Nitrogen-dependent metabolic regulation of lipid production in microalga Scenedesmus vacuolatus

Simultaneously enhancement in the assimilation of microalgal nitrogen and the accumulation of carbohydrate by Debaryomyces hansenii

Microalgae-based techniques are considered an alternative to traditional activated sludge processes for removing nitrogen from wastewater. Bacteria consortia have been broadly conducted as one of the most important partners. However, fungal effects on the removal of nutrients and changes in physiological properties of microalgae, and their impact mechanisms remain unclear. The current work demonstrates that, adding fungi increased the nitrogen assimilation of microalgae and the generation of carbohydrates compared to pure microalgal cultivation. The NH₄⁺-N removal efficiency was 95.0% within 48 h using the microalgae-fungi system. At 48 h, total sugars (glucose, xylose, and arabinose) accounted for 24.2 ± 4.2% per dry weight in the microalgae-fungi group. Gene ontology (GO) enrichment analysis revealed that, among various processes, phosphorylation and carbohydrate metabolic processes were more prominent. Gene encoding the key enzymes of glycolysis, pyruvate kinase, and phosphofructokinase were significantly up-regulated. Overall, for the first time, this study provides new insights into the art of microalgae-fungi consortia for producing value-added metabolites.

Co-Cultivation of Leptolyngbya tenuis (Cyanobacteria) and Chlorella ellipsoidea (Green alga) for Biodiesel Production, Carbon Sequestration, and Cadmium Accumulation

The co-cultivation approach using cyanobacteria-Leptolyngbya tenuis and green alga-Chlorella ellipsoidea demonstrated in the present study showed additive and synergistic effects on biomass yield, biomass productivity, lipid yield, lipid productivity, CO₂ fixation, and cadmium bioremediation efficiency. The results of co-culture in batch mode revealed about 2-3 times increase in biomass and two times increase in total lipid, when compared to the pure culture batches. The results revealed that co-cultures exhibited significantly high CO₂ fixation rate of 2.63  ±  0.09 g/L/d, which is 1.5-2 times better than monocultures (P < 0.05). To explore the bioaccumulation of cadmium by co-cultures and pure cultures, different concentrations of cadmium nitrate was used in flask trials. Cadmium accumulation was observed in the order: co-culture (74%, 0.37 mg/L) > Chlorella (58%, 0.29 mg/L) > Leptolyngbya (50%, 0.25 mg/L) (P < 0.05). In addition, fatty acid composition, CHNS analysis, biodiesel characterization, and biochemical compositions were also determined using co-culture method. The maximum biomass yield, productivity, lipid content, and CO₂ fixation rate in cadmium induced co-culture were 3.95  ±  0.13 g/L, 258.88  ±  15.75 mg/L/d, 41.43  ±  0.71%, and 3.21  ±  0.20 g/L/d, respectively which is 1.2, 1.3, 2.3, and 1.2 times higher than the control (P < 0.05). Cadmium induced changes in growth and lipid yield using co-culture suggests cost-effective and eco-friendly production of biodiesel and carbon mitigation.

Pretreatments for enhancing sewage sludge reduction and reuse in lipid production

BACKGROUND

Converting wastewater sludge to lipid is considered as one of the best strategies of sludge management. The current problem of lipid production from wastewater sludge is the low yield (0.10-0.16 g lipid/g dry sludge) due to the low availability of easily uptaken materials (such as soluble monosaccharide and oligosaccharide) in sludge to oleaginous microorganism (Rhodotorula glutinis, Trichosporon oleaginosus, Lipomyces starkeyi). Pretreatments are efficient methods to improve sludge bioavailability. This study is aimed to achieve high lipid production from sludge and high sludge reduction.

RESULTS

In this study, it was observed that the soluble chemical oxygen demand (SCOD) had significantly increased after different pretreatment. The SCOD in the supernatant was increased from 32.64 to 180.25 mg/L, 924.16 mg/L, 1029.89 mg/L and 3708.31 mg/L after acidic (pH 2 for 2 h), alkaline (pH 12 for 2 h), microwave irradiation (15 min with 5 min interval), and ultrasonication (30 min at 450 W and 20 kHz frequency with 5 s on and 2 s off mode) pretreatment, respectively. Pretreatments have also increased the release of total nitrogen (TN) and total phosphorus (TP) from solids. The sludge after different pretreatments were used as a medium for lipid production, and the highest lipid content (36.67% g/g) was obtained in the fermentation with ultrasonication pretreatment sludge, and the sludge reduction was 63.10%. For other pretreatments, the lipid content and sludge reduction were 18.42% and 32.63% in acid pretreatment case, 21.08% and 36.44% in alkaline pretreatment case, and 26.31% and 43.03% in microwave pretreatment case, respectively.

CONCLUSION

It was found that ultrasonication pretreatment was the most efficient way to increase the sludge biodegradability (SCOD) and to release TN and TP from solid phase to liquid phase. Pretreated sludge for lipid production achieved significant improvement in lipid yield and sludge reduction. Lipids produced from pretreated sludge were transesterified to biodiesel and the analysis showed that biodiesel had a similar composition as commercial biodiesel. The study reveals that pretreatment on sludge is a promising method for enhancing biological sludge management efficiency.

Characterization of Chlorella sorokiniana and Chlorella vulgaris fatty acid components under a wide range of light intensity and growth temperature for their use as biological resources

This study aims to characterize the potential of three strains of microalgal species (Chlorella sorokiniana KNUA114 and KNUA122; C. vulgaris KNUA104) for use as feedstock, based on their fatty acid compositions. Each strain was molecularly identified using four marker genes (ITS, SSU, rbcL, and tufA) and phylogenetically characterized. C. sorokiniana and C. vulgaris collected from Ulleung Island, South Korea, were homologous with other known species groups. Samples' fatty acid components were measured using GC/MS analysis in growth temperatures of 10 °C, 25 °C, and 35 °C. The growth rate of C. sorokiniana strains was higher than that of C. vulgaris under high-temperature conditions, confirming the potential industrial applicability of the former as feedstock material. Additionally, saturated fatty acid contents and productivities increased as biological resources of the C. sorokiniana strains were higher than those of C. vulgaris under high light intensity and temperature conditions. These results suggest that the fatty acid components of C. sorokiniana strains may potentially be used as biological resources (e.g., feedstock).

Resource recovery from wastewaters using microalgae-based approaches: A circular bioeconomy perspective

The basic concepts of circular bioeconomy are reduce, reuse and recycle. Recovery of recyclable nutrients from secondary sources could play a key role in meeting the increased demands of the growing population. Wastewaters of different origin are rich in energy and nutrients sources that can be recovered and reused in a circular bioeconomy perspective. Microalgae can effectively utilize wastewater nutrients for growth and biomass production. Integration of wastewater treatment and microalgal cultivation improves the environmental impacts of the currently used wastewater treatment methods. This review provides comprehensive information on the potential of using microalgae for the recovery of carbon, nitrogen, phosphorus and other micronutrients from wastewaters. Major factors influencing large scale microalgal wastewater treatment are discussed and future research perspectives are proposed to foster the future development in this area.