Many techniques are available in the fermentation medium designer’s toolbox (borrowing, component swapping, biological mimicry, one-at-a-time, statistical and mathematical techniques—experimental design and optimization, artificial neural networks, fuzzy logic, genetic algorithms, continuous fermentation, pulsed batch and stoichiometric analysis). Each technique has advantages and disadvantages, and situations where they are best applied. No one ‘magic bullet’ technique exists for all situations. However, considerable advantage can be gained by logical application of the techniques, combined with good experimental design.

Strategies for improving fermentation medium performance: a review

Polyhydroxybutyrate (PHB) is an attractive substitute for petrochemical plastic due to its similar properties, biocompatibility, and biodegradability. The cost of scaled-up PHB production inhibits its widespread usage. Intensive researches are growing to reduce costs and improve thermomechanical, physical, and processing properties of this green biopolymer. Among cheap substrates which are used for reducing total cost of PHB production, some C1 carbon sources, e.g., methane, methanol, and CO2 have received a great deal of attention due to their serious role in greenhouse problem. This article reviews the fundamentals of strategies for reducing PHA production and moves on to the applications of several cheap substrates with a special emphasis on methane, methanol, and CO2. Also, some explanation for involved microorganisms including the hydrogen-oxidizing bacteria and methanotrophs, their history, culture condition, and nutritional requirements are given. After description of some important strains among the hydrogen-oxidizing and methanotrophic producers of PHB, the article is focused on limitations, threats, and opportunities for application and their future trends.

Microbial production of poly(hydroxybutyrate) from C₁ carbon sources.

Renewable methanol and formate as microbial feedstocks

Methylobacterium extorquens ATCC 55366 was successfully cultivated at very high cell densities in a fed-batch fermentation system using methanol as a sole carbon and energy source and a completely minimal culture medium for the production of poly-β-hydroxybutyrate (PHB). Cell biomass levels were between 100 g/l and 115 g/l (dry weight) and cells contained between 40% and 46% PHB on a dry-weight basis. PHB with higher molecular mass values than previously reported for methylotrophic bacteria was obtained under certain conditions. Shake-flask and fermentor experiments showed the importance of adjusting the mineral composition of the medium for improved biomass production and higher growth rates. High-cell-density cultures were obtained without the need for oxygen-enriched air; once the oxygen transfer capacity of the fermentor was reached, methanol was thereafter added in proportion to the amount of available dissolved oxygen, thus preventing oxygen limitation. Controlling the methanol concentration at a very low level (less than 0.01 g/l), during the PHB production phase, led not only to prevention of oxygen limitation but also to the production of very high-molecular-mass PHB, in the 900–1800 kDa range. Biomass yields relative to the total methanol consumed were in the range 0.29–0.33 g/g, whereas PHB yields were in the range 0.09–0.12 g/g. During the active period of PHB synthesis, PHB yields relative to the total methanol consumed were between 0.2 g/g and 0.22 g/g. M. extorquens ATCC 55366 appears to be a promising organism for industrial PHB production.

High-cell-density production of poly-β-hydroxybutyrate (PHB) from methanol by Methylobacterium extorquens : production of high-molecular-mass PHB

The integration of electrochemical and microbial processes offers a unique opportunity to displace fossil carbon with CO2 and renewable energy as the primary feedstocks for carbon-based chemicals. Yet, it is unclear which strategy for CO2 activation and electron transfer to microbes has the capacity to transform the chemical industry. Here, we systematically survey experimental data for microbial growth on compounds that can be produced electrochemically, either directly or indirectly. We show that only a few strategies can support efficient electromicrobial production, where formate and methanol seem the best electron mediators in terms of energetic efficiency of feedstock bioconversion under both anaerobic and aerobic conditions. We further show that direct attachment of microbes to the cathode is highly constrained due to an inherent discrepancy between the rates of the electrochemical and biological processes. Our quantitative perspective provides a data-driven roadmap towards an economically and environmentally viable realization of electromicrobial production.

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Making quantitative sense of electromicrobial production

Methylobacterium organophilum, a facultative methylotroph was cultivated on a methanol as a sole carbon and energy source. The cell growth was affected by the various components of minimal synthetic medium and the medium composition was optimized with 0.5% (v/v) methanol at pH 6.8 and at 3. The maximum specific growth rate of M. organophilum was achieved to 0.26 hr in the optimized medium which has following composition: Methanol, 0.5% (v/v):(NH)SO, 1.0g/l:KHPO, 2.13g/l:KHPO, 1.305g/ι:MgSO.7. 45g/l and trace elements (CaCl.2, 3.3mg:FeSO.7, 1.3mg:MnSO.4, 130:ZnSO.5, 40:NaMoO.2, 40:CoCl.6, 40:HBO, 30 per liter). By the limitation of nitrogen and deficiency of Mn or Fe, the cell growth was significantly repressed. Methanol greatly repressed the cell growth and the complete inhibition was observed at concentration above 4% (v/v). In order to overcome the methanol inhibition and to prevent the methanol limitation, intermittent feeding of methanol was conducted by a D.O.-stat technique. PHB production by M. organophilum was stimulated by deficiency of nutrients such as NH, SO, , , or PO in the medium. The maximum PHB content was obtained as 58% of dry cell weight under deficiency of potassium ion in the optimized synthetic medium.

M. Daniel

Papers

Optimization of Growth Medium and Poly-$\beta$-hydroxybutyric Acid Production from Methanol in Methylobacterium organophilum