Photosynthetic and fermentative bio-hydrogen production by microalgae and fermenting bacteria

Abstract

A good understanding of both, photosynthetic and fermentative bio-hydrogen metabolism is essential in various research areas such as microbial metabolism, auto-/hetero- photosynthesis, technologies of both alternative energy production and conventional organic waste treatment, and biochemistry. Biological hydrogen production processes can be classified such as biophotolysis of water using algae and cyanobacteria, photodecomposition of organic compounds by photo-synthetic bacteria, fermentative hydrogen production from organic compounds, and hybrid systems using photosynthetic and fermentative bacteria. The demand for more efficient hydrogen production by dark hydrogen fermentation is particularly increasing. However, full scale bio-hydrogen plants fail to exist due to economical issues. A two-stage process to enhance the bio-hydrogen production is one possible solution for increasing the efficiency of dark fermentation process. However, anaerobic fermentation process involves the generation of CO2, a greenhouse gas, which is the drawback of this technology. Reduction of carbon dioxide emission is therefore of international concern, and the removal or reuse of carbon dioxide is demanded. Our another research aim is to use CO2 generated from the anaerobic bio-energy generating process as a carbon source for the algae and algal growths with other nutrient (short chain fatty acids) sources present in the effluent form fermentation process. Mostly, capability of hydrogen production by green algae has been demonstrated focusing on photo-heterotrophic and -mixotrophic production under anaerobic and limited aerobic conditions. In order to evaluate the optimal the potential directly photo-autotrophic hydrogen production during algae cultivation under aerobic system for algae plantation and photo-heterotrophic hydrogen production from short chain fatty acid (acetate- and butyrate-) enriched wastewater using microalgae, it would be beneficial to have a good understanding on microalgal activity during the hydrogen production for optimizing substrate utilization. This study over goal is developing the novel technologies for the best energy recovery using organic wastewater. The present study provides a fundamental scientific blueprint for the development of an integrated set of technologies designed to recover energy from organic wastewater resources. We were investigated optimal condition on fermentation process for bioenergy using various organic wastewater (i.e., sulfate wastewater and fruit waste) and direly hydrogen generation from microalgae using inorganic and organic carbon sources. This study showed the effects of varying sulfate concentrations with pH on continuous fermentative hydrogen production were studied to understand the relationship between sulfate reducing bacteria (SRB) and hydrogen (H2) production. Both Fe(II) and SO42- improved the H2 production efficiency from wastewater. In order to obtain an appropriate fermentation system using sulfate- and ferrous- enriched wastewater, it would be beneficial to quantitatively predict SRB activity during the H2 fermentative process for optimizing substrate utilization in the H2 reactor. The inhibition in hydrogen production by SRB at pH 6.2 diminished entirely by lowering the pH to 5.5, at which activity of SRB is substantially suppressed.Enhancement of fermentative bioenergy production is essential for efficient hydrogen production. A two-stage fermentation system was employed for combined hydrogen production to improve the energy efficiency. The energy efficiency (H2 conversion) obtained from mixed ripened fruits (RF) as substrate increased from 4.6% (in thefirst stage) to 15.5% (in the second stage). A direct hydrogen study showed that microalgal strains can upregulate the expression of mRNA synthesis of hydrogenase gene (hydA) and simultaneously produce hydrogen through photosynthesis using CO2 as the sole source of carbon with continuous illumination under aerobic conditions. We employed dissolved oxygen regimes typical of environmental conditions for microalgae in natural aquatic systems. The expression of hydA and the specific activity of hydrogenase evidenced that microalgae enzymatically produced hydrogen even under atmospheric conditions, which has been previously considered infeasible. Microalgal photo-heterotrophic hydrogen production by green algae organic carbon enriched (i.e., acetate and butyrate) wastewater effluent under anaerobic conditions was investigated. The wastewater was abundant in acetate and butyrate which caused different state of algae growth for phototrophic H2 production. The accumulative H2 production by microalgae with high organic concentration was almost 1.8 times higher than that from the control. These results demonstrate that AWE could be an effective substrate for enhancing the photo-heterotrophic H2 production from green algae.