Liquid-surface immobilization system and liquid–liquid interface bioreactor: Application to fungal hydrolysis

Microbial transformation of water-insoluble substrates by two types of novel interface bioprocesses, tacky liquid-liquid interface bioreactor and non-aqueous sporular bioconversion system

Although microbial transformation has been expected as a substitution technology for organic synthesis, microbial toxicity and water-insolubility of synthetic substrates prevent the practical application of the technology. For these problems, the authors have developed two types of interfacial bioprocesses, solid-liquid and liquid-liquid interface bioreactors and applied the systems to many microbial transformations. In the bioreactors, addition of substrates and accumulation of products were remarkably enhanced based on the toxicity alleviation effect on the interfaces and solubilization of substrates and/or products in an organic phase of the bioreactors. Recently, a novel tacky liquid-liquid interface bioreactor has been developed and applied to actinomycetes and yeasts. Furthermore, a novel bioconversion system with fungal spores in an organic phase has been constructed based on the combination of two facts as follows: (i) the fungal spores are never resting cells and are active ones like the vegetable cells, (ii) the fungal spores have the excellent solvent-tolerance. In this review, the tacky liquid-liquid interface bioreactor (L-L IBR_tac) and the non-aqueous sporular bioconversion system with immobilized fungal spores (NASB) are mainly given outlines.

Novel, Non-aqueous Bioconversion Systems Using Fungal Spores

Two novel types of non-aqueous bioconversion systems using fungal spores, either adsorbed on the surface of a filter pad or entrapped in calcium alginate beads, were constructed and applied for a model reaction: reduction of benzil to benzoin by Aspergillus sojae NBRC 32074. The spores adsorbed on a filter pad catalyzed the reduction in some toxic organic solvents, such as methylcyclohexane (log P: 3.61) and din-butyl ether (3.21). For the relationship between the reduction activity and the log P value of the organic solvent, a highly positive correlation (R²: 0.815) was observed. Surprisingly, the reduction proceeded in the more hydrophilic and toxic tert-butyl acetate (log P: 1.76). Glycerol was selected as the best hydride source. The higher the glycerol content, the more the benzoin was produced. While the production of benzil by spores was lower than that by mycelia in harmless di-n-hexyl ether (log P: 5.12), mycelia could not catalyze the reduction in the toxic tert-butyl acetate. In contrast, the spores entrapped in the calcium alginate beads could catalyze the reduction. Although the reduction by alginate-entrapped spores could be stably repeated 5 times in di-n-hexyl ether without a decline in the reduction activity, it was observed that the reduction activity of the spores gradually decreased after repeated reduction in tert-butyl acetate.

Production of Valuable Lipophilic Compounds by Using Three Types of Interface Bioprocesses: Solid-Liquid Interface Bioreactor, Liquid-Liquid Interface Bioreactor, and Extractive Liquid-Surface Immobilization System

Bioconversions such as enzymatic and microbial transformations are attractive alternatives to organic synthesis because of practical advantages such as resource conservation, energy efficiency, and environmentally harmonic properties. In addition, the production of secondary metabolites through microbial fermentation is also useful for manufacturing pharmaceuticals, agricultural chemicals, and aroma compounds. For microbial production of useful chemicals, the authors have developed three unique interfacial bioprocesses: a solid-liquid interface bioreactor (S/L-IBR), a liquid-liquid interface bioreactor (L/L-IBR), and an extractive liquid-surface immobilization (Ext-LSI) system. The S/L-IBR comprises a hydrophobic organic solvent (upper phase), a microbial film (middle phase), and a hydrophilic gel such as an agar plate (lower phase); the L/L-IBR and the Ext-LSI consist of a hydrophobic organic solvent (upper phase), a fungal mat with ballooned microspheres (middle phase), and a liquid medium (lower phase). All three systems have unique and practically important characteristics such as utilization of living cells, high concentration of lipophilic substrates/products in an organic phase, no requirement for aeration and agitation, efficient supply of oxygen, easy recovery of product, high regio- and stereoselectivity, and wide versatility. This paper reviews the principle, construction, characteristics, and application of these interfacial systems for producing lipophilic compounds such as useful aroma compounds, citronellol-related compounds, β-caryophyllene oxide, and 6-penty-α-pyrone.

Solvent-tolerance of fungi located on an interface between an agar plate and an organic solvent

While 6 by 20 of type culture fungi could grow on an interface between organic solvent (log P, 4.12) and agar plate, 13 by 20 of strains could form a large colony after the removal of more toxic solvent, such as styrene (log P, 2.95) and tert-butyl acetate (log P, 1.76) because of viability of spores on the interface.

Relationship between interfacial hydrophobicity and hydroxylation activity of fungal cells located on an organic-aqueous interface

In a liquid-liquid interface bioreactor, fungal cells locate in a hydrophilic polyacrylonitrile microsphere layer on an aqueous-organic interface. In this article, effects of hydrophobicity of the interface on n-decane hydroxylation activity of Monilliera sp. NAP 00702 was examined. (-)-4-Decanol production was significantly enhanced to 132% by addition of polytetrafluoroethylene.

Enhancement of 6-pentyl-α-pyrone fermentation activity in an extractive liquid-surface immobilization (Ext-LSI) system by mixing anion-exchange resin microparticles

The addition of anion-exchange resin microparticles into a polyacrylonitrile (PAN) ballooned microsphere layer drastically enhanced the fermentative activity of Trichoderma atroviride AG2755-5NM398 in an extractive liquid-surface immobilization (Ext-LSI) system. The production of 6-pentyl-α-pyrone (6PP), a fungicidal secondary metabolite, was 1.92-fold higher than the control (PAN alone).

Derepression of carbon catabolite repression in an extractive liquid-surface immobilization (Ext-LSI) system

An extractive liquid-surface immobilization (Ext-LSI) system with a fungal mat formed on the surface of a liquid medium effectively enabled derepression of carbon catabolite repression. In this system, a fungicidal secondary metabolite 6-pentyl-α-pyrone was efficiently produced by Trichoderma atroviride AG2755-5NM398 despite the addition of 25% glucose or fructose.

Synthesis of (-)-β-caryophyllene oxide via regio- and stereoselective endocyclic epoxidation of β-caryophyllene with Nemania aenea SF 10099-1 in a liquid-liquid interface bioreactor (L-L IBR)

Nemania aenea SF 10099-1, a basidiomycete isolated from a forest soil sample, regio- and stereoselectively epoxidized β-caryophyllene (Car) to (-)-β-caryophyllene oxide (Car-Ox) in a liquid-liquid interface bioreactor (L-L IBR) consisted of a liquid medium (a bottom phase), a fungus-ballooned microsphere (MS) mat (a middle phase), and an organic phase containing Car (a top phase). The cultivation conditions, such as carbon and nitrogen sources, kind of MS, initial medium pH and Car concentration, were optimized in the L-L IBR system. The best carbon and nitrogen sources were xylose and tryptone, respectively. The most suitable polyacrylonitrile MS was MMF-DE-1 (former MFL-80SDE; non-coated type). Although the strain could not grow below pH 5.5, the endocyclic epoxidation of Car efficiently proceeded at a wide range of initial medium pH (6.0 to 9.0). The bioconversion system exhibited an excellent alleviation effect toward substrate and product inhibitions. While Car could be added into an organic phase (KF-96L-1CS, dimethyl silicone oil) at 50% (w/v), the accumulation of Car-Ox reached over 30g/l in spite of these strong microbial toxicities. Moreover, the epoxidation reaction smoothly proceeded in a novel L-L IBR system, a multistory L-L IBR systems, consisted of 5 stacked reactor units. The optical rotation of Car-Ox produced was (-) and the enantiomeric excesses of (-)-β-Car-Ox purified by 1st and 2nd recrystallization from methanol reached 97.51 and 99.33%, respectively.

Efficient hydrolytic reaction of an acetate ester with fungal lipase in a liquid-liquid interface bioreactor (L-L IBR) using CaCO₃-coated ballooned microsphere

In a liquid-liquid interface bioreactor using a CaCO₃-coated ballooned microsphere, 2-ethylhexyl acetate was efficiently hydrolyzed to 2-ethyl-1-hexanol with Absidia coerulea NBRC 4423 compared with using talc-coated or non-coated ballooned microsphere. It was assumed that CaCO₃ brought about stabilization of lipase by Ca²⁺ and maintenance of medium pH.