RESUMEN
Light-dependent hydrogen production by microalgae attracts attention of researchers because of the potential practical application. It is generally recognized that Calvin-Benson-Bassham cycle competes with hydrogen production process for electrons, and substrate (CO2) limitation of the cycle can increase hydrogen production rate. Furthermore, photosystem II is not destroyed by CO2 deficiency. We studied photoautotrophic cultures of Chlamydomonas reimhardtii under CO2 deficiency. Under the flow of air with removed CO2 the cultures reached stationary phase of growth and the photosystem II was downregulated due to overreduction of plastoquinone pool followed by degradation of the entire photosynthetic machinery. Under the Ar flow in the absence of CO2 the cultures were brought to microaerobic conditions producing small amounts of hydrogen (5 ml H2 day-1 liter-1 culture). Similar to the case of incubation under air atmosphere, prolonged incubation of cultures under microaerobic conditions resulted in down-regulation of photosystem II due to overreduction of plastoquinone pool with following degradation of whole photosynthetic machinery. Following removal of CO2, transfer of cultures into dark anaerobic conditions (2.5 h), and illumination with low-intensity light resulted in the cultures producing H2 with high initial rate. Total microalgal hydrogen production under these conditions was 56 ml H2 liter-1 culture. Thus, the CO2-deprived photoautotrophic cultures produce hydrogen. Hydrogen production was limited by the toxic effect of oxygen on hydrogenase but not by the Calvin-Benson-Bassham cycle competition with hydrogen production process.
Asunto(s)
Chlamydomonas reinhardtii , Hidrogenasas , Chlamydomonas reinhardtii/metabolismo , Complejo de Proteína del Fotosistema II/metabolismo , Dióxido de Carbono/metabolismo , Hidrogenasas/metabolismo , Hidrogenasas/farmacología , Plastoquinona/farmacología , Azufre/metabolismo , Azufre/farmacología , Fotosíntesis/fisiología , Hidrógeno , Oxígeno/metabolismoRESUMEN
Certain microalgae species are capable of light-dependent hydrogen production under conditions of dark anaerobic incubation or nutrient deprivation. From the biotechnological point of view, this phenomenon is a process of synthesizing the energy carrier H2 while consuming light energy. Here, we overview the functional connection between the photosynthetic machinery and light-dependent hydrogen production and assess the physiological significance of this process. We characterize events involved in PSII downregulation, as well as the relationship between PSII regulation mechanisms and hydrogen generation. We suggest that the light-dependent hydrogen production forms part and parcel of the sophisticated regulatory network ensuring adaptation of microalgae to such environmental stresses as anaerobiosis or nutrient deprivation.