Microbial Fuel Cells as CO2 Source in the Autotrophic Cultivation of the Green Microalgae Tetraselmis subcordiformis: Impact on Biomass Growth, Nutrient Removal, and Hydrogen Production

Carbon dioxide (CO2) is often a limiting factor for the growth of microalgal biomass. Consequently, the search for new CO2 sources that do not contain components inhibitory to microalgal metabolism remains a priority. An alternative to the solutions tested thus far may involve the use of CO2-rich gas derived from microbial fuel cells (MFCs). This concept served as the basis for the original experimental work described in this study. The objective of the research was to evaluate the effect of using gases from the anode chamber of an MFC as a CO2 source in the autotrophic cultivation of Tetraselmis subcordiformis. The highest biomass growth efficiency was observed when the CO2 concentration in the culture medium was maintained at 220.0 ± 8.0 mg/L. Under these conditions, the microalga proliferation rate reached 0.52 ± 0.03 g VS/(L∙day) and 11.54 ± 0.42 mg chl-a/(L∙day), with a final biomass concentration of 2.68 ± 0.10 g VS/L and 63.53 ± 2.44 mg chl-a/L at the end of the cultivation cycle. Moreover, the highest total hydrogen (H2) production of 312 ± 38 mL was achieved in the same experimental variant, corresponding to an H2 production rate of 62.4 ± 6.1 mL/day. The removal efficiency of ammonium nitrogen (N-NH4) was notably high in experimental variants using MFC-derived biogas, ranging from 97.0 ± 2.2% to 98.2 ± 1.8%. Additionally, the growing microalgal biomass effectively utilized phosphate phosphorus (P-PO4) and iron, further highlighting its potential for nutrient recovery.