DEVELOPMENT OF AN ALGAE MIXOTROPHIC BIOFILM REACTOR FOR BIOMASS PRODUCTION
Rincon Miranda, Sandra Milena
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This research aims to grow algal biofilms under mixotrophic condition without external CO2 supply. CO2 can be internally generated using glycerol and urea, in a closed membrane biofilm reactor operated in flow through and fed-batch recycle mode. Under conditions of low light intensity and volume (<100 µmol m-2 s-1 and < 100 ml), 14 hours light and 10 hours dark cycle, 5 g L-1 glycerol concentration, and 1 ml min-1 liquid flow, Chlorella vulgaris biofilms effectively grew under mixotrophic conditions in the developed biofilm reactor. The biofilm (> 300 μm thickness) within 192 hours of operation obtained biomass productivity of 12.64±0.94 g DW m-2 d-1 and total lipid content of 23.91±0.03% w/w. The pH in the biofilm did not vary with the depth, and it was around 6.8 in the dark and 7.5 in the light. Inside the reactor, the liquid medium was in continuous contact with the biofilm. The biofilm was accumulated above a membrane filter that filled the reactor’s cross-sectional area. As the liquid medium was recirculated into the closed reactor, the accumulated CO2/O2 gases were dissolved in the medium and consumed by photosynthesis and respiration processes. Chlorophyll-a fluorescence results showed that the use of the bioreactor and medium favored both metabolisms of C. vulgaris and allowed the growth in biofilms without photochemistry inhibition. An average 0.65 Fv/Fm was obtained in the PAM analyses, thus light energy to chemical energy conversion by the PSII in the C. vulgaris biofilms was maintained throughout the evaluation time. After 168 hours of growth, the enzymes ACC and DGAT for lipid biosynthesis were upregulated as a result of the shift from exponential growth to stationary growth. This metabolic shift was also shown by the downregulation of genes for G3PD and PRK related to the Calvin-Benson cycle together with an MDH change in the carbohydrate metabolism. We observed that the lipid profile and the pH were affected by the organic carbon source used (glycerol, glucose, and acetate). In this new technology, the biomass solid content reached 22%, need for handling algae for lipid production and harvest was minimized, and oil quality was highly improved.