Eiichi Kojima

Growth and Hydrocarbon Production of Microalga Botryococcus braunii in Bubble Column Photobioreactors

Microalga Botryococcus braunii, which produces high levels of liquid hydrocarbons called botryococcenes, was cultivated in bubble column photobioreactors. Algal cells, adapted to low irradiance (3 klx) in the preculture, showed lower biomass and hydrocarbons productivity in the photobioreactor illuminated at high irradiance (10 klx) due to the effects of photoinhibition. The degree of photoinhibition was reduced by partial shading, the lighted volume ratio being varied from 25 to 100%. The algal cells adapted to high irradiance in the preculture showed a high biomass productivity at 10 klx: the cell concentration reached higher than 7 kg/m3 and the hydrocarbon content was 50% based on cell dry weight. Hydrocarbon production kinetics of this alga in the linear growth phase was found to be growth-associated irrespective of the experimental conditions of illumination and preculture.

Effect of Light Intensity on Colony Size of Microalga Botryococcus braunii in Bubble Column Photobioreactors

The variation in the colony size distribution with time of microalga Botryococcus braunii, which produces extracellular polysaccharides as well as high levels of liquid hydrocarbon, was measured in bubble column photobioreactors. The effect of average light intensity on the size of colonies was examined by fixing films impervious to light on the column wall and measuring the light intensity within the photobioreactors. The colony size distribution of this alga was found to fit the log-normal distribution. Under identical hydrodynamic conditions, the average colony size shifted to an equilibrium size, which was determined depending on the average light intensity within the photobioreactors.

Effect of partial shading on photoproduction of hydrogen by Chlorella.

The photoproduction of hydrogen by the green alga Chlorella pyrenoidosa was studied in 650-ml bubble columns made of glass with 7 cm diameter. Hydrogen was produced by adding sodium dithionite as an oxygen scavenger directly to an algal suspension. When a part of the wall of the bubble column was shaded with a sheet of plastic film impervious to light, the production rate and total volume of hydrogen increased as compared to those in the columns without partial shading. This relationship between the hydrogen photoevolution rate and the proportion of lighted region is contrary to normal photochemical reactions. This phenomenon is considered to be related to the regulation by light of the activity of the enzymes and/or the photosynthetic electron transport systems, which was examined by measuring fluorescence induction curves of dark-adapted Chlorella cells and also the distribution of light intensity within the bubble column bioreactors.

Photoproduction of hydrogen by adapted cells of Chlorella pyrenoidosa

Abstract Photoproduction of hydrogen gas by the green alga Chlorella pyrenoidosa was studied in a large scale culture of 2.1. Hydrogen was produced by adding sodium hydrosulfite directly to an algal suspension after anaerobiosis in darkness for activation of hydrogenase. The hydrogen production rate showed a characteristic course of an initial burst of gas then steady production, and this course appeared most clearly at cell concentrations around 0.6–0.7 kg/m3. In the final third phase, the hydrogen production rate gradually decreased until evolution ceased. The steady hydrogen evolution was inhibited 75% by a herbicide, DCMU, which blocks electron flow through photosystem II, indicating that the electron donor for hydrogen production was mainly water. The average light intensity within the culture vessel was measured with a diffusing sphere photoprobe. The rate of hydrogen evolution increased hyperbolically with the average light intensity. The duration of hydrogen photoproduction was shorter at higher light intensity due to the inhibition of hydrogenase by concomitantly released oxygen. The duration was shorter also at higher concentrations of algal suspension. It was foudd that the optimum concentration of algae, about 0.7 kg/m3 in this system, must be selected to maximize the yield of hydrogen.