<i>Ecological Engineering of Photoorganoheterotrophic Microalgal Mixed Cultures to Valorise Cheese Whey</i>

Cheese whey is a massive dairying residue world-wide (154 Mm3·y-1) with high organic load, 50% being discharged causing eutrophication. Microalgal technologies capture organics and nutrients yielding added-value biomass and products like triacylglycerol-based biofuel. We studied photoorganoheterotrophy in microalgal mixed cultures to valorise whey. A mother photolithoautotrophic culture was enriched at 35 °C and 80 μmolphotons·m−2·s−1 on BG-11 medium. Photoorganoheterotrophy was tested in opened/sealed batches, using 40% demineralized whey powder concentrate, lactose, or acetate. All organics sustained phototrophic growth. Acetate was most efficiently serving as energy and carbon substrate, metabolised via glyoxylate cycle and possibly bypassing Krebs’ cycle. The selection of photoorganoheterotrophs in mixed cultures benefits from volatile fatty acids supplied by acidogenic pre-fermentation of whey. Scaling up, a 2-L continuous-flow stirred-tank photobioreactor was stabilized for photolithoautotrophy, followed by photoorganoheterotrophic regime applying 18.9 mmolC·L-1 acetate, 96 μmolphotons·m-2·s-1 irradiance, and sparging 50 mL·min- 1 argon. At 0.25 d-1 dilution rate, phototrophs were outcompeted by chemotrophs. At 0.17 d-1, photoorganoheterotrophs established in symbiotic relationship with chemoorganoheterotrophs, by converting acetate, prior to photolithoautotrophic recycling of CO2. The biomass reached 125 mgVSS·L-1 with 95% acetate consumption. Microbiomes were characterized from photolithoautotrophy to photoorganoheterotrophy by bacterial V3-V4 16S and eukaryal V4 18S rRNA gene amplicon sequencing. Cyanobium, Synechocystis, Desmodesmus, Scenedesmus and Trebouxiophyceae family relatives dominated in batches, and Synechocystis, Chlorophyceae and Trebouxiophyceae affiliates in chemostat. The microecosystems harboured side-populations of aerobic and/or fermentative chemoorganoheterotrophs depending on organics. We currently resolve microbial and metabolic networks by genome-centric metagenomics. Using mathematical modelling that predicts phototrophic growth, we captured conditions to harness photomixotrophs over chemotrophs and simultaneously produce an added-value microalgal biomass encompassing triacylglycerol from pre-fermented and raw cheese whey in cyclic light-organic-feast/dark-organic-famine regime. Photoorganoheterotrophy can be ecologically engineered in mixed-culture biotechnologies to recover bioresources from cheese whey surplus, foster a green circular economy, and safeguard the aquatic environment.