overview of recent developments in microalgal biofuel production under stress conditions. Further, it has also been suggested that an integrated microalgal biorefinery that also produces high-value chemicals alongside biofuel might significantly enhance its economic feasibility. It is expected that this document will provide an understanding of how to develop commercially viable microalgae strains for biofuel applications, taking several types of stress into consideration.