Heterotrophic cultivation of Chlorella vulgaris using broken rice hydrolysate as carbon source for biomass and pigment production

Towards a circular economy - Repurposing side streams from the potato processing industry by Chlorella vulgaris

Common wastewater treatment strategies in the food industry do not include efficient remediation strategies for nitrogen, phosphorous and organic carbon. Incorporating microalgae in water treatment plants is rising in popularity because of their high nutrient and trace element uptake driven by light. In this study, four different side streams from an Austrian potato processing company have been screened for their applicability of microalgal cultivation. The side streams were assessed for Chlorella vulgaris growth and their requirement of any additional pretreatment or media supplementation. One side stream specifically, called blanching water II, a stream generated by boiling the potatoes for ease of peeling, turned out very useful to cultivate Chlorella vulgaris and concomitantly remedy the wastewater. Compared to a state-of-the-art cultivation in BG11, cultivating Chlorella vulgaris in blanching water II led to a 45 % increase in specific growth rate of 1.29 day-1 and a 48% increase in biomass productivity to 294.6 mg/L/day, while all nitrogen and phosphate present in the side stream were metabolized. Overall, the results demonstrate that the water remediation process for blanching water II shows vast potential in regard to water purification and waste to value approaches.

Heterotrophic Selenium Incorporation into Chlorella vulgaris K-01: Selenium Tolerance, Assimilation, and Removal through Microalgal Cells

Chlorella has been applied in the production of selenium (Se) enriched organic biomass. However, limited information exists regarding heterotrophic selenium tolerance and its incorporation into Chlorella. This study aimed to investigate the potential of using Chlorella vulgaris K-01 for selenium biotransformation. To assess the dose-response effect of Se stress on the strain, time-series growth curves were recorded, growth productivity parameters were calculated, and Gaussian process (GP) regression analysis was performed. The strain's carbon and energy metabolism were evaluated by measuring residual glucose in the medium. Characterization of different forms of intracellular Se and residual Se in the medium was conducted using inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometer (ICP-OES). The EC50 value for the strain in response to Se stress was 38.08 mg/L. The maximum biomass productivity was 0.26 g/L/d. GP regression analysis revealed that low-level Se treatment could increase the biomass accumulation and the carrying capacity of Chlorella vulgaris K-01 in a heterotrophic culture. The maximum organic Se in biomass was 154.00 μg/g DW. These findings lay the groundwork for understanding heterotrophic microalgal production of Se-containing nutraceuticals, offering valuable insights into Se tolerance, growth dynamics, and metabolic responses in Chlorella vulgaris K-01.

Unlocking microalgal host-exploring dark-growing microalgae transformation for sustainable high-value phytochemical production

Microalgae have emerged as a promising, next-generation sustainable resource with versatile applications, particularly as expression platforms and green cell factories. They possess the ability to overcome the limitations of terrestrial plants, such non-arable land, water scarcity, time-intensive growth, and seasonal changes. However, the heterologous expression of interested genes in microalgae under heterotrophic cultivation (dark mode) remains a niche area within the field of engineering technologies. In this study, the green microalga, Chlorella sorokiniana AARL G015 was chosen as a potential candidate due to its remarkable capacity for rapid growth in complete darkness, its ability to utilize diverse carbon sources, and its potential for wastewater treatment in a circular bioeconomy model. The aims of this study were to advance microalgal genetic engineering via dark cultivation, thereby positioning the strain as promising dark-host for expressing heterologous genes to produce high-value phytochemicals and ingredients for food and feed. To facilitate comprehensive screening based on resistance, eleven common antibiotics were tested under heterotrophic condition. As the most effective selectable markers for this strain, G418, hygromycin, and streptomycin exhibited growth inhibition rates of 98%, 93%, and 92%, respectively, ensuring robust long-term transgenic growth. Successful transformation was achieved through microalgal cell cocultivation with Agrobacterium under complete darkness verified through the expression of green fluorescence protein and β-glucuronidase. In summary, this study pioneers an alternative dark-host microalgal platform, using, Chlorella, under dark mode, presenting an easy protocol for heterologous gene transformation for microalgal host, devoid of the need for expensive equipment and light for industrial production. Furthermore, the developed genetic transformation methodology presents a sustainable way for production of high-value nutrients, dietary supplements, nutraceuticals, proteins and pharmaceuticals using heterotrophic microalgae as an innovative host system.

Biotechnological Approaches for Biomass and Lipid Production Using Microalgae Chlorella and Its Future Perspectives

Heavy reliance on fossil fuels has been associated with increased climate disasters. As an alternative, microalgae have been proposed as an effective agent for biomass production. Several advantages of microalgae include faster growth, usage of non-arable land, recovery of nutrients from wastewater, efficient CO₂ capture, and high amount of biomolecules that are valuable for humans. Microalgae Chlorella spp. are a large group of eukaryotic, photosynthetic, unicellular microorganisms with high adaptability to environmental variations. Over the past decades, Chlorella has been used for the large-scale production of biomass. In addition, Chlorella has been actively used in various food industries for improving human health because of its antioxidant, antidiabetic, and immunomodulatory functions. However, the major restrictions in microalgal biofuel technology are the cost-consuming cultivation, processing, and lipid extraction processes. Therefore, various trials have been performed to enhance the biomass productivity and the lipid contents of Chlorella cells. This study provides a comprehensive review of lipid enhancement strategies mainly published in the last five years and aimed at regulating carbon sources, nutrients, stresses, and expression of exogenous genes to improve biomass production and lipid synthesis.

Effects of C/N ratio on the growth and protein accumulation of heterotrophic Chlorella in broken rice hydrolysate

BACKGROUND

Microalgae protein is considered as a sustainable alternative to animal protein in the future. Using waste for microalgal culture can upgrade low-value raw materials into high-value products, helping to offset the cost of microalgal protein production. In this study we explored the feasibility of using microalgae heterotrophic fermentation to convert broken rice hydrolysate (BRH) into protein.

RESULTS

The results showed that the increase of BRH supplemental ratio was beneficial to the increase of biomass production but not beneficial to the increase of intracellular protein content. To further improve protein production, the effect of C/N ratio on intracellular protein accumulation was studied. It was found that low C/N ratio was beneficial to the synthesis of glutamate in microalgae cells, which in turn promoted the anabolism of other amino acids and further the protein. When the C/N ratio was 12:1, the biomass productivity and protein content could reach a higher level, which were 0.90 g/L/day and 61.56%, respectively. The obtained Chlorella vulgaris biomass was rich in essential amino acids (41.80%), the essential amino acid index was as high as 89.07, and the lysine content could reach up to 4.05 g/100 g.

CONCLUSIONS

This study provides a theoretical basis and guidance for using Chlorella vulgaris as an industrial fermentation platform to convert broken rice into products with high nutritional value.

Enhancement of Carbon Conversion and Value-Added Compound Production in Heterotrophic Chlorella vulgaris Using Sweet Sorghum Extract

High-cost carbon sources are not economical or sustainable for the heterotrophic culture of Chlorella vulgaris. In order to reduce the cost, this study used sweet sorghum extract (SE) and its enzymatic hydrolysate (HSE) as alternative carbon sources for the heterotrophic culture of Chlorella vulgaris. Under the premise of the same total carbon concentration, the value-added product production performance of Chlorella vulgaris cultured in HSE (supplemented with nitrogen sources and minerals) was much better than that in the glucose medium. The conversion rate of the total organic carbon and the utilization rate of the total nitrogen were both improved in the HSE system. The biomass production and productivity using HSE reached 2.51 g/L and 0.42 g/L/d, respectively. The production of proteins and lipids using HSE reached 1.17 and 0.35 g/L, respectively, and the production of chlorophyll-a, carotenoid, and lutein using HSE reached 30.42, 10.99, and 0.88 mg/L, respectively. The medium cost using HSE decreased by 69.61% compared to glucose. This study proves the feasibility and practicability of using HSE as a carbon source for the low-cost heterotrophic culture of Chlorella vulgaris.

Effect of macronutrients and of anaerobic digestate on the heterotrophic cultivation of Chlorella vulgaris grown with glycerol

The aim of this work was to investigate the kinetics of the heterotrophic growth of Chlorella vulgaris as a means of producing bio-oil for biodiesel production. Glycerol was used as the sole organic carbon substrate. Anaerobic digestate from a local plant was used to examine its effect on the kinetics and the protein and lipid content of the biomass. The effect of the initial carbon and nitrogen concentrations on the carbon uptake rate was studied independently. In the one set of five experiments, the organic carbon in the form of glycerol varied from 0.27 to 5.36 g L^-1, while the concentration of atomic nitrogen was held constant and equal to 45.4 mg L^-1. The Co/No ratio varied from 6 to 118.1. In the second set, also of five experiments, the organic carbon was held constant and equal to 3.3 g L^-1 and atomic nitrogen varied from 22.7 to 450 mg L^-1. The Co/No ratio varied from 7.3 to 145.4. In the third set of experiments, anaerobic digestate was added in increasing amounts into the culture media from 4 to 16%. It was found that the carbon uptake rate as well as the lipid and protein content depended on the Co/No ratio. Increasing ratios of Co/No led to higher carbon uptake rates, higher lipid content, and lower protein content. The initial nitrogen concentration was also found to affect the growth rate of C. vulgaris. The addition of anaerobic digestate did not affect appreciably the protein and lipid content of the biomass, while the addition of anaerobic digestate up to 16% in the culture medium increased the carbon uptake rate by about 24%.

Phosphorus uptake, distribution and transformation with Chlorella vulgaris under different trophic modes

Phosphorus (P) uptake, distribution and transformation are important processes associated with the growth and metabolism of microalgae. In this study, the fate of P in soluble microbial products (SMP), extracellular polymeric substances (EPS), and intracellular polymeric substances (IPS) of Chlorella vulgaris C7 in the form of inorganic P (IP) and organic P (OP) was studied under different trophic modes, including photoautotrophy, heterotrophy and mixotrophy. The results showed that mixotrophic cultivation of microalgae brought highest biomass yield, which was 2.09 times and 11.10 times higher than that of the photoautotrophic and heterotrophic conditions. Regarding P distribution and transformation, the trophic modes affected the transformation trends and rates of P in the form of IP and OP among SMP, EPS and IPS. Under photoautotrophic condition, EPS was the main P pool, and most of P in C. vulgaris was IP. While, under mixotrophic condition, IPS was the main P pool, and most of P was transformed to OP. The addition of glucose promoted the uptake of P by algal cells and the transformation of IP to OP, which accounted for 79.32% of total phosphorus at the end of cultivation. Collectively, mixotrophic cultivation was regarded as the optimum approach for microalgae cultivation and nutrient recovery.