Bio-refinery of xylose processing wastes for green polymalic acid production and l-malic acid recovery by engineered Aureobasidium pullulans in a non-waste-disposal system

Commercial carboxylic acid fermentation is susceptible to CaSO4 solid waste pollution owing to the heavy use of neutralizer CaCO3. Polymalic acid (PMA), a biodegradable polyester generated from the fungus Aureobasidium pullulans, has attracted attention in the bioplastic and biomedical fields and is easily hydrolyzed to release the monomer l-malic acid (L-MA). This study evaluated the feasibility of using waste xylose-containing mother liquor (WXML) for efficient PMA fermentation under low pH control and L-MA recovery in a non-waste-disposal system. Herein, the multi-adaptive evolution of A. pullulans under high concentrations of WXML, coupled with low pH, was demonstrated. Subsequently, an engineered calcineurin signaling pathway overexpressing the exogenous cnb gene from Beauveria bassiana was employed to further regulate cell tolerance. A mutant strain AE-59 was obtained, and a comparable PMA titer of 49.47 ± 0.48 g/L and a yield of 0.33 g/g at low pH control with Na2CO3 in a 5-L fermenter were achieved. Finally, the direct-aqueous-phase-crystal method was employed to obtain 78.55 % of the total L-MA (with a purity of 95.4 %) from the acidic downstream broth using a low dosage of sulfuric acid for acid hydrolysis. This result showed the feasibility of L-MA production in the low-pH control mode with Na2CO3 without solid waste CaSO4. In conclusion, this study overcomes the major obstacles of calcium waste pollution for future industrial fermentative L-MA production and provides a green and sustainable route from lignocellulosic biorefinery for the production of future biopolymers and carboxylic acids.