Ramos-Martinez Erick Miguel, Fimognari Lorenzo, Rasmussen Maria K, Sakuragi Yumiko
Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, Frederiksberg, Denmark.
Front Bioeng Biotechnol. 2019 Feb 28;7:35. doi: 10.3389/fbioe.2019.00035. eCollection 2019.
Microalgae offer a promising biological platform for sustainable biomanufacturing of a wide range of chemicals, pharmaceuticals, and fuels. The model microalga is thus far the most versatile algal chassis for bioengineering and can grow using atmospheric CO and organic carbons (e.g., acetate and pure cellulose). Ability to utilize renewable feedstock like lignocellulosic biomass as a carbon source could significantly accelerate microalgae-based productions, but this is yet to be demonstrated. We observed that was not able to heterotrophically grow using wheat straw, a common type of lignocellulosic biomass, likely due to the recalcitrant nature of the biomass. When the biomass was pretreated with alkaline, was able to grow using acetate that was released from the biomass. To establish an eco-friendly and self-sustained growth system, we engineered to secrete a fungal acetylxylan esterase (AXE) for hydrolysis of acetylesters in the lignocellulosic biomass. Two transgenic strains (CrAXE03 and CrAXE23) secreting an active AXE into culture media were isolated. Incubation of CrAXE03 with wheat straw resulted in an eight-fold increase in the algal cell counts with a concomitant decrease of biomass acetylester contents by 96%. The transgenic lines showed minor growth defects compared to the parental strain, indicating that secretion of the AXE protein imposes limited metabolic burden. The results presented here would open new opportunities for applying low-cost renewable feedstock, available in large amounts as agricultural and manufacturing by-products, for microalgal cultivation. Furthermore, acetylesters and acetate released from them, are well-known inhibitors in lignocellulosic biofuel productions; thus, direct application of the bioengineered microalga could be exploited for improving renewable biofuel productions.
微藻为多种化学品、药品和燃料的可持续生物制造提供了一个有前景的生物平台。到目前为止，模式微藻是生物工程中用途最广泛的藻类底盘，能够利用大气中的二氧化碳和有机碳（如醋酸盐和纯纤维素）生长。利用木质纤维素生物质等可再生原料作为碳源的能力可以显著加速基于微藻的生产，但这一点尚未得到证实。我们观察到，[微藻名称]无法利用常见的木质纤维素生物质小麦秸秆进行异养生长，这可能是由于生物质的顽固性。当生物质用碱预处理后，[微藻名称]能够利用从生物质中释放出来的醋酸盐生长。为了建立一个生态友好且自我维持的生长系统，我们对[微藻名称]进行基因工程改造，使其分泌一种真菌乙酰木聚糖酯酶（AXE），用于水解木质纤维素生物质中的乙酰酯。分离出了两种向培养基中分泌活性AXE的转基因菌株（CrAXE03和CrAXE23）。CrAXE03与小麦秸秆一起培养，导致藻类细胞数量增加了八倍，同时生物质乙酰酯含量降低了96%。与亲本菌株相比，转基因品系表现出轻微的生长缺陷，这表明AXE蛋白的分泌带来的代谢负担有限。本文的研究结果将为应用低成本的可再生原料（大量可作为农业和制造业副产品获得）进行微藻培养开辟新的机会。此外，从它们中释放出的乙酰酯和醋酸盐是木质纤维素生物燃料生产中众所周知的抑制剂；因此，这种生物工程微藻的直接应用可用于改善可再生生物燃料的生产。
