A comparative assessment on how molasses and CO2 gas prevent carbon limitation in the large-scale culture of freshwater macroalgae

Freshwater macroalgae are an attractive treatment option for waste streams that have very high concentrations of nutrients. However, the long water residence times required in these scenarios will result in carbon becoming a limiting nutrient that negatively impacts the rate of biomass productivity and, subsequently, the potential for nutrient uptake. This study examined how the rate of carbon supplementation influenced the biomass productivity of Oedogonium intermedium, during both winter and summer periods in high rate algal ponds (HRAPs) maintained under batch conditions. We also examined the novel use of molasses as a source of carbon for intensive algal cultures and compared its effect on biomass productivity to that of CO2 gas. Oedogonium intermedium responded positively to carbon supplementation from both molasses and CO2 gas, with biomass productivity increasing as the rate of carbon supplementation increased. In cultures with no carbon supplementation, the average productivity of O. intermedium was 2.2 (± 0.8) g m− 2 day− 1, and a maximum of 17.3 g m− 2 day− 1 and 20.8 g m− 2 day− 1 during summer when carbon was supplemented through the addition of molasses and CO2, respectively. The optimal rate of carbon supplementation was 0.06 g of carbon per liter (g (C) L− 1) in winter and 0.08 g (C) L− 1 during summer. The addition of an exogenous source of carbon resulted in a decline in the culture pH and increased the availability of the dissolved inorganic carbon (DIC) pool and subsequently, carbon uptake by the algae. This study has identified a novel method to supplement carbon to algal cultures, where the waste residue from the sugar industry can be used as viable source of inorganic carbon. Importantly, we have demonstrated that O. intermedium can be cultivated in HRAPs without requiring a high rate of water exchange, providing that its requirements of DIC are satisfied.