The effect of irradiance growing on hydrogen photoevolution and on the kinetic growth in Rhodopseudomonas palustris, strain 42OL
Introduction
Nowadays, an interesting world challenge is the production of renewable energy, instead of fossil fuels. Hydrogen is attracting considerable attention as a replacement for fossil fuels; some hydrogen production methods have been reported [1], [2]. Photobiological hydrogen produced from photosynthetic bacteria could be a promising method of solar energy conversion [3]. Although hydrogen is considered to be one of the most promising future energy sources and the technical aspects involved in using it have advanced considerably, the future supply of hydrogen from renewable sources is still unsolved [4]. Before the concept of hydrogen economy becomes a reality, a safe, economical and tenable way of producing it needs to be developed [5]. Biohydrogen technologies are still in their infancy. Existing technologies offer potential for practical application, but if biohydrogen systems are to become commercially competitive they must be able to synthesize H2 at rates that are sufficient to power fuel cells of sufficient size to do practical work [6]. Climate change issues, the reduced world reserves of fossils and higher and higher fuel prices play an important role in the development of clean technologies for producing renewable energy, such as hydrogen. The major critical drawback to hydrogen photo-production is the high cost of the process: (i) because the process is light-dependent; (ii) because hydrogen generation is inhibited by oxygen; (iii) because the hydrogen conversion efficiency is relatively low. The first point could be overcome by using solar radiation instead of artificial light; the second, by using an anoxic process; the third, by improving the closed reactor design and optimizing the operating parameters, such as pH, hydraulic retention time and temperature, which could increase hydrogen conversion efficiency [7]. On the other hand, enhancement of the hydrogen yield may be possible by using suitable microbial strains, process modification, efficient bioreactor design, and even genetic and metabolic engineering techniques to redirect the metabolic pathway [8]. Purple non-sulfur photosynthetic bacteria can decompose organic acids by using light energy and nitrogenase in a photofermentation process [9]. When hydrogen photo-production is achieved via photo-fermentative process, such as the photo-metabolism used by purple non-sulfur bacteria, we have to add one or more organic substances to the culture medium [10], [11]. To offset the addition of organic substances (e.g. fatty acids) to the synthetic medium and to limit the high cost of biohydrogen production, distillery waste or oil mill wastewater has been used [12], [13]. The organic carbon source used for this study is malic acid, which is a compound of wine distillery waste [14]. Use of cheaper raw materials and efficient biological hydrogen production processes will surely make them more competitive with the conventional H2 generation processes in near future; pilot plant trials of the photofermentation processes require more attention [15]. High hydrogen yield remains the ultimate goal and challenge for biohydrogen research and development [7]. Nevertheless, the efficiency of light conversion to hydrogen is the key factor in the development of a biological process for hydrogen production [16], [17]. The relationship of light intensity to nitrogenase synthesis and hydrogen photoevolution in a continuous culture of Rodopseudomonas capsulatus has been studied [18]. The purpose of our study is to investigate hydrogen photoevolution by means of Rhodopseudomonas palustris (strain 42 OL) cultured under artificial radiation. Kinetic analyses with regard to bioH2 production, from low to high irradiance (36–805 W m−2), are required in order to improve the efficiency of hydrogen photoevolution.
Section snippets
Culture system
A cylindrical glass photobioreactor with an internal diameter (id) of 9.6 cm and the working volume of 1.07 l (Fig. 1) was used for our experiments. The reactor was placed in a heat exchanger-Plexiglas water bath at a constant temperature, and the culture was stirred with a magnetic stirrer. Two needles were inserted in the silicone stopper of each cultural system: the first served for the addition of substrate inside the reactor; the second was used for the gas outlet before being trapped in a
Theory
The specific growth rate (μe) was determined from the log phase of the growth curve by means of exponential regression [21]. The equation to obtain the specific growth rate attained during exponential growth is the following:where μe is the specific growth rate (h−1); BC2 and BC1 are, respectively, the biomass concentrations (g l−1) at times t2 and t1 (h).
By plotting the specific growth rate (μe) against the irradiance (I), the half saturation intensity (Ik) could be
Results
Relationships of the hydrogen photoevolution rate (HPR) to, the dry-biomass, and the age of the culture were investigated under conditions ranging from low to high irradiance, and the results are shown in Fig. 2. At the end of the experimental sets, a relevant average dry-biomass (2.307 ± 0.161 g) was produced during each hydrogen photoevolution set. Two exceptions concern the lowest irradiance (36 W m−2) and the optimum irradiance (500 W m−2) at which 1.66 g dry-biomass and 3.38 g dry-biomass were
Discussion
Although we reached much lower hydrogen evolution rates (per unit of culture volume) than those obtained with R. capsulatus (on lactate) [25] and with R. palustris (on acetate-butyrate) [26], the hydrogen evolution rate we obtained using malic acid were within those limits reported in literature [27], [28], [29], [30]. The hydrogen evolution rate was increased from 20.9 ml l−1 h−1 to 38.2 ml l−1 h−1 (about 83%) when R. palustris WP3–5 was cultured with a combination of optical fibers, halogen lamps,
Conclusions
The specific growth rate observed from low (36 W m−2) to high irradiance (803 W m−2) values enabled us to find the key factors affecting μeMax and Ik during hydrogen generation. Fed-batch technique in photofermentation process such as malate and glutamate replenishment, described in this paper, was appropriate to study the effect of irradiance growing on bioH2 production and cellular growth to determine kinetic parameter as μH. As far as we know, this was the first time that Boltzmann's sigmoidal
Acknowledgements
We wish to thank Mr. Edoardo Pinzani for the management of the cultural system and Mr. Sandro Dodero for the technical assistance.
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