The establishment of new protein expression system using N starvation inducible promoters in Chlorella

Molecular design of microalgae as sustainable cell factories

Microalgae are regarded as sustainable and potent chassis for biotechnology. Their capacity for efficient photosynthesis fuels dynamic growth independent from organic carbon sources and converts atmospheric CO2 directly into various valuable hydrocarbon-based metabolites. However, approaches to gene expression and metabolic regulation have been inferior to those in more established heterotrophs (e.g., prokaryotes or yeast) since the genetic tools and insights in expression regulation have been distinctly less advanced. In recent years, however, these tools and their efficiency have dramatically improved. Various examples have demonstrated new trends in microalgal biotechnology and the potential of microalgae for the transition towards a sustainable bioeconomy.

Response of Chlorella vulgaris to exposure to CuO NPs: Contributions of particulate and dissolved metal forms as modulated by tannic acid and pH

A suspension of copper oxide nanoparticles (CuO NPs) is a mixture of dissolved and particulate Cu, the relative proportions of which highly depend on the water chemistry. However, the relationship between different proportions of particulate and dissolved Cu and the overall toxicity of CuO NPs is still unknown. This study investigated the response of Chlorella vulgaris to CuO NPs at varying solution pH and at different tannic acid (TA) additions, with a focus on exploring whether and how dissolved and particulate Cu contribute to the overall toxicity of CuO NPs. The results of the exposure experiments demonstrated the involvement of both dissolved and particulate Cu in inducing toxicity of CuO NPs, and the inhibition of CuO NPs on cell density of Chlorella vulgaris was found to be significantly (p < 0.05) alleviated with increased levels of TA and pH (< 8). Using the independent action model, the contribution to toxicity of particulate Cu was found to be enhanced with increasing pH values and TA concentrations. The toxic unit indicator better (R2 = 0.86, p < 0.001) explained impacts of CuO NPs on micro-algae cells than commonly used mass concentrations (R2 = 0.27-0.77, p < 0.05) across different levels of pH and TA. Overall, our study provides an additivity-based method to improve the accuracy of toxicity prediction through including contributions to toxicity of both dissolved and particulate Cu and through eliminating the uneven distribution of data due to large variations in total Cu, particulate Cu, dissolved Cu, Cu2+ activities, Cu-TA complexes and other Cu-complexes concentrations with varying water chemistry conditions.

Is Genetic Engineering a Route to Enhance Microalgae-Mediated Bioremediation of Heavy Metal-Containing Effluents?

Contamination of the biosphere by heavy metals has been rising, due to accelerated anthropogenic activities, and is nowadays, a matter of serious global concern. Removal of such inorganic pollutants from aquatic environments via biological processes has earned great popularity, for its cost-effectiveness and high efficiency, compared to conventional physicochemical methods. Among candidate organisms, microalgae offer several competitive advantages; phycoremediation has even been claimed as the next generation of wastewater treatment technologies. Furthermore, integration of microalgae-mediated wastewater treatment and bioenergy production adds favorably to the economic feasibility of the former process-with energy security coming along with environmental sustainability. However, poor biomass productivity under abiotic stress conditions has hindered the large-scale deployment of microalgae. Recent advances encompassing molecular tools for genome editing, together with the advent of multiomics technologies and computational approaches, have permitted the design of tailor-made microalgal cell factories, which encompass multiple beneficial traits, while circumventing those associated with the bioaccumulation of unfavorable chemicals. Previous studies unfolded several routes through which genetic engineering-mediated improvements appear feasible (encompassing sequestration/uptake capacity and specificity for heavy metals); they can be categorized as metal transportation, chelation, or biotransformation, with regulation of metal- and oxidative stress response, as well as cell surface engineering playing a crucial role therein. This review covers the state-of-the-art metal stress mitigation mechanisms prevalent in microalgae, and discusses putative and tested metabolic engineering approaches, aimed at further improvement of those biological processes. Finally, current research gaps and future prospects arising from use of transgenic microalgae for heavy metal phycoremediation are reviewed.

Unraveling metabolic alterations in Chlorella vulgaris cultivated on renewable sugars using time resolved multi-omics

The inherent metabolic versatility of Chlorella vulgaris that enables it to metabolize both inorganic and organic carbon under various trophic modes of cultivation makes it a promising candidate for industrial applications. To shed light on the metabolic flexibility of this microalga, time resolved proteomic and metabolomic studies were conducted in three distinct trophic modes (autotrophic, heterotrophic, mixotrophic) at two growth stages (end of linear growth at 6 days and during nutrient deprivation at 10 days). Sweet sorghum bagasse (SSB) hydrolysate was supplied to the cultivation medium as a renewable source of organic carbon mainly in the form of glucose. Integrated multi-omics data showed improved nitrogen assimilation, re-allocation, and recycling and increased levels of photosystem II (PS II) proteins indicating effective cellular quenching of excess electrons during mixotrophy. As external addition of organic carbon (glucose) to the cultivation medium decreases the cell's dependence on photosynthesis, an upregulation in the mitochondrial electron transport chain was recorded that led to increased cellular energy generation and hence higher growth rates under mixotrophy. Moreover, upregulation of the lipid-packaging proteins caleosin and 14_3_3 domain-containing protein resulted in maximum expression during mixotrophy suggesting a strong correlation between lipid synthesis, stabilization, and assembly. Overall, cells cultivated under mixotrophy showed better nutrient stress tolerance and redox balancing leading to higher biomass and lipid production. The study offers a panoramic view of the microalga's metabolic flexibility and contributes to a deeper understanding of the altered biochemical pathways that can be exploited to enhance algal productivity and commercial potential.

Establishment of a Genome Editing Tool Using CRISPR-Cas9 in Chlorella vulgaris UTEX395

To date, Chlorella vulgaris is the most used species of microalgae in the food and feed additive industries, and also considered as a feasible cell factory for bioproducts. However, the lack of an efficient genetic engineering tool makes it difficult to improve the physiological characteristics of this species. Therefore, the development of new strategic approaches such as genome editing is trying to overcome this hurdle in many research groups. In this study, the possibility of editing the genome of C. vulgaris UTEX395 using clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9) has been proven to target nitrate reductase (NR) and adenine phosphoribosyltransferase (APT). Genome-edited mutants, nr and apt, were generated by a DNA-mediated and/or ribonucleoprotein (RNP)-mediated CRISPR-Cas9 system, and isolated based on the negative selection against potassium chlorate or 2-fluoroadenine in place of antibiotics. The null mutation of edited genes was demonstrated by the expression level of the correspondent proteins or the mutation of transcripts, and through growth analysis under specific nutrient conditions. In conclusion, this study offers relevant empirical evidence of the possibility of genome editing in C. vulgaris UTEX395 by CRISPR-Cas9 and the practical methods. Additionally, among the generated mutants, nr can provide an easier screening strategy during DNA transformation than the use of antibiotics owing to their auxotrophic characteristics. These results will be a cornerstone for further advancement of the genetics of C. vulgaris.