Differing growth responses in four related microalgal genera grown under autotrophic, mixotrophic and heterotrophic conditions

Microalgae are increasingly recognised as valuable feedstock for novel product production and biotechnological applications, but high costs associated with both the culturing and harvesting of microalgae remain one of the major bottlenecks. Mixotrophy and heterotrophy are increasingly being recognised as potentially cost-effective growth strategies for mass culture, but the response of microalgae to alternative growth strategies is not universal amongst species. This study assessed the response of four species from related genera (Acutodesmus acutiformis, Desmodesmus communis, Scenedesmus obtusus, and Tetradesmus dimorphus) to the different growth conditions, autotrophic, mixotrophic, and heterotrophic with inorganic nutrients and heterotrophic with organic nutrients. While all four related genera were capable of growing mixotrophically and heterotrophically, the response was not universal across all four species. Biomass production was highest under heterotrophy for three out of the four species, while production was inhibited in T. dimorphus, relative to autotrophy. The percentage lipids and proteins varied amongst all four species under the four different growth strategies. Percentage lipids were highest under autotrophy and mixotrophy for both A. acutiformis and D. communis, while percentage lipids were lowest in S. obtusus under autotrophic conditions. For T. dimorphus percentage, lipids did not vary across treatments. The reverse occurred for percentage proteins, being highest for both A. acutiformis and D. communis under heterotrophy and highest for S. obtusus under autotrophic conditions. Percentage protein did not vary across treatments for T. dimorphus. Coenobium size and total numbers varied amongst treatments for all four species, with no consistent between the species. Microalgal performance under either heterotrophic or mixotrophic growth was species dependent. For commercial applications, these results demonstrate the importance of assessing the response of a given species to alternative growth strategies and if any resultant enhanced biomass, or product, yield is cost-effective compared to autotrophy.