Attached Biofilm Cultivation (ABC) of Mixotrophic Microalgae for the Sustainable Supply of Innovative New Bioproducts

The industrial utility of microalgae is critically constrained by high-volume, monotrophic suspension approaches to biomass cultivation. Suspension cultivation suffers from high capital costs, labor-intensive culture maintenance, low biomass productivity, and prohibitively high energy demands for biomass mixing and harvesting. Attached Biofilm Cultivation (ABC) of aquatic mixotrophic microalgae is attracting increasing interest as a solution to many of these challenges. Aquatic microalgae biofilms constitute a hidden bank of chemical diversity and functional potential. Primarily phototrophic, biofilm-forming microalgae exhibit high metabolic flexibility with the capacity for complementary heterotrophy (mixotrophy) under low light conditions reported in several species. Biomass yields up to five-fold higher than those of suspension systems are also reported. ABC focusses on biofilm-forming species of diatoms, cyanobacteria, and chlorophytes species naturally rich in high-value organic and inorganic materials. In this review, authors report on ABC as a means for more consistent and cost-effective industrial biomass production. Principle physiochemical parameters affecting biofilm metabolism and metabolite biosynthesis are discussed, with special attention given to photobiology and the use of artificial LED light to enhance productivity in synthetic mixotrophic microalgae biofilms. Finally, inter-species microbe–microbe mutualism in natural biofilms is discussed and the challenges in harnessing positive interactions are reviewed.