Closing the loop in bioproduction: Spent microbial biomass as a resource within circular bioeconomy.

Successful transition to a circular bioeconomy relies on the availability and efficient use of organic feedstocks such as agricultural and food waste. Advances in industrial biotechnology provide novel tools to valorize these feedstocks differently. Less attention, however, has been directed towards assessment of the organic side-residues arising from industrial biotechnology, such as spent microbial biomass (SMB). This study aims to reflect the current state of SMB within bioeconomy and create awareness of this growing industrial resource. Data from a range of published fermentation processes is used to estimate the amount of SMB formed per product (weight per weight, wt/wt) across different types of bioproducts, namely organic acids, alcohols, polymers, amino acids, antibiotics, protein and vitamins. Varying amounts of SMB are generated depending on the bioproducts and bioprocess, where bulk bioproducts, e.g. alcohols, generate less SMB than bioproduction of high-value low-volume specialty products, e.g. vitamins. It is estimated that more than 50 million tons of nutrient-rich SMB was generated in 2013, with SMB from bulk and specialty bioproduction accounting for roughly equal amounts. Furthermore, the composition of six industrially relevant organisms is summarized and compared, highlighting the general features of SMB as a carbon-rich substrate mainly consisting of protein. The results indicate that SMB is a growing resource with a reliable supply and predictable composition. The predictable nature of SMB could make it a favorable substrate for further innovation in industrial applications and nutrient circulation within the bioeconomy, for example, by using it as a co-substrate for valorization of other biomasses.

Towards compartmentalized photocatalysis: multihaem proteins as transmembrane molecular electron conduits.

The high quantum efficiency of natural photosynthesis has inspired chemists for solar fuel synthesis. In photosynthesis, charge recombination in photosystems is minimized by efficient charge separation across the thylakoid membrane. Building on our previous bioelectrochemical studies of electron transfer between a light-harvesting nanoparticle (LHNP) and the decahaem subunit MtrC, we demonstrate photo-induced electron transfer through the full transmembrane MtrCAB complex in liposome membranes. Successful photoelectron transfer is demonstrated by the decomposition of a redox dye, Reactive Red 120 (RR120), encapsulated in MtrCAB proteoliposomes. The photoreduction rates are found to be dependent on the identity of the external LHNPs, specifically, dye-sensitized TiO2, amorphous carbon dots (a-CD) and graphitic carbon dots with core nitrogen doping (g-N-CDs). Agglomeration or aggregation of TiO2 NPs likely reduces the kinetics of RR120 reductive decomposition. In contrast, with the dispersed a-CD and g-N-CDs, the kinetics of the RR120 reductive decomposition are observed to be faster with the MtrCAB proteoliposomes and we propose that this is due to enhancement in the charge-separated state. Thus, we show a proof-of-concept for using MtrCAB as a lipid membrane-spanning building block for compartmentalised photocatalysis that mimics photosynthesis. Future work is focused on incorporation of fuel generating redox catalysts in the MtrCAB proteoliposome lumen.