Green phototrophs such as microalgae and cyanobacteria have been proven to be able to perform photoorganoheterotrophic metabolism. The use of organic carbon can result in higher biomass production and increased concentrations of valuable compounds. Organic waste streams can serve for this purpose and lower the biomass production costs. Cheese whey, a by-product of the dairy industry with a high organic content, can be a suitable organic carbon source.
Previous studies have focused on axenic cultures, determining the species that can perform this metabolic pathway and the best substrates for their growth. However, the use of organic substrates by phototrophs in mixed microbial communities is less understood, as it is the result of multiple metabolic processes. As mixed cultures are economically preferable to pure ones, further research is needed to understand the competition mechanisms taking place in mixed-culture processes and how they can be engineered to promote the selection of phototrophs.
The first part of this work focused on the production and spectrum of volatile fatty acids from the acidogenic fermentation of 40% demineralized cheese whey. The maximum degree of acidification (77±7%) of 40% demineralized cheese whey (DWP40) was obtained when thermal (90°C) pre-treatment of the inoculum was applied and combined with a F/M ratio of 0.5 g COD/g VS.
The second part aimed to assess the selection for green phototrophs in a photoorganoheterotrophic mixed culture, using organic carbon sources derived from cheese whey, namely DWP40, lactose (as a model constituent of cheese whey), and acetate (as model volatile fatty acid derived from acidogenic fermentation of cheese whey). The cultivations were carried out in shake-flasks prior to implementation in a continuous-flow stirred-tank photobioreactor. DWP40, lactose, and acetate sustained the growth of green phototrophs in the mixed culture. Amongst the organic carbon sources, the use of acetate resulted in the highest biomass growth (170 mg VSS/L) and pigment content (87 μg/mg VSS). The selection for phototrophic organisms was possible both in batch and continuous mode.
The results obtained showed that the conversion of the lactose inside cheese whey to acetate could improve its uptake by phototrophs. Their selection inside a photoorganoheterotrophic mixed culture can be improved by higher pH and inorganic nutrient concentrations, and lower dissolved oxygen levels.