State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 73 Huanghe Road, P,O, Box 2614, Harbin, 150090, China.
Biotechnol Biofuels. 2013 May 3;6(1):64. doi: 10.1186/1754-6834-6-64.
Hydrogen recovered from organic wastes and solar energy by photo-fermentative bacteria (PFB) has been suggested as a promising bioenergy strategy. However, the use of PFB for hydrogen production generally suffers from a serious biomass washout from photobioreactor, due to poor flocculation of PFB. In the continuous operation, PFB cells cannot be efficiently separated from supernatant and rush out with effluent from reactor continuously, which increased the effluent turbidity, meanwhile led to increases in pollutants. Moreover, to replenish the biomass washout, substrate was continuously utilized for cell growth rather than hydrogen production. Consequently, the poor flocculability not only deteriorated the effluent quality, but also decreased the potential yield of hydrogen from substrate. Therefore, enhancing the flocculability of PFB is urgent necessary to further develop photo-fermentative process.
Here, we demonstrated that L-cysteine could improve hydrogen production of Rhodopseudomonas faecalis RLD-53, and more importantly, simultaneously trigger remarkable aggregation of PFB. Experiments showed that L-cysteine greatly promoted the production of extracellular polymeric substances, especially secretion of protein containing more disulfide bonds, and help for enhancement stability of floc of PFB. Through formation of disulfide bonds, L-cysteine not only promoted production of EPS, in particular the secretion of protein, but also stabilized the final confirmation of protein in EPS. In addition, the cell surface elements and functional groups, especially surface charged groups, have also been changed by L-cysteine. Consequently, absolute zeta potential reached a minimum value at 1.0 g/l of L-cysteine, which obviously decreased electrostatic repulsion interaction energy based on DLVO theory. Total interaction energy barrier decreased from 389.77 KT at 0.0 g/l of L-cysteine to 127.21 kT at 1.0 g/l.
Thus, the strain RLD-53 overcame the total energy barrier and flocculated effectively. After a short settlement, the biomass rush out will be significantly reduced and the effluent quality will be greatly improved in the continuous operation. Furthermore, aggregation of PFB could enable high biomass hold-up of photobioreactor, which allows the photobioreactor to operate at low hydraulic retention time and high organic loading rate. Therefore, the described flocculation behaviour during photo-hydrogen production is potentially suitable for practicable application.
通过光发酵细菌（PFB）从有机废物和太阳能中回收的氢气已被认为是一种很有前途的生物能源策略。然而，由于 PFB 的絮凝性能较差，用于生产氢气的 PFB 一般会遭受来自光生物反应器的生物质严重洗出。在连续运行中，PFB 细胞不能有效地从上清液中分离出来，并随着反应器中流出物不断冲出，这增加了流出物的浊度，同时也导致污染物的增加。此外，为了补充生物质洗出，底物不断地被用于细胞生长而不是氢气生产。因此，较差的絮凝性能不仅恶化了流出物的质量，而且降低了底物的潜在氢气产量。因此，迫切需要提高 PFB 的絮凝能力，以进一步发展光发酵工艺。
在这里，我们证明了 L-半胱氨酸可以提高 Rhodopseudomonas faecalis RLD-53 的氢气产量，更重要的是，同时引发 PFB 的显著聚集。实验表明，L-半胱氨酸大大促进了胞外聚合物的产生，特别是含有更多二硫键的蛋白质的分泌，并有助于增强 PFB 絮体的稳定性。通过形成二硫键，L-半胱氨酸不仅促进了 EPS 的产生，特别是蛋白质的分泌，而且稳定了 EPS 中蛋白质的最终构象。此外，细胞表面元素和官能团，特别是表面带电基团，也被 L-半胱氨酸改变。因此，在 1.0 g/L 的 L-半胱氨酸时，绝对 Zeta 电位达到最小值，这明显降低了基于 DLVO 理论的静电排斥相互作用能。总相互作用能垒从 0.0 g/L 的 L-半胱氨酸时的 389.77 KT 下降到 1.0 g/L 时的 127.21 kT。
因此，RLD-53 菌株克服了总能量障碍并有效地絮凝。在短时间的沉降后，连续运行中生物质的冲出量将显著减少，流出物的质量将得到极大改善。此外，PFB 的聚集可以使光生物反应器中的生物质保持高浓度，从而使光生物反应器能够在低水力停留时间和高有机负荷率下运行。因此，所描述的光氢生产过程中的絮凝行为可能适用于实际应用。
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