Microbial substrate utilisation

In any quantitative assessment of growth and/or product formation, it is essential to link formation of microbial biomass and products with the utilisation of substrate and nutrients. In the case of microbial biomass production, the total amount of cell mass yield formed is often proportional to the mass of substrate utilised. Mathematically this is coefficient expressed as the corresponding ratio, or yield coefficient ... 

M. C. Rillig, K. M. Scow, J. N. Klironomos, and M. F, Allen, Microbial carbon-substrate utilisation in the rhizosphere of Cutierrezia sarothrae grown in elevated atmospheric carbon dioxide. Soil Biology and Biochemistry 29 1387 (1997). 

The MRB shown in Figure 3.26 consists of a reaction vessel and a membrane (UF or MF) module. The biocatalyst is suspended in the main reaction vessel, and the feed is pumped into the reaction vessel at the same rate as the permeate (product) flow rate. The reaction mixture is pumped continuously through the membrane module where the products of the reaction and other low molecular weight compounds are removed in the permeate, while the biocatalyst and other rejected compounds are recycled back to reaction vessel for further reaction. MRBs generate higher product concentration because of higher cell concentrations that allow high dilution rates with complete substrate utilisation. UF membranes with a MWCO of 10,000—100,000 Da are used to retain the enzymes, and MF membranes are used when microbial cells are involved. 

Research on microbial production of isobutene is just starting to gain momentum. Besides aerobic pathways from Rhodotorula, pathways over 3-hydroxyisovalerate and isobutanol will enable higher yields and substrate utilisation. 

Figure 16.2 Basics for the microbial utilisation of carbon substrates...

First, there is a lag phase during which the microbial population adapts to the available test C-substrate. Then, there is the biodegradation phase during which the adapted microbial population begins to utilise the carbon substrate for its cellular life processes, as measured by the conversion of the carbon in the test material to CO2. Finally, the output reaches a plateau when all of the substrate is completely utilised. 

Biodegradability is an EOL option for single-use disposable, packaging and consumer plastics that harnesses microbes to completely utilise the carbon substrate and remove it from the environmental compartment through the microbial food chain. 

Peaks in bacterial population occur soon after rainfall , indicating that the available substrates that accumulated during the preceding dry spell are rapidly being utilised, and P and other nutrients immobilised in fresh microbial tissue. 

It has been pointed out that some actinomyctes can utilise CO2 as a source of carbon . It is therefore necessary to equate microbial growth and associated formation of protein to loss of weight of the substrate. Table 2, taken from the work of Heisey shows that there is indeed a broad correlation between mass loss and protein formation and Delort and co-workers have shown that loss of carboxylic acids formed during abiotic peroxidation of PE correlates with the formation of protein and polysaccharides, almost certainly associated with the cross-linked bacterial cell wall structure. 

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