Scoma Alberto, Tóth Szilvia Z
Center for Microbial Ecology and Technology (CMET), Faculty of Bioengineering, University of Gent, Coupure Links 653, 9000 Ghent, Belgium.
Center for Geomicrobiology, Aarhus University, Ny Munkegade 116, 8000 Aarhus, Denmark.
Biotechnol Biofuels. 2017 May 4;10:116. doi: 10.1186/s13068-017-0800-6. eCollection 2017.
Under low O concentration (hypoxia) and low light, cells can produce H gas in nutrient-replete conditions. This process is hindered by the presence of O, which inactivates the [FeFe]-hydrogenase enzyme responsible for H gas production shifting algal cultures back to normal growth. The main pathways accounting for H production in hypoxia are not entirely understood, as much as culture conditions setting the optimal redox state in the chloroplast supporting long-lasting H production. The reducing power for H production can be provided by photosystem II (PSII) and photofermentative processes during which proteins are degraded via yet unknown pathways. In hetero- or mixotrophic conditions, acetate respiration was proposed to indirectly contribute to H evolution, although this pathway has not been described in detail.
Recently, Jurado-Oller et al. (Biotechnol Biofuels 8: 149, 7) proposed that acetate respiration may substantially support H production in nutrient-replete hypoxic conditions. Addition of low amounts of O enhanced acetate respiration rate, particularly in the light, resulting in improved H production. The authors surmised that acetate oxidation through the glyoxylate pathway generates intermediates such as succinate and malate, which would be in turn oxidized in the chloroplast generating FADH and NADH. The latter would enter a PSII-independent pathway at the level of the plastoquinone pool, consistent with the light dependence of H production. The authors concluded that the water-splitting activity of PSII has a minor role in H evolution in nutrient-replete, mixotrophic cultures under hypoxia. However, their results with the PSII inhibitor DCMU also reveal that O or acetate additions promoted acetate respiration over the usually dominant PSII-dependent pathway. The more oxidized state experienced by these cultures in combination with the relatively short experimental time prevented acclimation to hypoxia, thus precluding the PSII-dependent pathway from contributing to H production.
In , continuous H gas evolution is expected once low O partial pressure and optimal reducing conditions are set. Under nutrient-replete conditions, the electrogenic processes involved in H photoproduction may rely on various electron transport pathways. Understanding how physiological conditions select for specific metabolic routes is key to achieve economic viability of this renewable energy source.
在低氧浓度（缺氧）和低光照条件下，细胞在营养充足的环境中能够产生氢气。氧气的存在会阻碍这一过程，因为氧气会使负责氢气产生的[FeFe]-氢化酶失活，从而使藻类培养物恢复正常生长。尽管人们对缺氧条件下氢气产生的主要途径以及设定叶绿体中支持持续氢气产生的最佳氧化还原状态的培养条件了解并不完全清楚。光合作用系统II（PSII）和光发酵过程可以提供产生氢气所需的还原力，在这些过程中，蛋白质通过尚不明确的途径被降解。在异养或混合营养条件下，尽管尚未对该途径进行详细描述，但有人提出乙酸呼吸可能间接促进氢气的释放。
最近，Jurado-Oller等人（《生物技术与生物燃料》8：149，2015）提出，在营养充足的缺氧条件下，乙酸呼吸可能对氢气产生有显著支持作用。添加少量氧气可提高乙酸呼吸速率，尤其是在光照条件下，从而使氢气产量增加。作者推测，通过乙醛酸途径进行的乙酸氧化会产生琥珀酸和苹果酸等中间体，这些中间体进而会在叶绿体中被氧化生成FADH和NADH。后者会在质体醌库水平进入一条不依赖PSII的途径，这与氢气产生对光照的依赖性是一致的。作者得出结论，在营养充足的混合营养培养物缺氧条件下，PSII的水裂解活性在氢气释放中作用较小。然而，他们使用PSII抑制剂敌草隆（DCMU）的实验结果还表明，添加氧气或乙酸会促进乙酸呼吸，使其超过通常占主导地位的依赖PSII的途径。这些培养物所经历的氧化程度更高，再加上实验时间相对较短，阻碍了对缺氧环境的适应，从而使依赖PSII的途径无法对氢气产生做出贡献。
在[具体条件未提及]中，一旦设定了低氧分压和最佳还原条件，预计会持续产生氢气。在营养充足的条件下，氢气光产生过程中涉及的产电过程可能依赖于各种电子传输途径。了解生理条件如何选择特定的代谢途径是实现这种可再生能源经济可行性的关键。
