The enhancement of hydrogen photoproduction in Chlorella protothecoides exposed to nitrogen limitation and sulfur deprivation

H2 photoproduction, hydrogenase activities and PSII photochemical activities in Chlorella protothecoides under sulfur (S–) or nitrogen (N–) deprivation or simultaneous N-limitation and S-deprivation were studied. C. protothecoides pre-cultured in full nutrient TAP medium containing 7 mM NH4Cl was found to produce a detectable but low level of H2, once the cells were inoculated either in S-free or N-free medium. However, cells pre-grown in a low concentration of NH4Cl (0.35 and 0.7 mM) generated a large amount of H2 after transfer to N-limited and S-free medium. The maximal H2 outputs of ∼233.7 and ∼129.1 ml/l were obtained within 100 h in the cultures exposed to S-deprived medium containing 0.35 mM and 0.7 mM NH4Cl, with the average H2 production rates being ∼2.19 and ∼1.37 ml/l/h, respectively. Our studies further indicated that N-limitation resulted in considerable starch accumulation, chlorophyll synthesis reduction, photosynthetic electron transfer block and oxygen evolving complex (OEC) injury, as well as attenuation in PSII oxygenic activity. Significant starch degradation was not observed during the H2 photoevolution process. Attenuation of PSII O2 evolution favored a rapid establishment of anaerobiosis for hydrogenase induction. Meanwhile, a constant high level of hydrogenase activities in C. protothecoides exposed to simultaneous N-limitation and S-deprivation were measured. Based on the above results, a possible mechanism of high H2 photoproduction in C. protothecoides exposed to N-limitation and S-deprivation was discussed. Low net photosynthetic oxygenic rates, together with high hydrogenase activities were thought to contribute to the enhancement of H2 photoproduction by C. protothecoides.