Controlling light-use byRhodobacter capsulatus continuous cultures in a flat-panel photobioreactor

Technological tools and strategies for culturing human gut microbiota in engineered in vitro models

The gut microbiota directly impacts the pathophysiology of different human body districts. Consequently, microbiota investigation is an hot topic of research and its in vitro culture has gained extreme interest in different fields. However, the high sensitivity of microbiota to external stimuli, such as sampling procedure, and the physicochemical complexity of the gut environment make its in vitro culture a challenging task. New engineered microfluidic gut-on-a-chip devices have the potential to model some important features of the intestinal structure, but they are usually unable to sustain culture of microbiota over an extended period of time. The integration of gut-on-a-chip devices with bioreactors for continuous bacterial culture would lead to fast advances in the study of microbiota-host crosstalk. In this review, we summarize the main technologies for the continuous culture of microbiota as upstream systems to be coupled with microfluidic devices to study bacteria-host cells communication. The engineering of integrated microfluidic platforms, capable of sustaining both anaerobic and aerobic cultures, would be the starting point to unveil complex biological phenomena proper of the microbiota-host crosstalks, paving to way to multiple research and technological applications.

Transcriptome analysis of Rhodobacter capsulatus grown on different nitrogen sources

This study investigated the effect of different nitrogen sources, namely, ammonium chloride and glutamate, on photoheterotrophic metabolism of Rhodobacter capsulatus grown on acetate as the carbon source. Genes that were significantly differentially expressed according to Affymetrix microarray data were categorized into Clusters of Orthologous Groups functional categories and those in acetate assimilation, hydrogen production, and photosynthetic electron transport pathways were analyzed in detail. Genes related to hydrogen production metabolism were significantly downregulated in cultures grown on ammonium chloride when compared to those grown on glutamate. In contrast, photosynthetic electron transport and acetate assimilation pathway genes were upregulated. In detail, aceA encoding isocitrate lyase, a unique enzyme of the glyoxylate cycle and ccrA encoding the rate limiting crotonyl-CoA carboxylase/reductase enzyme of ethylmalonyl-coA pathway were significantly upregulated. Our findings indicate for the first time that R. capsulatus can operate both glyoxylate and ethylmalonyl-coA cycles for acetate assimilation.

A process economic assessment of hydrocarbon biofuels production using chemoautotrophic organisms

Economic analysis of an ARPA-e Electrofuels (http://arpa-e.energy.gov/?q=arpa-e-programs/electrofuels) process is presented, utilizing metabolically engineered Rhodobacter capsulatus or Ralstonia eutropha to produce the C30+ hydrocarbon fuel, botryococcene, from hydrogen, carbon dioxide, and oxygen. The analysis is based on an Aspen plus® bioreactor model taking into account experimentally determined Rba. capsulatus and Rls. eutropha growth and maintenance requirements, reactor residence time, correlations for gas-liquid mass-transfer coefficient, gas composition, and specific cellular fuel productivity. Based on reactor simulation results encompassing technically relevant parameter ranges, the capital and operating costs of the process were estimated for 5000 bbl-fuel/day plant and used to predict fuel cost. Under the assumptions used in this analysis and crude oil prices, the Levelized Cost of Electricity (LCOE) required for economic feasibility must be less than 2¢/kWh. While not feasible under current market prices and costs, this work identifies key variables impacting process cost and discusses potential alternative paths toward economic feasibility.

The stoichiometry and energetics of oxygenic phototrophic growth

The values of gross metabolic flows in cells are essentially interconnected due to conservation laws of chemical elements and interrelations of biochemical coupling. Therefore, the overall stoichiometry of cellular metabolism, such as the biomass quantum yield, the ratio between linear and circular flows via the electron transport chain, etc., can be calculated using balances of metabolic flows in the network branching points and coupling ratios related to ATP formation and expenditures. This work has studied the energetic stoichiometry of photosynthetic cells by considering the transfer of reductivity in the course of biochemical reactions. This approach yielded rigorous mathematical expressions for biomass quantum yield and other integral bioenergetic indices of cellular growth as functions of ATP balance parameters. The effect of cellular substance turnover has been taken into account. The obtained theoretical estimation of biomass quantum yield is rather close to experimental data which confirms the predictive capacity of this approach.

Maximum photosynthetic yield of green microalgae in photobioreactors

The biomass yield on light energy of Dunaliella tertiolecta and Chlorella sorokiniana was investigated in a 1.25- and 2.15-cm light path panel photobioreactor at constant ingoing photon flux density (930 µmol photons m⁻² s⁻¹). At the optimal combination of biomass density and dilution rate, equal biomass yields on light energy were observed for both light paths for both microalgae. The observed biomass yield on light energy appeared to be based on a constant intrinsic biomass yield and a constant maintenance energy requirement per gram biomass. Using the model of Pirt (New Phytol 102:3-37, 1986), a biomass yield on light energy of 0.78 and 0.75 g mol photons⁻¹ and a maintenance requirement of 0.0133 and 0.0068 mol photons g⁻¹ h⁻¹ were found for D. tertiolecta and C. sorokiniana, respectively. The observed yield decreases steeply at low light supply rates, and according to this model, this is related to the increase of the amount of useable light energy diverted to biomass maintenance. With this study, we demonstrated that the observed biomass yield on light in short light path bioreactors at high biomass densities decreases because maintenance requirements are relatively high at these conditions. All our experimental data for the two strains tested could be described by the physiological models of Pirt (New Phytol 102:3-37, 1986). Consequently, for the design of a photobioreactor, we should maintain a relatively high specific light supply rate. A process with high biomass densities and high yields at high light intensities can only be obtained in short light path photobioreactors.

Next-generation biofuels: Survey of emerging technologies and sustainability issues

Next-generation biofuels, such as cellulosic bioethanol, biomethane from waste, synthetic biofuels obtained via gasification of biomass, biohydrogen, and others, are currently at the center of the attention of technologists and policy makers in search of the more sustainable biofuel of tomorrow. To set realistic targets for future biofuel options, it is important to assess their sustainability according to technical, economical, and environmental measures. With this aim, the review presents a comprehensive overview of the chemistry basis and of the technology related aspects of next generation biofuel production, as well as it addresses related economic issues and environmental implications. Opportunities and limits are discussed in terms of technical applicability of existing and emerging technology options to bio-waste feedstock, and further development forecasts are made based on the existing social-economic and market situation, feedstock potentials, and other global aspects. As the latter ones are concerned, the emphasis is placed on the opportunities and challenges of developing countries in adoption of this new industry.

Coenzyme Q10 production in a 150-l reactor by a mutant strain of Rhodobacter sphaeroides

For the commercial production of CoQ(10), batch-type fermentations were attempted in a 150-l fermenter using a mutant strain of R. sphaeroides. Optimum temperature and initial aeration rate were found to be 30 degrees C and 2 vvm, respectively. Under optimum fermentation conditions, the maximum value of specific CoQ(10) content was achieved reproducibly as 6.34 mg/g DCW after 24 h, with 3.02 g/l of DCW. During the fermentation, aeration shift (from the adequate aeration at the early growth phase to the limited aeration in active cellular metabolism) was a key factor in CoQ(10) production for scale-up. A higher value of the specific CoQ(10) content (8.12 mg/g DCW) was achieved in fed-batch fermentation and comparable to those produced by the pilot-scale fed-batch fermentations of A. tumefaciens, which indicated that the mutant strain of R. sphaeroides used in this study was a potential high CoQ(10) producer. This is the first detailed study to demonstrate a pilot-scale production of CoQ(10) using a mutant strain of R. sphaeroides.

Exploration of the hydrogen producing potential of Rhodobacter capsulatus chemostat cultures: The application of deceleration-stat and gradient-stat methodology

In this work, the dependency of the volumetric hydrogen production rate of ammonium-limited Rhodobacter capsulatus chemostat cultures on their imposed biomass concentration and dilution rate was investigated. A deceleration-stat experiment was performed by lowering the dilution rate from 1.0 d(-1) to zero aimed at a constant biomass concentration of 4.0 g L(-1) at constant incident light intensity. The results displayed a maximal volumetric hydrogen production rate of 0.6 mmol m(-3) s(-1), well below model predictions. Possibly the high cell density limited the average light availability, resulting in a sub-optimal specific hydrogen production rate. To investigate this hypothesis, a gradient-stat experiment was conducted at constant dilution rate of 0.4 d(-1) at constant incident light intensity. The biomass concentration was increased from 0.7 to 4.0 g L(-1) by increasing the influent ammonium concentration. Up to a biomass concentration of 1.5 g L(-1), the volumetric hydrogen production rate of the system increased according to model predictions, after which it started to decline. The results obtained provide strong evidence that the observed decline in volumetric hydrogen production rate at higher biomass concentrations was at least partly caused by a decrease in light availability.