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
The aim of this work was a comparative study of S-repleted and S-depleted photoautotrophic cultures of Chlamydomonas reinhardtii under aerobic and anoxic conditions with the main focus on PSII activity. For that we used photobioreactor with short light path connected on-line to PAM fluorometer and cultivated microalgae in twice concentrated HS medium to avoid any uncontrolled limitation by mineral elements. Photoautotrophic cultures grown under Ar + CO2 gas mixture did not reach the same Chl (a + b) concentration as control culture (grown under air + CO2). At pO2 40% of air saturation (96 µM O2), the actual quantum yield of PSII started to decrease. Under microaerobic conditions when cultures stopped growing, the most significant changes in PSII function were observed. Maximum quantum yield Fv/Fm decreased significantly along with performance index, PIabs. It was accompanied by increase of fluorescence at J point, Vj. Results indicate that microaerobic conditions are stressful for photoautotrophic cultures. Photoautotrophic cultures of microalgae under S-deprivation in aerobic or anaerobic conditions showed similar behavior as photoheterotrophic ones described earlier. However, photoautotrophic cultures during anaerobiosis establishment did not show sharp "switch off" effect of actual quantum yield. We show also that S-deprivation under air or argon as well as the growth under Ar + CO2 cause significant increase of initial rise of fluorescence, which indicates that PSII and oxygen-evolving complex might be disintegrated.