Electron spin resonance in microalgae whole-cells to monitor hydrogen production

Unicellular algae can produce pure hydrogen gas from water and sun-light. We observed Chlorella vulgaris whole cells when they produce hydrogen using X-band continuous-wave electron spin resonance (ESR). Whole-cell spectroscopy is particularly useful in those cases where purified enzymes are sensitive to oxidant air conditions. By tuning cell preparation, the microwave power, the temperature, the time of air exposure, we could isolate from the background signal candidate markers of hydrogen production. Our observations indicate the presence of a species consistent mainly with an intermediate ${\text{Fe}}_3{\text{S}}_4_{}^{+}$ cluster when hydrogen production is high, but not maximal, and when FeS cluster oxidation has just begun. The optimal conditions to detect the above marker by ESR have been identified. Our investigation paves the way to extensive statistical analysis of cellular conditions in future studies using whole-cell ESR.