Microbubble Intensification of Bioprocessing : The role of direct microorganism and bubble interactions

Abstract

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.