Climate change is an inevitable phenomenon that occurs under the influence of various factors such as oceanic processes, solar radiation, and volcanic eruptions. These changes are often slow, giving enough time for most of the species to adapt. But faster climate change may cause dangerous and irreversible effects [1]. The increased CO2 emission is the main reason for the temperature elevation as it can absorb the thermal radiation emitted from the earth's surface. Due to global warming and limited fossil fuel supplies, the use of renewable energy has gained increasing popularity [2, 3]. Biofuel is a type of renewable energy whose energy content is derived from biological sources (organic constituents of the body of living organisms). The idea of using microalgae in the production of biofuels and the removal of greenhouse gases through photosynthesis is not new [4–6]. About 50% of the dry weight of microalgae biomass contains carbon. Hence, 100 tons of microalgae biomass production can fix 183 tons of CO2 [2]. Moreover, the biological production of hydrogen using these microorganisms is a new horizon to reach hydrogen through renewable sources. The hydrogen gas produced by biological processes can be easily converted into electrical energy; therefore, it can be considered as a clean and suitable fuel for transportation purposes [7, 8]. Biohydrogen is produced through photosynthesis) Biophotolysis direct and indirect) and photofermentation and dark fermentation [8, 9]. A significant number of bacteria, cyanobacteria and green algae such as Chlamydomonas, Scenedesmus, Chlorella, Tetraspora are able to produce biological hydrogen [10, 11]. Anaerobic cultures of microorganisms such as Clostridium, whose carbon source is supplied by carbohydrates such as glucose, are able to produce hydrogen gas during the fermentation process [12, 13]. Photosynthetic bacteria such as Rhodobacter have the ability to use carbon sources such as lactic acid and acetic acid as energy sources and produce hydrogen gas under light conditions [14]. Cyanobacteria among other microorganisms are suitable for hydrogen production due to their simple nutritional, water, light, and carbon dioxide requirements [15]. Some of its species, especially Anabaena variabilis and Anabaena sp. are the most potent CO2 consumers and H2 producers [16, 17]. Microalgae and cyanobacteria can be grown in open systems and closed systems which called photobioreactors. Photobioreactors provide a closed culture medium that is safe against invasion and competition by other microorganisms and controls the culture of microalgae more effectively [18]. In addition, more diverse species of microalgae can be grown in such closed environments under a large portion of the light not only shines directly on the surface of the culture medium but also passes through the transparent walls of the reactor and reaches the cultured cells [19, 20]. Researchers have been using many photobioreactors in hydrogen production for years [21]. Some studies presented a two-stage photobioreactor alternating between stages of growth and production of hydrogen. In the growth phase, cyanobacteria stabilize atmospheric CO2 and nitrogen for their own growth and carbohydrate production, and in the hydrogen production phase, they produce hydrogen using stored carbohydrates [22, 23]. It is reported that to produce hydrogen by Anabaena variabilis a two-step discontinuous process was used [24]. Among photobioreactors airlift reactors have been widely used in separation processes, especially for the cultivation of microalgae and biohydrogen production [25]. Compared to other similar devices such as stirred tanks and bubble columns, airlift photobioreactors (APBRs) offer lower and more uniform shear stress [26, 27], which has led to their widespread use in biological operations. Besides, the growth of cyanobacteria is significantly affected by superficial gas velocity, which is called aeration rate [28] since the culture medium is fed by a mixture of air and gas with different ratio or with pure gas which increases the mass transfer, avoid scarcity of carbon dioxide, dead zones, microalgae sedimentation [29] and control the amount of O2 and CO2 which inhibits cyanobacteria growth [30, 31]; Also, aeration rate improves photosynthesis with an optimized light/dark cycle [32] and influence on Oxygen mass transfer that is an inhibitory factor for biological H2 production. To achieve high amount of biohydrogen and cyanobacteria growth, the hydrodynamic parameters and volumetric mass transfer coefficient (kLa), which is the essential factor of the PBRs, must provide culture requirements especially sufficient light absorption [33].
On this matter, the present study investigated biohydrogen production in an airlift photobioreactor by Anabaena sp. in different superficial gas velocities to find out the impact of bioreactor hydrodynamic on H2 production. Also, the result of this study might be applied in further researches on biohydrogen production from microalgae.