Other than direct photolysis, photosynthetic hydrogen can be produced through the use of green algae that can directly produce hydrogen under the condition of sulfur depri­vation (Manis and Banerjee, 2008). Deprivation of sulfur nutrients in the growth medium causes a reversible inhibition in the activity of oxygenic photosynthesis in green algae
(Melis et al., 2000). Protein biosynthesis is impeded in the absence of sulfur, and the green algae are unable to perform the required turnover of the D1/32-kD reaction center protein of PSII (known as the psbA chloroplast gene product) in the thylakoid membrane of algae (Wykoff et al., 1998). Under sulfur deprivation, the photochemical activity of PSII declines, and the absolute activity of photosynthesis becomes less than that of respiration. As a result, the rates of photosynthetic oxygen evolution drop below those of oxygen consumption by respiration (Melis et al., 2000). Such imbalance in the photosynthesis-respiration relationship by sulfur deprivation resulted in net consumption of oxygen by the cells, causing anaerobic conditions in the growth medium. Consequently, an anaerobic condition prevails in the sealed light-dependent algal cultures. With energy derived from light under deprivation of sulfur, the anaerobic algal cultures would elicit the [Fe]-hydrogenase pathway of electron transport in the chloroplast to photosynthetically produce hydrogen (Melis, 2002).

In essence, hydrogen can be produced under sulfur deprivation by circumventing the sensitivity of the [Fe]-hydrogenase to molecular oxygen through a temporal separation of the reactions of oxygen and hydrogen photoproduction. In the course of such a hydrogen pro­duction condition (sulfur-deprivation), algal cells consumed significant amounts of internal starch and protein (Zhang et al., 2002). Such catabolic reactions apparently indirectly sustain the hydrogen production process.

Hydrogen production via indirect photolysis by algae is deemed feasible if photon conver­sion efficiency can be improved for large-scale applications. Algal bioreactors can provide an engineering approach to regulate light inputs to the culture to improve the photon conversion efficiency of algal cell. The improvement of photosynthesis efficiency is too difficult to achieve for conventional crop plants (Hankamer et al., 2007). Recent research has reported a substan­tial increase in light utilization efficiency of up to 15%, compared with the previous utilization of around 5% (Tetali et al., 2007; Laurinavichene et al., 2008). Some researchers claimed that efficiency between 10% and 13% is attainable by engineering the microorganisms to better utilize the solar energy (Turner et al., 2008). However, improvements must be made to opti­mize the solar conversion efficiency of the algae under mass culture conditions. Optical short­comings associated with the chlorophyll antenna size and the light-saturation drawback of photosynthesis need to be addressed before high photosynthetic solar conversion efficiencies in mass culture can be achieved (Melis et al., 1999). Additional challenges that must be tackled include finding ways to recycle photobioreactor components and minimize the chemical cost of the nutrients to support algal growth, since these two items constitute 80-85% of the overall cost of commercial hydrogen production (Melis, 2002).

Photoproduction of hydrogen at a rate of about 12.5 ml H2/h per gram cell dry weight was reported in a study on indirect biophotolysis with cyanobacterium anabaena variabilis (Markov et al., 1997). In another study on indirect biophotolysis with cyanobacterium gloeocapsa alpicola, it was found that maintaining the culture at pH value between 6.8 and 8.3 yielded optimal hydrogen production (Troshina et al., 2002). Increasing the temperature from 30°C to 40°C resulted in twofold increase in the hydrogen production. The hydrogen production rate through indirect biophotolysis is comparable to hydrogenase-based hydrogen production by green algae.

Currently, less than 10% of the algae photosynthetic capacity was utilized for biohydrogen production. Research is underway to further improve algal photosynthetic capacity using a molecular engineering approach. Mutant algae with less chlorophyll could be manipulated for large-scale commercial applications that disperse more light to deeper algae layers in the bioreactor (Hankamer et al., 2007; Beer et al., 2009). Hence, sunlight is made available for more algal cells to generate hydrogen, thus improving the production rate. With technology ad­vancement, biohydrogen production via algae bioreactors will offer a sustainable alternative energy resource in future.