A screening model to predict microalgae biomass growth in photobioreactors and raceway ponds.

A microalgae biomass growth model was developed for screening novel strains for their potential to exhibit high biomass productivities under nutrient-replete conditions in photobioreactors or outdoor ponds. Growth is modeled by first estimating the light attenuation by biomass according to Beer-Lambert's Law, and then calculating the specific growth rate in discretized culture volume slices that receive declining light intensities due to attenuation. The model uses only two physical and two species-specific biological input parameters, all of which are relatively easy to determine: incident light intensity, culture depth, as well as the biomass light absorption coefficient and the specific growth rate as a function of light intensity. Roux bottle culture experiments were performed with Nannochloropsis salina at constant temperature (23°C) at six different incident light intensities (10, 25, 50, 100, 250, and 850 µmol/m(2) s) to determine both the specific growth rate under non-shading conditions and the biomass light absorption coefficient as a function of light intensity. The model was successful in predicting the biomass growth rate in these Roux bottle batch cultures during the light-limited linear phase at different incident light intensities. Model predictions were moderately sensitive to minor variations in the values of input parameters. The model was also successful in predicting the growth performance of Chlorella sp. cultured in LED-lighted 800 L raceway ponds operated in batch mode at constant temperature (30°C) and constant light intensity (1,650 µmol/m(2) s). Measurements of oxygen concentrations as a function of time demonstrated that following exposure to darkness, it takes at least 5 s for cells to initiate dark respiration. As a result, biomass loss due to dark respiration in the aphotic zone of a culture is unlikely to occur in highly mixed small-scale photobioreactors where cells move rapidly in and out of the light. By contrast, as supported also by the growth model, biomass loss due to dark respiration occurs in the dark zones of the relatively less well-mixed pond cultures. In addition to screening novel microalgae strains for high biomass productivities, the model can also be used for optimizing the pond design and operation. Additional research is needed to validate the biomass growth model for other microalgae species and for the more realistic case of fluctuating temperatures and light intensities observed in outdoor pond cultures.

A fluorescence spectroscopic study of phenanthrene sorption on porous silica.

Fluorescence spectroscopic characteristics of sorbed phenanthrene in porous silica provide information about its chemical state such as monomer vs dimer or higher aggregates, as well as a basis for high sensitivity detection. In this study, the chemical state and distribution of phenanthrene sorbed in two types of porous silica particles, mesoporous silica (365 microns particle diameter, 150 A average pore diameter) and microporous silica (custom synthethized, 1 micron particle diameter, 20 A pore diameter), is determined by fluorescence spectroscopy, fluorescence lifetime measurements, and scanning two-photon excitation fluorescence profiling. From the characteristic fluorescence emission spectra, it is found that at loading levels of < or = 4.7 mg/g (phenanthrene/silica) phenanthrene exists as monomers in both meso- and microporous silica particles for phenanthrene loaded from super critical CO2 (SCF). Two-photon excitation fluorescence intensity distribution profiles indicate that for the mesoporous silica particles phenanthrene is adsorbed throughout the entire silica particle. Introduction of water into phenanthrene-loaded mesoporous silica particles causes instantaneous conversion of phenanthrene from monomer to crystalline form at phenantherene loading levels > or = 4.7 micrograms/g due to hydration of the silica surface. In this process, sorption of water molecules expels phenanthrene from the surface sorption sites and causes localized phenanthrene concentration beyond its solubility limit, resulting in crystallization. In comparison this fast conversion is not observed for phenanthrene-loaded microporous silica particles that show extremely slow conversion even for phenanthrene loading levels as high as 4.7 mg/g. This difference is interpreted as reflecting hindered diffusion of phenanthrene in the nearly monodispersed micropores with pore sizes close to the molecular diameter of phenanthrene.

Artificial aging of phenanthrene in porous silicas using supercritical carbon dioxide.

Expedited artificial aging is described and demonstrated using a novel system that circulates a solution of supercritical carbon dioxide and a hydrophobic organic sorbate (phenanthrene) through a closed loop containing a porous substrate. Unlike traditional methods used to simulate the natural aging process, our approach allows for real-time monitoring of sorption equilibria, and the process is highly accelerated due to the unique physical properties of supercritcal carbon dioxide. The effectiveness of the system to simulate aging was demonstrated with a series of experiments in which three silicas with varying particle and pore sizes were loaded with phenanthrene. Batch aqueous desorption experiments were used to evaluate the extent of the aging process. For the two types of particles containing the largest pores (i.e., mean diameters of 202 and 66 A), 95% and 86%, respectively, of the phenanthrene was released to the aqueous fraction within 3 h. In contrast, only 16% of the phenanthrene was released from particles having a mean pore diameter of 21 A after 24 h. These results were confirmed by the results from an aqueous column desorption experiment. Confounding factors that might contribute to slow aqueous desorption such as the hydration state of the particles' surfaces, the chemical form of the loaded phenanthrene, and the organic carbon content were investigated and/or normalized for all three particle types. Consequently, we were able to attribute the slow desorption behavior and the presence of the resistant fraction in the 21 A silica to pore effects. With properly designed experiments, the results of this study suggest that the supercritical fluid system could be extended to the study of contaminant aging and bioavailability in natural soils and sediments.

Transport of substrates and metabolites and their effect on cell metabolism (in butyric-acid and methylotrophic fermentations).

The two components, delta pH and delta psi, of the membrane protonmotive force (delta p) effect and are affected by the transport of many substrates and metabolites. Because the integrity (or restoration) of the delta p requires the expenditure of metabolic energy, such transport processes affect the overall cell bioenergetics. However, the transport or high concentrations of certain substrates and metabolites can have more serious effects on cell metabolism because they partially or completely abolish either or both the delta pH and delta psi. If the cells cannot eventually restore the collapsed component(s) of the delta p, complete growth inhibition and cell death become inevitable. In the butanol/acetone fermentation of Clostridium acetobutylicum, the transport and the presence of key metabolites (acetic and butyric acids, and butanol) have serious and some necessary effects on the delta p. Acetic and butyric acids act as uncouplers of the delta pH, thereby reducing the internal pH. Using other acid uncouplers (such as acetoacetate, which is metabolized by the cells, or FCCP, which is not metabolized by the cells), we found that a lower pHo combined with the metabolic-energy drain of the uncoupling effect and high internal acid concentrations are implicated in the mechanism(s) of solventogenesis. Thus, the production or presence (or both) of the two acids (acetic and butyric) is beneficial to the initiation of solvent production. The transport mechanisms of CH3OH, CH2O, and HCOOH in obligate CH3OH utilizers (methylotrophs) were also discussed in detail. We showed that CH3OH is actively transported by the cells at the expense of metabolic energy and that its transport significantly affects the dynamics of continuous bioreactors. The accumulation of CH2O was found to be driven by the membrane delta p. Finally, formate was accumulated by the delta pH according to the general transport mechanism of short-chain fatty acids. The inhibition of growth by formate was explained by its uncoupling effect on the cells. Growth inhibition by CH3OH appeared to be related to the severe reduction of the membrane delta pH and cell pHi by relatively low CH3OH concentrations.