Effects of dual-emission peak spectral conversion on photo-fermentative biohydrogen production from giant reed

Biological hydrogen production via photo-fermentation can play a key role in carbon reduction and the circular economy. Spectral conversion has been recognized as an effective approach to enhance its photosynthetic efficiency. In this study, a dual-emission peak spectral converter was developed to improve the photosynthetic rate and biohydrogen production. The high-energy light was selected from the inefficient utilization by the photosynthetic bacterium HAU-M1. Characteristic absorption peaks were converted at 490 and 592 nm in the visible region. Thereby, synergistic enhancement of effective light absorption was achieved after conversion. Dual-emission peak spectral converters were fabricated to mix phosphors with emission peaks at 482 and 596 nm at five mass ratios (4∶1, 2∶1, 1∶1, 1∶2, and 1∶4). The transmittance and haze of 5 dual-emission peak spectral converters were in the ranges of 70.1%-81.4% and 80.0%-90.0%, respectively. The average optical power intensity loss and light intensity loss were 13.0% and 14.5%, respectively. The results show that the dual-emission peak spectral converter with a mixing ratio of 1∶4 shared the maximum optical power intensity loss and light intensity loss of 17.0% and 20.9%, respectively, while that with a mixing ratio of 4∶1 achieved the minimum optical power intensity loss of 10.0%. There was no effect of the mixing ratio of phosphors with emission peaks at 482  and 596 nm on the positions of the absorption and emission peaks of the liquid or solid. A systematic investigation was implemented to explore the effects of five spectral converters on bacterial dry weight, photosynthetic pigment contents (bacteriochlorophyll and carotenoids), photosynthetic rate, cumulative hydrogen production, hydrogen production rate, and light transmission characteristics. The bacterial dry weight, bacterial chlorophyll content, carotenoid content, and photosynthetic rate of the spectral converter with a mixing ratio of 1∶2 increased by 33.5%, 35.2%, 33.1%, and 63.6%, respectively, compared with the control group. In contrast, the spectral converter with a mixing ratio of 2∶1 achieved the highest cumulative hydrogen production of 261.61 mL, which was 20.3% higher over the control group, followed by the 1∶1 experimental group with 248.78 mL, the 4∶1 experimental group with 240.53 mL, the 1∶2 experimental group with 238.74 mL, and the 1∶4 experimental group with 228.29 mL, indicating increases of 14.3%, 10.6%, 10.0%, and 5.0%, respectively. Furthermore, the difference between bacterial dry weight and hydrogen production represented the competition between Calvin-Benson-Bassham and the tricarboxylic acid cycle during hydrogen production. In addition, transmission performance of light in the bacterial suspension was realized to regulate the growth and pigment synthesis of photosynthetic bacteria. A spectral converter with a mixing ratio of 2∶1 exhibited the highest light absorption, which was aligned with its optimal performance of hydrogen production. Moreover, the dual-emission peak spectral conversion can be expected to optimize the metabolic pathway, particularly for the high amount of efficient light with the absorption peaks of photosynthetic bacteria at 482 and 596 nm in the visible light region. The findings can provide a new approach to enhance biohydrogen production via photo-fermentation.