Role of Mitochondria in Regulating Lutein and Chlorophyll Biosynthesis in Chlorella pyrenoidosa under Heterotrophic Conditions

GhLCYε-3 characterized as a lycopene cyclase gene responding to drought stress in cotton

Drought stress significantly affects crop productivity. Carotenoids are essential photosynthetic pigment for plants, bacteria, and algae, with signaling and antioxidant functions. Lutein is a crucial branch product in the carotenoid synthesis pathway, which effectively improves the stress tolerance of higher plants. lycopene cyclase, a central enzyme for lutein synthesis, holds great significance in regulating lutein production. This research establishes a correlation between lutein content and stress resistance by measuring the drought resistance and lutein content of various cotton materials. To identify which crucial genes are associated with lutein, the lycopene cyclase family (LCYs) was analyzed. The research found that LCYs form a highly conserved family divided into two subfamilies, LCY-ε (lycopene ε-cyclase) and LCY-β (lycopene β-cyclase). Most members of the LCY family contain photoresponsive elements and abscisic acid elements. qRT-PCR demonstrates showed that most genes responded positively to drought stress, and GhLCYε-3 was expressed significantly differently under drought stress. Virus-induced gene silencing (VIGS) assay showed that the content of GhLCYε-3 was significantly increased with MDA and PRO, and the contents of chlorophyll and lutein were significantly decreased in pYL156 plants. The decrease in GhLCYε-3 expression is speculated to lead to reduced lutein content in vivo, resulting in the accumulation of reactive oxygen species (ROS) and decreased drought tolerance. This research enriched the understanding of LCY gene family and lutein function, and provided a new reference for cotton planting in arid areas.

Synopsis

Lycopene cyclase plays an important role in enhancing the ability of scavenging ROS and drought resistance of plants.

Evolving perspectives on lutein production from microalgae - A focus on productivity and heterotrophic culture

Increased consumer awareness for healthier and more sustainable products has driven the search for naturally sourced compounds as substitutes for chemically synthesized counterparts. Research on pigments of natural origin, such as carotenoids, particularly lutein, has been increasing for over three decades. Lutein is recognized for its antioxidant and photoprotective activity. Its ability to cross the blood-brain barrier allows it to act at the eye and brain level and has been linked to benefits for vision, cognitive function and other conditions. While marigold flower is positioned as the only crop from which lutein is extracted from and commercialized, microalgae are proposed as an alternative with several advantages over this terrestrial crop. The main barrier to scaling up lutein production from microalgae to the commercial level is the low productivity compared to the high costs. This review explores strategies to enhance lutein production in microalgae by emphasizing the overall productivity over lutein content alone. Evaluation of how culture parameters, such as light quality, nitrogen sufficiency, temperature and even stress factors, affect lutein content and biomass development in batch phototrophic cultures was performed. Overall, the total lutein production remains low under this metabolic regime due to the low biomass productivity of photosynthetic batch cultures. For this reason, we describe findings on microalgal cultures grown under different metabolic regimes and culture protocols (fed-batch, pulse-feed, semi-batch, semi-continuous, continuous). After a careful literature examination, two-step heterotrophic or mixotrophic cultivation strategies are suggested to surpass the lutein productivity achieved in single-step photosynthetic cultures. Furthermore, this review highlights the urgent need to develop technical feasibility studies at a pilot scale for these cultivation strategies, which will strengthen the necessary techno-economic analyses to drive their commercial production.

A Holistic Approach to Circular Bioeconomy Through the Sustainable Utilization of Microalgal Biomass for Biofuel and Other Value-Added Products

Emissions from transportation and industry primarily cause global warming, leading to floods, glacier melt, and rising seas. Widespread greenhouse gas emissions and resulting global warming pose significant risks to the environment, economy, and society. The need for alternative fuels drives the development of third-generation feedstocks: microalgae, seaweed, and cyanobacteria. These microalgae offer traits like rapid growth, high lipid content, non-competition with human food, and growth on non-arable land using brackish or waste water, making them promising for biofuel. These unique phototrophic organisms use sunlight, water, and carbon dioxide (CO2) to produce biofuels, biochemicals, and more. This review delves into the realm of microalgal biofuels, exploring contemporary methodologies employed for lipid extraction, significant value-added products, and the challenges inherent in their commercial-scale production. While the cost of microalgae bioproducts remains high, utilizing wastewater nutrients for cultivation could substantially cut production costs. Furthermore, this review summarizes the significance of biocircular economy approaches, which encompass the utilization of microalgal biomass as a feed supplement and biofertilizer, and biosorption of heavy metals and dyes. Besides, the discussion extends to the in-depth analysis and future prospects on the commercial potential of biofuel within the context of sustainable development. An economically efficient microalgae biorefinery should prioritize affordable nutrient inputs, efficient harvesting techniques, and the generation of valuable by-products.

Enhanced Carotenoid Production in Chlamydomonas reinhardtii by Overexpression of Endogenousand Exogenous Beta-Carotene Ketolase (BKT) Genes

Chlamydomonas reinhardtii is a unicellular green alga that can grow heterotrophically by using acetate as a carbon source. Carotenoids are natural pigments with biological activity and color, which have functions such as antioxidant, anti-inflammatory, vision protection, etc., and have high commercial value and prospects. We transformed Chlamydomonas reinhardtii with the BKT genes from Phaffia rhodozyma (PrBKT) and Chlamydomonas reinhardtii (CrBKT) via plasmid vector, and screened out the stable transformed algal strains C18 and P1. Under the condition that the cell density of growth was not affected, the total carotenoid content of C18 and P1 was 2.13-fold and 2.20-fold higher than that of the WT, respectively. CrBKT increased the levels of β-carotene and astaxanthin by 1.84-fold and 1.21-fold, respectively, while PrBKT increased them by 1.11-fold and 1.27-fold, respectively. Transcriptome and metabolome analysis of C18 and P1 showed that the overexpression of CrBKT only up-regulated the transcription level of BKT and LCYE (the gene of lycopene e-cyclase). However, in P1, overexpression of PrBKT also led to the up-regulation of ZDS (the gene of ζ-carotene desaturase) and CHYB (the gene of β-carotene hydroxylase). Metabolome results showed that the relative content of canthaxanthin, an intermediate metabolite of astaxanthin synthesis in C18 and P1, decreased. The overall results indicate that there is a structural difference between CrBKT and PrBKT, and overexpression of PrBKT in Chlamydomonas reinhardtii seems to cause more genes in carotenoid pathway metabolism to be up-regulated.

Mitochondria and chloroplasts function in microalgae energy production

Microalgae are organisms that have the ability to perform photosynthesis, capturing CO₂ from the atmosphere to produce different metabolites such as vitamins, sugars, lipids, among others, many of them with different biotechnological applications. Recently, these microorganisms have been widely studied due to their possible use to obtain clean energy. It has been postulated that the growth of microalgae and the production of high-energy metabolites depend on the correct function of cellular organelles such as mitochondria and chloroplasts. Thus, the development of different genetic tools to improve the function of these organelles is of high scientific and technological interest. In this paper we review the recent advances in microalgae engineering and the role of cellular organelles in order to increase cell productivity and biomass.

Target of Rapamycin Regulates Photosynthesis and Cell Growth in Auxenochlorella pyrenoidosa

Auxenochlorella pyrenoidosa is an efficient photosynthetic microalga with autotrophic growth and reproduction, which has the advantages of rich nutrition and high protein content. Target of rapamycin (TOR) is a conserved protein kinase in eukaryotes both structurally and functionally, but little is known about the TOR signalling in Auxenochlorella pyrenoidosa. Here, we found a conserved ApTOR protein in Auxenochlorella pyrenoidosa, and the key components of TOR complex 1 (TORC1) were present, while the components RICTOR and SIN1 of the TORC2 were absent in Auxenochlorella pyrenoidosa. Drug sensitivity experiments showed that AZD8055 could effectively inhibit the growth of Auxenochlorella pyrenoidosa, whereas rapamycin, Torin1 and KU0063794 had no obvious effect on the growth of Auxenochlorella pyrenoidosaa. Transcriptome data results indicated that Auxenochlorella pyrenoidosa TOR (ApTOR) regulates various intracellular metabolism and signaling pathways in Auxenochlorella pyrenoidosa. Most genes related to chloroplast development and photosynthesis were significantly down-regulated under ApTOR inhibition by AZD8055. In addition, ApTOR was involved in regulating protein synthesis and catabolism by multiple metabolic pathways in Auxenochlorella pyrenoidosa. Importantly, the inhibition of ApTOR by AZD8055 disrupted the normal carbon and nitrogen metabolism, protein and fatty acid metabolism, and TCA cycle of Auxenochlorella pyrenoidosa cells, thus inhibiting the growth of Auxenochlorella pyrenoidosa. These RNA-seq results indicated that ApTOR plays important roles in photosynthesis, intracellular metabolism and cell growth, and provided some insights into the function of ApTOR in Auxenochlorella pyrenoidosa.

Sustainable lipid and lutein production from Chlorella mixotrophic fermentation by food waste hydrolysate

Bioconversion of food waste into value-added products is a promising way to tackle the global food waste management problem. In this study, a novel valorisation strategy for bioenergy and lutein production via microalgal fermentation was investigated. Significant amount of glucose was recovered from enzymatic hydrolysis of food waste. The resultant hydrolysate was then utilised as culture medium in mixotrophic cultivation of Chlorella sp. to obtain high levels of lipid and lutein, whose accumulation patterns were consistent with molecular analyses. The resultant algal lipid derived from microalgal biomass using food hydrolysate was at high quality in terms of biodiesel properties. Further, in semi-continuous fermentation, the average algal biomass was 6.1 g L^-1 with 2.5 g L^-1 lipid and 38.5 mg L^-1 lutein using hydrolysate with an initial glucose concentration of 10 g L^-1. Meanwhile, the resultant algal biomass was 6.9 g L^-1 with 1.8 g L^-1 lipid and 63.0 mg L^-1 lutein using hydrolysate with an initial glucose concentration of 20 g L^-1, which suggests food waste hydrolysate could trigger algal products preferences. The experimental results of this study suggested the potential of microalgae as a platform for bioconversion of food waste into high-value products, especially sustainable bioenergy.

Bioengineering of Microalgae: Recent Advances, Perspectives, and Regulatory Challenges for Industrial Application

Microalgae, due to their complex metabolic capacity, are being continuously explored for nutraceuticals, pharmaceuticals, and other industrially important bioactives. However, suboptimal yield and productivity of the bioactive of interest in local and robust wild-type strains are of perennial concerns for their industrial applications. To overcome such limitations, strain improvement through genetic engineering could play a decisive role. Though the advanced tools for genetic engineering have emerged at a greater pace, they still remain underused for microalgae as compared to other microorganisms. Pertaining to this, we reviewed the progress made so far in the development of molecular tools and techniques, and their deployment for microalgae strain improvement through genetic engineering. The recent availability of genome sequences and other omics datasets form diverse microalgae species have remarkable potential to guide strategic momentum in microalgae strain improvement program. This review focuses on the recent and significant improvements in the omics resources, mutant libraries, and high throughput screening methodologies helpful to augment research in the model and non-model microalgae. Authors have also summarized the case studies on genetically engineered microalgae and highlight the opportunities and challenges that are emerging from the current progress in the application of genome-editing to facilitate microalgal strain improvement. Toward the end, the regulatory and biosafety issues in the use of genetically engineered microalgae in commercial applications are described.

Distinct roles of alternative oxidase pathway during the greening process of etiolated algae

The vital function of mitochondrial alternative oxidase (AOX) pathway in optimizing photosynthesis during plant de-etiolation has been well recognized. However, whether and how AOX impacts the chloroplast biogenesis in algal cells remains unclear. In the present study, the role of AOX in regulating the reassembly of chloroplast in algal cells was investigated by treating Auxenochlorella protothecoides with salicylhydroxamic acid (SHAM), the specific inhibitor to AOX, in the heterotrophy to autotrophy transition process. Several lines of evidences including delayed chlorophyll accumulation, lagged reorganization of chloroplast structure, altered PSI/PSII stoichiometry, and declined photosynthetic activities in SHAM treated cells indicated that the impairment in AOX activity dramatically hindered the development of functioning chloroplast in algal cells. Besides, the cellular ROS levels and activities of antioxidant enzymes were increased by SHAM treatment, and the perturbation on the balance of NAD⁺/NADH and NADP⁺/NADPH ratios was also observed in A. protothecoides lacking AOX activity, indicating that AOX was essential in promoting ROS scavenging and keeping the redox homeostasis for algal chloroplast development during greening. Overall, our study revealed the essentiality of mitochondrial AOX pathway in sustaining algal photosynthetic performance and provided novel insights into the physiological roles of AOX on the biogenesis of photosynthetic organelle in algae.