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

Abstract

The main bottleneck in scale-up of phototrophic fermentation is the low efficiency of light energy conversion to the desired product, which is caused by an excessive dissipation of light energy to heat. The photoheterotrophic formation of hydrogen from acetate and light energy by the microorganism Rhodobacter capsulatus NCIMB 11773 was chosen as a case study in this work. A light energy balance was set up, in which the total bacterial light energy absorption is split up and attributed to its destinations. These are biomass growth and maintenance, generation of hydrogen and photosynthetic heat dissipation. The constants defined in the light energy balance were determined experimentally using a flat-panel photobioreactor with a 3-cm optical path. An experimental method called D-stat was applied. Continuous cultures were kept in a so-called pseudo steady state, while the dilution rate was reduced slowly and smoothly. The biomass yield and maintenance coefficients of Rhodobacter capsulatus biomass on light energy were determined at 12.4 W/m(2) (400-950 nm) and amounted to 2.58 x 10(-8) +/- 0.04 x 10(-8) kg/J and 102 +/- 3.5 W/kg, respectively. The fraction of the absorbed light energy that was dissipated to heat at 473 W/m(2) depended on the biomass concentration in the reactor and varied between 0.80 and 0.88, as the biomass concentration was increased from 2.0 to 8.0 kg/m(3). The process conditions were estimated at which a 3.7% conversion efficiency of absorbed light energy to produced hydrogen energy should be attainable at 473 W/m(2).

Integrating dark and light bio-hydrogen production strategies: towards the hydrogen economy

“…Process intensification is required to overcome the problem of light delivery to the second stage photofermentations, which would push bio-H 2 production to competitive levels. A review of photobioreactor designs to achieve effective light transfer into high-activity cultures is outside the scope of this overview; the reader is referred to recent reviews (Tsygankov 2001;Hoekema et al 2002;Kondo et al 2002;Hoekema et al 2006;. (Zhu et al 2002) NG: The productivity of carbohydrate accumulation was not given and could not be calculated from given data.…”

Photofermentative Hydrogen Production in Outdoor Conditions

“…Generally, higher light conversion efficiencies were obtained at low light intensities [56]. Light conversion efficiency reduced from 1.11% to 0.25% as the light intensity increased from 88 to 405 W/m 2 [62].…”

“…These enzymatic processes rely on chemical bond energy (ATP) [25] generated by the conversion of absorbed light energy to ATP as discussed in Section 3. This energy is utilized in different basic cellular metabolic activities such as biomass formation, biomass maintenance, hydrogen production and the excess is dissipated as heat [4,56]. Indoor studies have demonstrated…”

“…There are limited numbers of outdoor photofermentative hydrogen production studies using continuous systems reported in literature [80,81]. Most of the ones described were carried out in indoor conditions [34,46,56,82,83].…”

Advanced continuous cultivation methods for systems microbiology

“…Wijffels and colleagues first used the De-stat approach for optimizing the photosynthetic efficiency of the purple nonsulphur bacterium Rhodobacter capsulatus for the production of hydrogen from acetate and light energy by determining important components of the light energy balance: biomass growth and maintenance, generation of hydrogen, and photosynthetic heat dissipation (Hoekema et al, 2006). They saw that culture characteristics were strongly dependent on m.…”