Direct microbial conversion

In direct microbial conversion of lignocellulosic biomass into ethanol that could simplify the ethanol production process from these materials and reduce ethanol production costs, Clostridium thermocellum, a thermoanaerobe was used for enzyme production, hydrolysis and glucose fermentation (755). Cofermentation with C thermosaccharolyticum simultaneously converted the hemicellulosic sugars to ethanol. However, the formations of by-products such as acetic acid and low ethanol tolerance are some drawbacks of the process. Neurospora crassa produces extracellular cellulase and xylanase and has the ability to ferment cellulose to ethanol 139). 

A direct microbial conversion of glucose and UMP to UDP-Glc has also been explored [78] as has the production of a-galactosyl phosphate from bacteria [79]. 

Photosynthetic products are synthesized by plants from water and carbon dioxide, and therefore renewable monomers obtained from plants are very attractive. Though natural polymers from plants are also very attractive for the same reason, monomers are preferentially focused on in this chapter due to limitations of space (monomers obtained by digestion of natural polymers are included here). Types of renewable monomers obtained from plants without any chemical, enzymatic, and microbial conversion are limited, since plant cells are mainly composed of polymers. The principal monomer compounds that can be directly extracted from plants are oils triglycerides of fatty acids and essential oils. 

One more important economic factor is the costs for the recovery of the fuel from the fermentation broth. Additionally, it is possible that the separated molecule is a precursor that needs to be modified to transform it into an engine fuel. Ideally, biomass feedstock would be directly transformed, through microbial conversion, into fuel-grade molecules, which would then be secreted into the fermentation broth and separated relatively easily due to their immiscibility in water (Rude and Schirmer 2009). 

Acetic acid is a two-carbon monocarboxylic acid with many applications in food and as a building block for a wide range of industrial chemicals (Table 7.1). While much of the industrial acetic acid production is used directly for applications such as solvents, its primary use by volume is as a feedstock for the production of other chemicals such as vinyl acetate, which is used in plastics, adhesives and paints. Other products include cellulose acetate, acetic anhydride and acetate esters. The synthetic production route has been the dominant process since 1950 [31] using either hydrocarbon-derived ethylene or methanol as the feedstock. The 10% currently produced by microbial conversion is directed towards food applications. 

The ease of the Strecker synthesis from aldehydes makes a-aminonitriles an attractive and important route to a-amino acids. Fortunately, the microbial world offers a number of enzymes for carrying out the necessary conversions, some of them highly stereoselective. Nitrilases catalyze a direct conversion of nitrile into carboxylic acid (Equation (11)), whereas nitrile hydratases catalyze formation of the amide, which can then be hydrolyzed to the carboxylic acid in a second step (Equation (12)). In a recent survey, with a view to bioremediation and synthesis, Brady et al have surveyed the ability of a wide range of bacteria and yeasts to grow on diverse nitriles and amides as sole nitrogen source. This provides a rich source of information on enzymes for future application. 

The strength of the bioassay approach is that it directly estimates the fraction of natural DOC that can be used by a natural microbial assemblage under defined conditions. However, there are numerous manipulations of water samples during bioassay incubations, and the effects of these manipulations on the measured parameters are not well known. For example, containment of water samples can rapidly alter microbial population structure. Nutrients, rather than carbon, can be limiting for microbial utilization of DOM. Moreover, there are no standard protocols for bioassay experiments. Different indicators of DOM utilization are measured by different investigators, and many of the measured parameters rely on conversion factors that are also quite variable. The extent of DOM utilization also depends upon the duration and temperature of the bioassay experiment. Despite these shortcomings, the bioassay experiment remains the best approach for estimating the bioavailability of DOM. 

There are four major transformation pathways leading from the DOM pool into the microbial loop direct uptake and photolysis-, ectoenzyme-, and sorption-mediated uptake (Fig. 1). Each of these pathways or processes is regulated by a combination of intrinsic and extrinsic factors. Intrinsic factors are elements of the pathway itself and include DOM characteristics, enzyme kinetics, and microbial diversity. For instance, the uptake characteristics of the resident microbial community will affect which monomers are assimilated from the pool of DOM. Conversely, the composition of the DOM pool is likely to affect which microbial consortia are present and active at any given time. 

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