Energetic yields associated with hydrocarbon fermentations

Bioconversion and biodegradation of aliphatic hydrocarbons

Aliphatic hydrocarbons represent a substantial energy reserve but also constitute a useful feedstock for the biotechnological production of various alkane-derived commodity chemicals. In addition, the biodegradation of aliphatic hydrocarbons continues to pose problems for fuel stocks with associated corrosion and eventual motor filter blocking. A relatively high number of yeasts and filamentous fungi have been described that degrade n-alkanes, but relatively few have received thorough investigation. Early work exploiting hydrocarbons as a potential substrate for unicellular protein production, though never commercially successful, enabled high-performance fermentation strategies to be developed that overcame many of the inherent problems caused by the use of high energy content insoluble liquid substrates. The biochemical pathways and physiological characteristics have been sufficiently established, as have the subcellular localization of the alkane-specific pathways, though many of the regulatory phenomena remain obscure. Currently, interest lies in the exploitation of such species, or their enzymes, in bioconversion processes and the unicellular yeasts, whose amenability to rational genetic engineering strategies exceeds that of filamentous species, are currently attracting renewed research interest. In view of this, the existing knowledge and potential for alkane-based biotechnology will be reviewed. Key words: alkane metabolism, bioconversion, biotechnology, aliphatic hydrocarbons, yeasts, filamentous fungi.

Energy transformation and entropy production in living systems I. Applications to embryonic growth

Generalized material and energy balances are presented for biological systems that experience negligible kinetic, elastic, and potential energy changes. The balances are used to characterize the mass changes and energy transformations that occur in the developing avian embryo, using as an example a consistent set of data for the chicken egg. It is shown that the rate of total chemical energy turnover by the embryo is a quantity of interest and that this rate is not necessarily equivalent to the metabolic rate that is predicted from heat transfer measurements or oxygen consumption rates. The energy required for evaporative water loss is accounted for in the overall energy balance. Using the results of the energy calculations and a generalized expression for the rates of internal and total entropy production, the Prigogine-Wiame hypothesis is examined for the developing embryo with two different assumptions regarding the efficiency of biomass conversion. An order of magnitude analysis of the internal heat-conduction term is performed to show that the chemical reaction term dominates the entropy production relation. The constant efficiency case is shown to be in agreement with the Prigogine-Wiame hypothesis for the data used in the analysis.

Thermodynamics of growth. Non-equilibrium thermodynamics of bacterial growth. The phenomenological and the mosaic approach

Microbial growth is analyzed in terms of mosaic and phenomenological non-equilibrium thermodynamics. It turns out that already existing parameters devised to measure bacterial growth, such as YATP, mu, and Q substrate, have as thermodynamic equivalents flow ratio, output flow and input flow. With this characterisation it becomes possible to apply much of the already existing knowledge of phenomenological non-equilibrium thermodynamics to bacterial growth. One of the conclusions is that the frequent observation that YATP is only 50% of its theoretical maximum does not mean that the microbe corresponds to a thermodynamic system that has been optimized for maximal output power, as has been suggested. Rather, at least in some cases, it corresponds to a system that has been optimized towards maximum growth rate. When the degree of reduction of the (single) carbon source is significantly smaller than that of the biomass produced, the efficiency of biomass synthesis has been kept as high (i.e., about 24%) as is consistent with maximization of the growth rate at optimal efficiency. Mosaic thermodynamics allows an analysis of processes which in microbial metabolism may be responsible for any particular growth behaviour. Equations are derived that predict the effect of uncoupling through leaks, futile cycling, or 'slip' on microbial growth. It turns out that uncoupling is expected to affect both the growth rate-independent and the growth rate-dependent 'maintenance coefficient'. The effect on the latter is different when catabolic substrate limits growth than when anabolic substrate limits growth. In the latter case, the growth rate-dependent maintenance coefficient is negative. It is concluded that mosaic non-equilibrium thermodynamics will be a powerful theoretical tool especially in future experimental analyses of the metabolic basis for microbial growth characteristics and growth regulation.