Johnson Matthey Technology Review - Volume 67, Issue 4, 2023

Microbubbles are famed for their large surface area-to-volume ratio, with the promise of intensification of interfacial phenomena, highlighted by more rapid gas exchange. However, for bioprocessing, it has been recognised for many decades that surfactant-rich fermentation media hinders mass transfer and possibly other interfacial processes due to surfactant loading on the interface. This article focuses on the roles of microbubble size and bubble bank, dispersed microbubbles that are sufficiently small to be non-buoyant, in mediating other modes of interfacial transfer via collisions with microorganisms and self-assembled clusters of microorganisms and microbubbles. These provide a more direct route of mass transfer for product gases that can be released directly to the microbubble with ~10⁴ faster diffusion rates than liquid mediated gas exchange. Furthermore, secreted external metabolites with amphoteric character are absorbed along the microbubble interface, providing a faster route for liquid solute transport than diffusion through the boundary layer. These mechanisms can be exploited by the emerging fields of symbiotic or microbiome engineering to design self-assembled artificial lichen dispersed structures that can serve as a scaffold for the selected constituents. Additionally, such designed scaffolds can be tuned, along with the controllable parameters of microbubble mediated flotation separations or hot microbubble stripping for simultaneous or in situ product removal. Staging the product removal thus has benefits of decreasing the inhibitory effect of secreted external metabolites on the microorganism that produced them. Evidence supporting these hypotheses are produced from reviewing the literature. In particular, recent work in co-cultures of yeast and microalgae in the presence of a dispersed bubble bank, as well as anaerobic digestion (AD) intensification with dispersed, seeded microbubbles, is presented to support these proposed artificial lichen clusters.

Hydrogen offers a source of energy that does not produce any greenhouse gas (GHG) when combusted. However, some hydrogen manufacturing methods consume large amounts of energy and produce carbon dioxide as a byproduct. The production of hydrogen by bacteria is an attractive alternative because it is not energy intensive and, under the right conditions, does not release GHG. In this review, we introduce the five known ways by which bacteria can evolve hydrogen. We then describe methods to encapsulate living bacteria in synthetic layers, called biocoatings, for applications in bioreactors. We review the few examples in which biocoatings have been used to produce hydrogen via the photofermentation method. Although not used in biocoatings so far, the dark fermentation method of hydrogen production avoids the need for illumination while offering a high yield with low oxygen evolution. We identify the potential for using genetically-modified bacteria in future research on biocoatings.

The Biotechnology and Biological Sciences Research Council (BBSRC) funded six Networks in Industrial Biotechnology and Bioenergy (NIBB) for a second phase in 2019 and previously 13 BBSRC NIBB for a first phase in 2014. These networks promote interactions between academia and industry to advance research along the technology readiness levels (TRLs). The networks fund collaborations that start in the region of TRL2 and Johnson Matthey contributes to the management of one of these BBSRC NIBB entitled ‘The Elements of Bioremediation, Bio-Manufacturing and Bioenergy (E3B)’. This network brings together communities working on metals in biology.

Sustainability has been one of the main issues in the world in recent years. The decrease of resources in the world, along with the growing world population and the resulting environmental waste, present a fairly significant problem. As an alternative solution to this problem, insects are put forward as an ideal resource. Due to the enzymes and microorganisms in their intestinal microbiota, the biotransformation processes of insects are capable of converting wastes, organic materials and residues into valuable products that can be used for various industrial applications such as pharmaceuticals, cosmetics and functional foods. Some species of insects are in an advantageous position because of the simplicity of their lifecycle, the ease of their production and their ability to feed on organic materials to make valuable products. From a sustainability perspective, utilisation of the microorganisms or enzymes isolated from these microorganisms available in the microbiota of insects may allow novel insect-based biotransformation processes that promise a more sustainable world and novel green technologies.

Chiral amines are important building blocks in the pharmaceutical, agrochemical and chemical industries. There is a drive to augment traditional transition metal catalysts with ‘green’ alternatives such as biocatalysts. Transaminase (TA) biocatalysts can be used in combination with ‘smart’ sacrificial amine donors to synthesise a variety of aliphatic and aromatic amines from the corresponding aldehydes and ketones. Despite their enormous potential, the unfavourable reaction equilibrium often limits the widespread application of TAs for industrial synthesis. Recently we disclosed a new biomimetic amine donor N-phenyl putrescine (NPP), which was inspired by the biosynthesis of the dipyrroloquinoline alkaloids. NPP was demonstrated to have good activity with a library of commercial and wild-type TAs (total 25 TAs). This work focused on exploring the use of NPP with the Johnson Matthey TA kit (17 biocatalysts; eight S-selective and nine R-selective) and three different amine acceptors (vanillin, benzaldehyde and acetophenone). NPP worked well with all 17 TAs and gave the corresponding amine products vanillylamine, benzylamine and methylbenzylamine (MBA) in up to 85% high-performance liquid chromatography (HPLC) yield. From the screen, STA-14 was identified as a good biocatalyst for further analysis and used in a comparative screen of NPP versus the commonly used donor iPrNH2. It was found that NPP was the best amine donor and used to prepare S-methylbenzylamine in >99.5% enantiomeric excess (e.e.). This work, combined with our previous study, highlights the potential of NPP in the biocatalytic synthesis of amines.

The biosynthesis of palladium nanoparticles supported on microbial cells (bio-Pd) has attracted much recent interest, but the effect of solution chemistry on the process remains poorly understood. Biological buffers can be used to maintain physiological pH during the bioreduction of Pd(II) to Pd(0) by microbial cells, however, buffer components have the potential to complex Pd(II), and this may affect the subsequent microbe-metal interaction. In this study, a range of Pd(II) salts and biological buffers were selected to assess the impact of the solution chemistry on the rate of bioreduction of Pd(II) by Geobacter sulfurreducens, and the resulting biogenic palladium nanoparticles. The different buffer and Pd(II) combinations resulted in changes in the dominant Pd(II) species in solution, and this affected the amount of palladium recovered from solution by the microbial cells. The physical properties of the bio-Pd nanoparticles were altered under different solution chemistries; only slight variations were observed in the mean particle size (<6 nm), but significant variations in particle agglomeration, the extent of Pd(II) bioreduction and subsequent catalytic activity for the reduction of 4-nitrophenol (4-NP) were observed. The combination of sodium tetrachloropalladate and bicarbonate buffer resulted in bio-Pd with the smallest mean particle size, and the fastest initial rate of reaction for 4-NP reduction (0.33 min^–1). Other solution chemistries appeared to damage the cells and result in bio-Pd with relatively poor catalytic performance. This work emphasises that future studies into bio-Pd synthesis should consider the importance of solution chemistry in controlling the speciation of Pd(II) and its impact on both the bioreduction process and the resulting properties of the nanoparticles produced, in order to maximise Pd(II) biorecovery and optimise catalytic properties.

Bacterial cellulose (BC) has attracted much research interest, delivering a combination of exclusive properties, such as flexibility, hydrophilicity, crystallinity and a three-dimensional network. In this study, the effects of carbon source and cultivation conditions on BC production by the bacterium Acetobacter xylinum subsp. sucrofermentans DSM 15973 were assessed. Fructose was the most suitable carbon source and high BC concentrations up to 31 g l^–1 were achieved in substrates with 60 g l^–1 fructose under static culture conditions. Notably, BC production was equally high under the same fermentation conditions in agitated cultures (~30 g l^–1). Moreover, the effectiveness of sodium hydroxide and sodium hypochlorite solutions in BC purification and their potential impact on BC structure and properties were explored. The combination of weak NaOH and NaOCl proved an effective purification method, preserving the fibre structure and crystallinity of BC.