Development of Bacterial Platforms for Upgrading of Technical Lignins

Lignocellulosic feedstocks from agriculture and forestry, provide an inexpensive and sustainable raw material for biofuels and chemicals production. Microorganisms, such as certain bacteria, can utilize complex substrate mixtures making them interesting candidates for biocatalysis of lignocellulose. Further strain engineering can improve biosynthesis of industrially relevant products. However, the recalcitrant nature of lignin and the presence of toxic compounds has posed challenges for traditional bioprocessing.

In this Doctor of Science thesis, lignin and lignocellulosic biomass valorization by bacterial systems was investigated. Molecular tools were developed for monitoring lignin-derived aromatics from bioprocesses containing technical lignin liquors. The tools were based on antibodies that allow identification of target compounds even from complex matrixes, such as technical lignins, with high specificity. The potential of native and non-native product biosynthesis from lignocellulosic substrates was demonstrated by a flexible and robust bacterial chassis, Acinetobacter baylyi ADP1. The substrate preferences and tolerance towards selected lignin derived aromatic acids were profiled. Industrially relevant medium- and long chain carbon compounds, such as wax esters (C34), alkanes (C17) and α-olefins (C11) were produced from abundant, yet challenging materials, such as aromatic acids, softwood originated technical lignin and cellulose, as well as less difficult substrates including glucose, acetate and lactate. The products were obtained by exploiting natural catabolism and lipid production pathways of ADP1. The non-native product synthesis (alka(e)nes) was enabled by genetically modifying the natural fatty acid pathway. The α-olefins are industrially interesting molecules as they can be further polymerized to poly-α-olefins (C33) by catalytic oligomerization reactions.

Additionally, the substrate and product range were expanded by a multispecies system that combined fermentation metabolism of anaerobic bacteria to the synthesis pathways of engineered aerobic bacteria ADP1. The multispecies approach by the Clostridium species and ADP1 enabled cellulose hydrolysis, hydrogen gas (H2) production, α-olefin or wax ester production.

In this work, a proof-of-principle bacterial platform for upgrading of technical lignin was established. In the future, further improvements are required related to the production efficiencies. This could be achieved, for example, by metabolic pathway and bioprocess optimization and increasing tolerance towards different inhibiting compounds.