Engineering strategies for enhancing photofermentative biohydrogen production by purple non-sulfur bacteria using dark fermentation effluent

Most energy fuels, chemicals, and raw materials in our daily lives are derived from petroleum-based refineries. However, depleting fossil fuel reserves and increasing greenhouse gas wmissions and severe pollution problems as the consequences of by-products from fossil fuel utilization are driving interests toward biorefineries for the production of energy and useful chemicals (Cherubini 2010; Menon and Rao 2012). In the energy and environmental sector, hydrogen (H2) has gained considerable interest owing to its higher specific energy content (122 MJ/kg), as well as water and energy being the sole oxidative reaction by-products (Balat and Kırtay 2010). At present, H2 production for industrial applications is mainly derived from thermocatalytic and gasification processes, which are highly dependent on fossil fuels. In comparison with the energy-intensive physicochemical routes for H2 production, biological processes can be operated at ambient conditions and are advantageous, as they can utilize renewable biomass (Das and Veziroglu 2001; Ghimire et al. 2015). Based on light dependency as an energy source for biochemical reactions, biological H2 production pathways can be broadly categorized into light-dependent and -independent processes (Das and Veziroglu 2008; Hallenbeck and Ghosh 2009). The light-dependent photohydrogen production systems can be further classified into (1) direct photolysis, where water is broken down into H2 and O2 gas by algae and cyanobacteria; (2) indirect photolysis, in which cyanobacteria or cyanophytes synthesize H2 in the presence of light and inorganic carbon; and (3) photofermentation (PF), carried out by photosynthetic bacteria where photodecomposition of organic compounds occurs. The light-independent processes include (1) dark fermentation (DF), which involves fermentative hydrogen production from carbohydrate-rich organic biomass, and (2) H2 from bioelectrochemical systems or microbial electrolysis cells.