Mixotrophy, a more promising culture mode: Multi-faceted elaboration of carbon and energy metabolism mechanisms to optimize microalgae culture

Principles, challenges, and optimization of indigenous microalgae-bacteria consortium for sustainable swine wastewater treatment

Indigenous microalgae-bacteria consortium (IMBC) offers significant advantages for swine wastewater (SW) treatment including enhanced adaptability and resource recovery. In this review, the approaches for enriching IMBC both in situ and ex situ were comprehensively described, followed by symbiotic mechanisms for IMBC which involve metabolic cross-feeding and signal transmission. Strategies for enhancing treatment efficiencies of SW-originated IMBC were then introduced, including improving SW quality, optimizing system operating conditions, and adjusting microbial activities. Recommendations for maximizing treatment efficiencies were particularly proposed using a decision tree approach. Moreover, removal/recovery mechanisms for typical pollutants in SW using IMBC were critically discussed. Ultimately, a technical route termed SW-IMBC-Crop-Pig was proposed, to achieve a closed-loop economy for pig farms by integrating SW treatment with crop cultivation. This review provides a deeper understanding of the mechanism and strategies for IMBC's resource recovery from SW.

A Response Surface Methodology Study for Chlorella vulgaris Mixotrophic Culture Optimization

Glycerol is a carbon source that produces good biomass under mixotrophic conditions. Enhancing the composition of culture media in algae biomass production improves growth rates, biomass yield, nutrient utilization efficiency, and overall cost-effectiveness. Among the key nutrients in the medium, nitrogen plays a pivotal role. Urea can be effectively used as a nitrogen source and is considered a low-cost form of nitrogen compared to other sources. Urea metabolism releases some CO2 in photosynthesis, and magnesium plays a major role in urea uptake. Magnesium is another key nutrient that is key in photosynthesis and other metabolic reactions. To maximize glycerol consumption in the mixotrophic system and to obtain high biomass and lipid productions, the variations in MgSO4·7H2O and urea concentrations were evaluated in the growth medium of the microalgae. A response surface methodology (RSM) using a central composite design (CCD) was designed to maximize glycerol consumption at the initial cellular growth rates (up to four days). The magnesium and urea supply varied from 0.3 to 1.7 g L-1. Response surface methodology was utilized to analyze the results, and the highest glycerol consumption rate, 770.2 mg L-1 d-1, was observed when C. vulgaris was grown at 1.7 g L-1 urea, 1.0 g L-1 MgSO4·7H2O. Using the optimal urea and magnesium concentrations with acetate, glucose, and glycerol as carbon sources, the same lipid content (10% average) was achieved on day 4 of mixotrophic C. vulgaris culture. Overall, the results show that mixotrophic growth of C. vulgaris using urea with an optimum magnesium concentration yields large amounts of fatty acids and that the carbon source greatly influences the profile of the fatty acids.

Mechanism of enhanced microalgal biomass and lipid accumulation through symbiosis between a highly succinic acid-producing strain of Escherichia coli SUC and Aurantiochytrium sp. SW1

Microalgae, known for rapid growth and lipid richness, hold potential in biofuels and high-value biomolecules. The symbiotic link with bacteria is crucial in large-scale open cultures. This study explores algal-bacterial interactions using a symbiotic model, evaluating acid-resistant Lactic acid bacteria (LAB), stress-resilient Bacillus subtilis and Bacillus licheniformis, and various Escherichia coli strains in the Aurantiochytrium sp. SW1 system. It was observed that E. coli SUC significantly enhanced the growth and lipid production of Aurantiochytrium sp. SW1 by increasing enzyme activity (NAD-IDH, NAD-ME, G6PDH) while maintaining sustained succinic acid release. Optimal co-culture conditions included temperature 28 °C, a 1:10 algae-to-bacteria ratio, and pH 8. Under these conditions, Aurantiochytrium sp. SW1 biomass increased 3.17-fold to 27.83 g/L, and total lipid content increased 2.63-fold to 4.87 g/L. These findings have implications for more efficient microalgal lipid production and large-scale cultivation.

Microalgae cultivation with recycled harvesting water achieved economic and sustainable production of biomass and lipid: Feasibility assessment and inhibitory factors analysis

This study was conducted to achieve economic and sustainable production of biomass and lipids from Chlorella sorokiniana by recirculating cultivation with recycled harvesting water, to identify the major inhibitory factors in recirculating culture, and to analyze accordingly economic benefits. The results showed that recirculating microalgae cultivation (RMC) could obtain 0.20-0.32 g/L biomass and lipid content increased by 23.1 %-38.5 %. Correlation analysis showed that the extracellular polysaccharide (PSext), chemical oxygen demand (COD) and chromaticity of recirculating water inhibited photosynthesis and induced oxidative stress, thus inhibiting the growth of C. sorokiniana. In addition, the economic benefits analysis found that circulating the medium twice could save about 30 % of production cost, which is the most economical RMC solution. In conclusion, this study verified the feasibility and economy of RMC, and provided a better understanding of inhibitory factors identification in culture.

Microalgae-assisted heterotrophic nitrification-aerobic denitrification process for cost-effective nitrogen and phosphorus removal from high-salinity wastewater: Performance, mechanism, and bacterial community

A microalgae-assisted heterotrophic nitrification-aerobic denitrification (HNAD) system for efficient nutrient removal from high-salinity wastewater was constructed for the first time as a cost-effective process in the present study. Excellent nutrient removal (∼100.0 %) was achieved through the symbiotic system. The biological removal process, biologically induced phosphate precipitation (BIPP), microalgae uptake, and ammonia stripping worked together for nutrient removal. Furthermore, the biological removal process achieved by biofilm contributed to approximately 55.3-71.8 % of nitrogen removal. BIPP undertook approximately 45.6-51.8 % of phosphorus removal. Batch activity tests confirmed that HNAD fulfilled an extremely critical role in nitrogen removal. Microalgal metabolism drove BIPP to achieve efficient phosphorus removal. Moreover, as the main HNAD bacteria, OLB13 and Thauera were enriched. The preliminary energy flow analysis demonstrated that the symbiotic system could achieve energy neutrality, theoretically. The findings provide novel insights into strategies of low-carbon and efficient nutrient removal from high-salinity wastewater.

Mixotrophic culture of Chaetoceros sp. and the synergistic carbon and energy metabolism

This research studied the metabolic mechanism of the mixotrophic Chaetoceros sp. The results showed this alga had the highest cell density and growth rate of 47.72 × 105 cells mL-1 and 0.41 d-1, respectively, with a maximum dry weight of 2.90 g/L, when compared to photoautotrophic and photoheterotrophic modes. Compared to photoheterotrophy, transcriptomics results showed the Rubisco, PGK, and GAPDH related genes were separately up-regulated by 1.03, 2.36, and 1.36 times in CBB cycle in mixotrophic mode, suggesting intermediate metabolites of EMP and PPP can enter the chloroplast via transporter proteins, or membrane permeation, and feedback inhibition regulates the reduction of multiple reactions in CBB cycle. Chaetoceros sp. achieves high biomass by utilizing ATP and carbon structures from EMP and PPP pathways, and the addition of NaHCO3 leads to an up-regulation of CBB cycle for the mixotrophic alga, resulting in higher biomass compared to the photoheterotrophic mode.