Organic acids from lignocellulose: Candida lignohabitans as a new microbial cell factory

Biorefineries require microbes for the conversion of various lignocellulose derived sugars into value-added chemicals or fuels. The organism of choice has to fulfill some prerequisites in terms of substrate range, tolerance to inhibitors derived from biomass pretreatment, pH-spectrum and accessibility to genetic modifications. We tested various yeast strains for growth on lignocellulosic sugar monomers like glucose, galactose, mannose, arabinose and xylose. Candida lignohabitans turned out to grow very well on all of these substrates in pure form or mixtures thereof. Furthermore, C. lignohabitans performed very well on hydrolysates of miscantus, sawdust, hard- and softwood chips thereby accumulating biomass and varying amounts of ethanol as a natural fermentation product. In addition to a broad substrate range, the ideal yeast for biorefinery applications has to be accessible for genetic modifications. Genetic engineering of C. lignohabitans with the isolated endogenous GAP promoter and terminator was successful. Expression of lactate dehydrogenase (L-LDH) and cis-aconitate decarboxylase (CAD) resulted in stable production of lactic acid and itaconic acid, respectively. The desired organic acids were accumulated converting pure sugars as well as lignocellulosic hydrolysates. In addition, C. lignohabitans proved to be very tolerant to growth at low pH-levels, which is an important feature when organic acids are the desired product. Thus C. lignohabitans proves to be a promising and reliable microbial host for production of organic acids from lignocellulosic material.