Microalgae can contribute to more than 40% of global primary biomass production and are suitable candidates for various biotechnology applications such as food, feed products, drugs, fuels, and wastewater treatment. However, the primary limitation for large-scale algae production is the fact that algae requires large amounts of fresh water for cultivation. To address this issue, scientists around the world are working on ways to reuse the water to grow microalgae so that it can be grown in successive cycles without the need for fresh water. In this study, we present the results when we cultivate microalgae with cultivation water that is purified and reused. Specifically, we purify the cultivation water using an ultrafiltration membrane (UFM) treatment and investigate how this treatment affects: the biomass and biochemical components of the microalgae; characteristics of microalgae growth inhibitors; the mechanism whereby potential growth inhibitors are secreted (followed using metabolomics analysis); the effect of activated carbon (AC) treatment and advanced oxidation processes (AOPs) on the removal of growth inhibitors of Euglena gracilis. Firstly, the results show that E. gracilis can be only cultivated through two growth cycles with water that has been filtered and reused, and the growth of E. gracilis is significantly inhibited when the water is used a third time. Secondly, as the number of reused water cycles increases, the Cl− concentration gradually increases in the cultivation water. When the Cl− concentration accumulates to a level of fivefold higher than that of the control, growth of E. gracilis is inhibited as the osmolality tolerance range is exceeded. Interestingly, the osmolality of the reused water can be reduced by replacing NH4Cl with urea as the source of nitrogen in the cultivation water. Thirdly, E. gracilis secretes humic acid (HA)—which is produced by the metabolic pathways for valine, leucine, and isoleucine biosynthesis and by linoleic acid metabolism—into the cultivation water. Because HA contains large fluorescent functional groups, specifically extended π(pi)-systems containing C=C and C=O groups and aromatic rings, we were able to observe a positive correlation between HA concentration and the rate of inhibition of E. gracilis growth using fluorescence spectroscopy. Moreover, photosynthetic efficiency is adversely interfered by HA, thereby reductions in the synthetic efficiency of paramylon and lipid in E. gracilis. In this way, we are able to confirm that HA is the main growth inhibitor of E. gracilis. Finally, we verify that all the HA is removed or converted into nutrients efficiently by AC or UV/H2O2/O3 treatments, respectively. As a result of these treatments, growth of E. gracilis is restored (AC treatment) and the amount of biomass is promoted (UV/H2O2/O3 treatment). These studies have important practical and theoretical significance for the cyclic cultivation of E. gracilis and for saving water resources. Our work may also provide a useful reference for other microalgae cultivation.

中文翻译：

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微藻Euglena gracilis培养的水回用及生长抑制机制

微藻可占全球初级生物质产量的 40% 以上，是各种生物技术应用的合适候选者，例如食品、饲料产品、药物、燃料和废水处理。然而，大规模藻类生产的主要限制是藻类需要大量淡水进行培养。为了解决这个问题，世界各地的科学家正在研究如何重新利用水来种植微藻，以便它可以在不需要淡水的情况下连续循环生长。在本研究中，我们展示了使用净化和再利用的培养水培养微藻时的结果。具体来说，我们使用超滤膜 (UFM) 处理净化栽培用水，并研究这种处理如何影响：微藻的生物量和生化成分；微藻生长抑制剂的特性；潜在生长抑制剂的分泌机制（随后使用代谢组学分析）；活性炭 (AC) 处理和高级氧化工艺 (AOP) 对去除眼虫生长抑制剂的影响。首先，结果表明，只有经过两次过滤和重复利用的水才能培养纤细纤毛虫，并且当第三次使用该水时，纤细纤毛虫的生长受到显着抑制。其次，随着回用水循环次数的增加，培养水中的Cl-浓度逐渐增加。当 Cl− 浓度累积到比对照高 5 倍的水平时，E. 当超过渗透压容限范围时，gracilis 被抑制。有趣的是，通过用尿素代替 NH4Cl 作为培养水中的氮源，可以降低再利用水的渗透压。第三，E. gracilis 将腐植酸 (HA) 分泌到培养水中，腐植酸 (HA) 由缬氨酸、亮氨酸和异亮氨酸生物合成的代谢途径以及亚油酸代谢产生。由于 HA 包含大的荧光官能团，特别是包含 C=C 和 C=O 基团和芳香环的扩展 π(pi)-系统，我们能够观察到 HA 浓度与 E. gracilis 生长抑制率之间存在正相关关系使用荧光光谱。此外，光合效率受到 HA 的不利干扰，从而降低了大肠杆菌中副淀粉和脂质的合成效率。薄薄的。通过这种方式，我们能够确认 HA 是 E. gracilis 的主要生长抑制剂。最后，我们验证了通过 AC 或 UV/H2O2/O3 处理分别有效地去除了所有 HA 或将其转化为营养物质。作为这些处理的结果，E. gracilis 的生长得到恢复（AC 处理）并促进了生物质的量（UV/H2O2/O3 处理）。这些研究对薄薄纱的循环栽培和节约水资源具有重要的实践和理论意义。我们的工作也可为其他微藻培养提供有益的参考。恢复 E. gracilis 的生长（AC 处理）并促进生物量的数量（UV/H2O2/O3 处理）。这些研究对薄薄纱的循环栽培和节约水资源具有重要的实践和理论意义。我们的工作也可为其他微藻培养提供有益的参考。恢复 E. gracilis 的生长（AC 处理）并促进生物量的数量（UV/H2O2/O3 处理）。这些研究对薄薄纱的循环栽培和节约水资源具有重要的实践和理论意义。我们的工作也可为其他微藻培养提供有益的参考。