**4.2 Metabolic flexibility of microalgae**

Microalgae have three different metabolic pathways, namely, autotrophy, heterotrophy and mixotrophy. While all algae species are autotrophic, some stains have the ability to exhibit heterotrophy and mixotrophy; and any of the chosen photosynthetic metabolic depends on the microalgae species, and the quality of the biomass desired. Autotrophic metabolism utilizes inorganic carbon in the form of CO2, gas and light energy. This mode of fixing CO2 produces low density microalgal biomass. Heterotrophic metabolism takes advantage of the presence of organic carbon and utilizes it both as a source of carbon and energy. This is the dominant pathway during the night or dark phases. Some microalgae do metabolize mixotrophically. Under mixotophic mode, light energy is not the absolute growth limiting factor as organic carbon sources are also accessed and utilized for microalgal biomass production. Photoinhibition, a phenomenon that describes excessive light intensity thereby arresting photosynthetic metabolism, is overcome under the mixotrophic metabolic mode. Consequently, the growth rate is not interrupted and high density biomass is produced with recorded higher productivities when compared to autotrophic and heterotrophic metabolic scenarios [6].
