Wine Lees Valorisation via Pilot-Scale Production of Phototrophic Purple Bacteria for Aquaculture Applications

Phototrophic purple bacteria (PPB) are a ubiquitous group of anoxygenic phototrophs, that can use organic/inorganic electron donors for anaerobic growth under heterotrophic and autotrophic conditions. PPB can collect light via bacteriochlorophylls (BChls) and carotenoids (Crts), also in non-sterile environments, and can contain approximately 60% of crude protein. These factors make PPB an ideal microorganisms’ group for assimilation of nutrients from waste/wastewater, yielding a product with potential interest as aquaculture feed or fish-feed additive. In this work, wine lees (WL) was exploited as renewable carbon source for PPB production in three tubular pilot-scale photobioreactors (PBRs; 50 L each) under different operating conditions, which were set to investigate the impact of the light availability (24h/day and 12h/day) and hydraulic retention time (HRT; 3 – 6 days). The organic loading rate was set at 1.0 g COD/(L d) (as soluble COD); wine lees was previously fermented to produce a volatile fatty acids (VFA) rich stream mainly composed by acetic, butyric and propionic acid (65%, 29% and 8% respectively). Each PBR was maintained in a dark thermostatic (25°C) container, illuminated by 4 LED lamps (122W in total) in far-red and infrared spectrum. The continuous light availability supported the growth of PPB and the synthesis of pigments better than intermittent light, with a growth yield of 0.43 CODPPB/CODSOL (20% higher than PBR with 12 h light/day). However, intermittent light promoted the storage of intracellular polyhydroxyalkanoates (PHA; up to 6% of cells’ dry weight), presumably as response to the stress caused by discontinuous illumination. On the contrary, PHA was not detected in the PPB grown under continuous light. Regarding the effect of HRT, when it was set at 6 days a 100% VFA removal was observed, which decreased to 75% when the HRT was reduced to 3 days. This suggested the necessity to investigate intermediate HRT values. In terms of nutritional value, the crude protein content of the three biomasses (separately collected from the three PBRs) was in the range 44-56 wt%. The highest value was obtained in the PBR operated under continuous light and 3 days as HRT. Other analyses are ongoing and addressed to measure carbohydrates, lipids and aminoacidic profile highlighting the potential of PPB biomass as a high protein, pigment rich ingredient for aquafeeds