Photobiological hydrogen as a renewable fuel

The huge global energy consumption has raised concerns over the depletion in readily available conventional energy resources. Besides, there are harmful atmospheric effects of fossil fuels and the qualms of future energy resources. The world hence is in dire need of new renewable energy sources that are cheap, non-polluting, environmentally friendly, and clean. This is the only way we can stop using fossil. Hydrogen is considered as an ideal fuel for the future because of its high energy content and its clean combustion to water. However, extensive technologies are required to introduce hydrogen as an alternative clean and cost-effective future fuel, which brings about the relevance of the exploitation of the microorganisms for large-scale renewable energy production. Reports of photobiological hydrogen production by oxygenic photosynthetic microbes, such as green algae and cyanobacteria and by anaerobic photosynthesis, are summarized in this paper, with a focus on the major obstacles that must be overcome by scientific and technical breakthroughs to make way for commercially feasible energy. The principle, progress, and prognosis of photobiological hydrogen as a renewable energy source are reviewed.


INTRODUCTION
Concerns about environmental pollution, looming climate change, and dwindling fossil fuels are compelling reasons to switch to renewable energy sources sufficiently large in scale to meet worldwide demand. A variety of renewable energy sources have been proposed and are presently under study, but hydrogen appears to have several advantages. First, its use in fuel cells is innately more efficient than the combustion that is currently required for the conversion of other potential fuels to mechanical energy. Secondly, its breakdown generates no pollutants unlike ethanol whose large scale use is predicted to release large amounts of carcinogenic acetaldehyde with the generation of large amounts of smog.
Thirdly, all presently studied alternative sources are largely carbon neutral since the carbon released by their combustion is derived, directly or indirectly, from recently fixed atmospheric CO2 [1]. Since it is the lightest carbon-neutral fuel rich in energy per unit mass and easy to store, hydrogen has attracted worldwide attention as a secondary energy carrier. Biological hydrogen production could even be carbon negative.
Biological production of hydrogen is a method for its renewable production. It could be through direct capture of solar energy or utilization of energy-rich organic material for photosynthetic fixation of carbon. Each approach has advantages and disadvantages along with challenging technical barriers to practical application.
In photolytic biological systems, water is used as a substrate. Microorganisms, such as green microalgae or cyanobacteria, use sunlight to split water into oxygen and hydrogen ions [2]. Its effectiveness is limited by low rates of hydrogen production and the fact that splitting water also produces oxygen, which quickly inhibits the hydrogen production reaction. The highly explosive hydrogen-oxygen mixture could be a safety issue; this process is also a very oxygen-sensitive process.
In photofermentative biological systems, organic matter is used as a substrate. Its effectiveness is limited by a very low hydrogen production rate and low solar-tohydrogen efficiency [3].
Photobiological production technologies may provide economical hydrogen production from sunlight with low to net-zero carbon emissions. The algae and bacteria could be grown in water that cannot be used for drinking or agriculture and could potentially even use wastewater [4].

PRINCIPLE AND PROGRESS
In the long run, the highest efficiencies of using hydrogen as a fuel can be achieved with technologies such as photoelectrochemistry or photochemistry, which can produce hydrogen directly from solar energy. Biological hydrogen production can be categorized into five different groups: (1) direct biophotolysis of water, (2) indirect biophotolysis of water, (3) biophotofermentation, (4) hydrogen production by water gas reaction, and (5) dark fermentation [5].
Direct hydrogen gas production can be done by the activity of hydrogenase without intermediate molecules, such as carbohydrates. Indirectly, hydrogen gas could be produced after the storage of carbohydrates or glycogen. Direct photobiological H2 production from water uses solar energy for largescale production of hydrogen gas by photosynthesis, in which solar energy is used to split water into H2 gas [6].
Different species of green algae (freshwater and marine) have been used for this. Examples include single-cell cyanobacteria (Synechocystis), multicellular cyanobacteria (Nostoc sp.), and green algae (Chlamydomonas sp.) [7]. H2 production by algae is a desired option since it produces hydrogen from easily available water, with no accumulation of CO2 and theoretical solar energy conversion efficiency of about 80 %. Though the process has many advantages, the hydrogenase enzyme involved in hydrogen production is inhibited by oxygen. Overall, the reaction of direct biophotolysis can be described as following [8]: Indirect biophotolysis is composed of two stages: carbohydrates synthesis in the light and dark fermentation of carbohydrates for H2 production. Cyanobacteria are ideal candidates for this process since they have the simplest nutrient requirements [9]. Examples of various species used include Anabaena, Oscillatoria, Callithrix and Gloeocapsa. Theoretical light conversion efficiency is 16.3 % while in actual practice is at 1 -2 % [10]. Indirect biophotolysis can be described by the following reactions: Hydrogen gas can also be produced by photofermentation. Organic compounds like acetic acids, lactic acids, and butyric acids are converted into H2 and CO2 by photosynthetic bacteria in the presence of sunlight in anaerobic conditions. During photofermentation both hydrogenase and nitrogenase enzymes are involved in H2 production [11]. The overall pathway is: (CH2O)2 + NADPH (Nicotinamide adenine dinucleotide phosphate (reduced form))→ Ferredoxin + ATP (Adenosine triphosphate) → Nitrogenase →H2 In photosynthetic bacteria, light intensity, light wavelength, duration of light and temperature are important for photofermentative H2 production. Factors that limit nitrogenasemediated photofermentation in purple nonsulphur bacteria are: (1) the occurrence of an H2 uptake enzyme, (2) low photofermentation efficiency for H2 production, (3) the low turnover rate of nitrogenase, (4) a low rate of carbon conversion, and (5) the availability of organic acids [12].
Research has been conducted into optimizing feedstock for photobiological H2 production; to produce H2, photosynthetic bacteria Rhodopseudomonas used sewage and wastewater, Rhodopseudomonas and Cyanobacterium anacystis used dairy and sugarcane wastewater, Rhodobacter sphaeroides used feedstock, sugar refinery and brewery wastewater [13 -15].
Photofermentation can be coupled with dark fermentation or used as a wastewater technique [16]. Dark fermentation is a carbonneutral process for the production of H2 and CO2 from biomass by facultative and obligate anaerobic fermentative bacteria. It does not require light energy as input. The reaction is the following: The main advantage of dark fermentation is that the hydrogen evolution rate is higher in contrast to other processes. The drawback is the low yield of H2 per substrate consumed. Pre-treatment techniques can inhibit bacterial activity and due to this H2 consumption decreases and yield increases.
Oxygenic photosynthesis of unicellular green algae generates biomass and, via the [Fe]hydrogenase enzyme, quantities of H2 gas. The algal biomass is used as feedstock for dark fermentation, producing hydrogen and organic acids [17]. The fermentative broth is used for photofermentation to produce H2. While green algae absorb visible light, photosynthetic bacteria absorb the infrared portion of solar radiation. This combination can increase total light conversion efficiency as well [18].
Algal hydrogen production is naturally inhibited by the presence of oxygen, which is a major product of photosynthesis. This leads to the transient production of hydrogen.
Overcoming that inhibition is a major focus of photobiological hydrogen production research. The inhibition depends on oxygen's ability to physically diffuse into the enzyme's catalytic centre and irreversibly bind to it, thus halting further catalytic activity. Strategies for extending the catalytic lifetime of such enzymes (hydrogenases) include: (1) molecular engineering of pathways for oxygen gas diffusion into the catalytic site of hydrogenases to block O2 from reaching the catalytic site, (2) mutagenizing the hydrogenase gene and then screening for an oxygen-tolerant version of this enzyme, and (3) searching for more oxygen-tolerant hydrogenases from nature and then transferring such genes into green algae and cyanobacteria [19]. It is necessary to separate the hydrogen and oxygen being produced to avoid flammable mixtures. One could also resort to a two-stage method to temporally separate O2 evolution and H2 production activities, thereby allowing H2 production for extended periods without resorting to the use of any mechanical or chemical manipulations. The method demonstrates the successful operation of a single organism, a two-stage photobiological H2 evolution process in a green alga. It is based on the concept of substrate S as a reversible switch to metabolically regulate the activity of the O2evolving PSII (Photosystem II) complex. The method was also checked reversible [20].
Hydrogen production by anaerobic bacteria (e.g. Clostridium pasteurianum) through fermentations results in the generation of an abundance of small organic acids, such as malate, lactate, propionate, butyrate, and/or acetate besides of H2. The further conversion of these small organic acids to H2 is not an energetically favourable reaction. Hence, small organic acids accumulate in the growth medium. This inhibits the rate of growth and limits the yield of H2 production by the anaerobes.
Sulphur-deprived Chlamydomonas reinhardtii was able to induce photobiological H2 production under photoautotrophic, photoheterotrophic growth conditions. Hence, hydrogen production can be sustained artificially by inhibiting photosynthesis. Sulphate deprivation hampers the production of a key enzyme for photosynthesis. As the sulphate is used up, photosynthesis slows and less oxygen is produced than consumed for respiration, and the culture becomes anaerobic and switches from carbon fixation to a combination of hydrogen-production and starch degradation [21]. Starch degradation supports the consumption of the low amount of oxygen that is still derived from residual photosynthesis and contributes reductants to hydrogen production. So, the culture starts accumulating the hydrogen gas.

CONCLUSION
Biological systems offer a variety of ways to generate renewable energy. Biohydrogen is one such potentially useful way to fulfil energy consumption, which currently met by fossil fuels. That makes photobiological hydrogen production using photosynthetic microorganisms a useful as well as exciting topic for research. It also shows promise for generating carbon-free, clean fuel free of greenhouse gas emissions from abundant natural resources, such as water and sunlight. For a more feasible and a better yield of hydrogen, a few ideas need to be explored, improved, and incorporated. Integrating hydrogenase and nitrogenase into suitable microorganisms, enhancing culture conditions, sustainable use of feedstocks, analysing various techniques of photobiological H2 production, integrating multiple mechanisms of photobiological hydrogen production in the photobioreactors are among those ideas. The use of bioreactors is a good way to minimize energy loss and harvest light energy more easily. After the identification of suitable strains of algae and cyanobacteria, various bioreactors can be used for photobiological H2 production by microorganisms in which light energy is converted into biochemical energy [22]. Factors influencing the performance of bioreactors are area-volume ratio, temperature, agitation and gas exchange.