On the choice of the optimal periodic operation for a continuous fermentation process

In this contribution we investigate the impact of the forcing waveform on the productivity of a continuous bioreactor governed by an unstructured, nonlinear kinetic model. The (periodic) forcing is applied on the substrate concentration in the feed. To this end, some alternative waveforms commonly encountered in practice are evaluated and their performance is compared. An analytical/numerical approach is used. The preliminary analytical step is based on the π‐criterion that gives useful information for small amplitudes. The extension to larger amplitudes, when significant improvements are expected, is then performed through a continuation‐optimization procedure. It is found that the choice of the specific waveform has an impact on the performance of the process and there is no unique best forcing for any process condition, but its choice depends on the operating parameters and the forcing amplitude and frequency values. Further, the influence of the waveform functions on the wash‐out conditions are extensively examined. The analysis shows that all the waveforms examined in this work may lead to significant enlargement of the nontrivial regime with respect to a steady state operation. In particular, square‐wave forcing leads in practice to the extinction of the wash‐out conditions for any feed substrate concentration and for a well defined choice of the forcing parameters. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Optimizing the expression of chimeric genes in plant cells

Summary The Serratia marcescens chiA gene encodes a secreted chitinase activity which contributes to the fungal growth inhibition exhibited by this bacterium. The coding region from the chiA gene was fused to the promoter and 3 polyadenylation region of the Agrobacterium nopaline synthase gene. Site-directed mutagenesis of specific nucleotides surrounding the initiating AUG of the coding sequence of this chimeric gene resulted in up to an eight-fold increase in the amount of chitinase protein detected in transformed plant tissue. Analysis of the chiA mRNA indicated that these nucleotides also affected mRNA levels. At least 50% of the chitinase protein produced in transformed tobacco cells was the same molecular weight as the S. marcescen secreted protein.     

Preparative resolution of some 5,5-hydantoin derivatives via formation of diastereoisomeric salts

Several preparative resolutions of 5,5-disubstituted hydantoins have been achieved via fractional crystallization of diastereoisomeric salts. The process can be extended by making use of the difference between the variation of solubilities of the hydantoins and their salts with α-methylbenzylamine as a function of the alkalinity of the medium. Optimization for each resolution procedure involves a refinement of the excess amount of base needed. © 1992 Wiley-Liss, Inc.     

Ka'apor ritual hunting

Several investigators cite population control, frequent settlement relocation, trekking, expansion of diet breadth, and food taboos as strategic responses to game depletion by indigenous populations living in interfluvial zones of lowland South America. The Ka'apor Indians of the interfluvial forest of northern Maranhão, Brazil, employ other means of optimizing hunting efficiency, partly based on ritual. Menstruating women, pubescent girls, and parents of newborns can consume meat only of the tortoise (Gochelone denticulata), the first prey species to be hunted out of an area. Tortoise capture requires, on average, one full day of hunting. This means that the full potential of hunting pressure does not materialize near the settlement. Meat productivity is unusually high even near old settlements. I argue that ritual tortoise hunting helps to regulate environmental utilization, to maintain a sustained yield of meat protein, and to expand the catchment area gradually.     

THE EFFECTS OF PREDATION ON THE AGE AND SIZE OF MATURITY OF PREY

The effects of nonselective predation on the optimal age and size of maturity of their prey are investigated using mathematical models of a simple life history with juvenile and adult stages. Fitness is measured by the product of survival to the adult stage and expected adult reproduction, which is usually an increasing function of size at maturity. Size is determined by both age at maturity and the value of costly traits that increase mean growth rate (growth effort). The analysis includes cases with fixed size but flexible time to maturity, fixed time but flexible size, and adaptively flexible values of both variables. In these analyses, growth effort is flexible. For comparison with previous theory, models with a fixed growth effort are analyzed. In each case, there may be indirect effects of predation on the prey's food supply. The effect of increased predation depends on (1) which variables are flexible; (2) whether increased growth effort requires increased exposure to predators; and (3) how increased predator density affects the abundance of food for juvenile prey. If there is no indirect effect of predators on prey food supply, size at maturity will generally decrease in response to increased predation. However, the indirect effect from increased food has the opposite effect, and the net result of predation is often increased size. Age at maturity may either increase or decrease, depending on functional forms and parameter values; this is true regardless of the presence of indirect effects. The results are compared with those of previous theoretical analyses. Observed shifts in life history in response to predation are reviewed, and the role of size-selective predation is reassessed.     

A medium optimization strategy for improvement of growth and methane production by Methanobacterium thermoautotrophicum

Methanobacterium thermoautotrophicum, strain Hveragerdi, has been cultivated in a completely defined mineral salts medium, under strictly anaerobic conditions with CO₂ and H₂ as sole carbon and energy source, respectively. During optimization of the medium an iron limitation was identified that could not be overcome by the simple addition of iron—or iron complexed with nitrilotriacetate (NTA)—to the medium, due to the formation of insoluble FeS complexes. In order to define a medium optimization strategy, and to avoid laborious empirical optimization procedures, a theoretical model has been developed in order to describe the solubility of iron and other mineral species in the medium as a function of the concentration of sulfur and NTA. This model may be applied for the optimization of any medium component. With this information sulfide has been replaced by a combination of cysteine and thiosulphate in conjunction with a non-toxic reducing agent (titanium (III) citrate). Using this defined medium precipitation was avoided and an iron limitation was overcome resulting in a 5-fold improvement of the final biomass concentration from 2–3 g l⁻¹ to 11.2 g l⁻¹ together with a 2-fold increase (from 45 to almost 100%) in the conversion of CO₂ and H₂ to CH₄, even at gas flow rates as high as 6 l min⁻¹.     

Heuristic optimization of the general life history problem: A novel approach

Summary The general life history problem concerns the optimal allocation of resources to growth, survival and reproduction. We analysed this problem for a perennial model organism that decides once each year to switch from growth to reproduction. As a fitness measure we used the Malthusian parameterr, which we calculated from the Euler-Lotka equation. Trade-offs were incorporated by assuming that fecundity is size dependent, so that increased fecundity could only be gained by devoting more time to growth and less time to reproduction. To calculate numerically the optimalr for different growth dynamics and mortality regimes, we used a simplified version of the simulated annealing method. The major differences among optimal life histories resulted from different accumulation patterns of intrinsic mortalities resulting from reproductive costs. If these mortalities were accumulated throughout life, i.e. if they were senescent, a bangbang strategy was optimal, in which there was a single switch from growth to reproduction: after the age at maturity all resources were allocated to reproduction. If reproductive costs did not carry over from year to year, i.e. if they were not senescent, the optimal resource allocation resulted in a graded switch strategy and growth became indeterminate. Our numerical approach brings two major advantages for solving optimization problems in life history theory. First, its implementation is very simple, even for complex models that are analytically intractable. Such intractability emerged in our model when we introduced reproductive costs representing an intrinsic mortality. Second, it is not a backward algorithm. This means that lifespan does not have to be fixed at the begining of the computation. Instead, lifespan itself is a trait that can evolve. We suggest that heuristic algorithms are good tools for solving complex optimality problems in life history theory, in particular questions concerning the evolution of lifespan and senescence.     

Modeling approach to control of carbohydrate metabolism during citric acid accumulation by Aspergillus niger: II. Sensitivity analysis

Steady state sensitivity analysis of a model of carbohydrate metabolism and anaplerotic synthesis of oxalacetate were, in Aspergillus niger under conditions of citric acid accumulation, carried out. The flux and metabolite concentration control structure of the system obtained shows that the hexokinase/substrate transport step is the main controlling step of the pathway. The quantitative contribution of the other enzyme catalyzed or transport steps are also discussed. These results allow the design of a proper strategy of biotechnological manipulation aimed at improvement of the process. (c) 1994 John Wiley & Sons, Inc.     

Predictions for oxygen supply control to enhance population stability of engineered production strains

The simultaneous growth and product formation in a microbial culture is an important feature of several laboratory, industrial, and environmental bioprocesses. Metabolic burden associated with product formation in these bioprocesses may lead to growth advantage of a nonproducing mutant leading to a loss of the producing population over time. A simple population dynamics model demonstrates the extreme sensitivity of population stability to the engineered productivity of a strain. Here we use flux balance analysis to estimate the effects of the metabolic burden associated with product secretion on optimal growth rates. Comparing the optimal growth rates of the producing and nonproducing strains under a given processing condition allows us to predict the population stability. In order to increase stability of an engineered strain, we determine processing conditions that simultaneously maximize the growth rate of the producing population while minimizing the growth rate of a nonproducing population. Using valine, tryptophan, and lysine production as specific examples, we demonstrate that although an appropriate choice of oxygenation may increase culture longevity more than twofold, total production as governed by economic criterion can be increased by several orders of magnitude. Choice of optimal nutrient and oxygen supply rates to enhance stability is important both for strain screening as well as for culture of engineered strains. Appropriate design of the culture environment can thus be used to enhance the productivity of bioprocesses that use engineered production strains. (c) 1994 John Wiley & Sons, Inc.