Roles of illumination on distribution of phosphorus in Chlorella vulgaris under mixotrophic cultivation

Self-assembly of algal-bacterial granules induced by bacterial N-acyl-homoserine lactone variation in response to high-strength pyridine

Algal-bacterial granules (ABGs) system represents a promising technology for organic wastewater treatment due to its high settleability, efficient oxygen transfer, and low-energy consumption. However, the secretion of extracellular polymeric substances (EPS) in algae, which played a key role in self-assembly of ABGs, would be inhibited by concentrated organic wastewater. This study proposed a novel strategy for developing ABGs by inducing bacterial N-acyl-homoserine lactone (AHL) variation through high-strength pyridine application. Results showed that bacterial long-chain AHL concentrations significantly increased in response to high-strength pyridine at 550 mg L-1, inducing the secretion of algal extracellular aromatic proteins and facilitating ABGs construction. The ABGs system achieved over 99 % pyridine removal efficiency and 82 % settleability. Moreover, the proportions of β-sheet and α-helix structures in the extracellular aromatic proteins of ABGs increased, while the random coil structures decreased. This shift in protein structure lowered the surface free energy and energy barriers, which in turn enhanced the surface hydrophobicity and promoted cell adhesion. Furthermore, based on metatranscriptomic analysis, the mechanism for AHL-regulated physiological and behavioral responses between algae and bacteria in ABGs was proposed. This study provides an economically feasible approach to develop efficient and sustainable ABGs systems for industrial wastewater treatment.

Intensified performance and mechanism of nitrogen removal in constructed wetland incorporating algal pond for treating low carbon nitrogen ratio wastewater

Insufficient carbon sources limit efficient total nitrogen (TN) removal in traditional constructed wetlands (CWs) especially for low C/N ratio wastewaters. This study applied an algal pond combined with CW (AP-CW) to amplify nitrogen removal from such wastewaters. Results demonstrated that both AP-CW and CW effectively removed chemical oxygen demand (COD, 84-87 %) and ammonia nitrogen (NH4+-N, over 98 %). Furthermore, AP-CW improved TN removal efficiency by 12 % compared to CW. Algal cells achieved removal efficiencies of 74 % for COD and 31 % for NH4+-N, while their biomass reached 605.2 cells mL-1 under low C/N conditions. The addition of algae enhanced microbial diversity and richness in the wetland, particularly in the upper substrate layer. Denitrifying bacteria positively correlated with nitrate through Mantel analysis, and nitrate reductase activity was increased, promoting denitrification. These findings would provide insights into the mechanism of AP-CW technology for low C/N wastewater treatment and offer guidance for practical applications.

Effects of nitrogen phosphorus ratio and light on phosphorus removal by microalgae in high-phosphorus wastewater

The removal of phosphorus from wastewater has consistently posed a major focus in the field of wastewater treatment. Microalgae-based phosphorus removal is widely acknowledged as an effective biological approach. However, ensuring the microalgae-mediated high phosphorus concentration removal remains a persistent challenge. In this study, a kind of multicellular microalgae, Klebsormidium sp., was used to explore its ability to remove phosphorus in high-phosphorus wastewater. The phosphorus removal rate by Klebsormidium sp. in highly concentrated (>20 mgP/L) wastewater can exceed 90%. To investigate the phosphorus absorption process, various nitrogen and phosphorus concentrations along with light conditions were employed. The results showed that 50% to 80% of the total phosphorus absorbed by microalgae entered the intracellular polymer. The phosphorus concentration and light intensity did not exert any significant effects on the absorption of phosphorus by microalgae. However, the nitrogen concentration and the light-to-dark ratio significantly influenced the storage of phosphorus by microalgae. At a nitrogen concentration over 300 mgN/L, phosphorus absorption by microalgae was inhibited. A higher light-to-dark ratio increased phosphorus transfer by microalgae, while the light duration exceeds 16 h inhibited it. Microalgae have emerged as promising materials for phosphorus removal in high-phosphorus sewage, the study offering potential solutions for a cleaner and more sustainable future.

Microalgal extracellular polymeric substances (EPS) and their roles in cultivation, biomass harvesting, and bioproducts extraction

Microalgae extracellular polymeric substances (EPS) are complex high-molecular-weight polymers and the physicochemical properties of EPS strongly affect the core features of microalgae cultivation and resource utilization. Revealing the key roles of EPS in microalgae life-cycle processes in an interesting and novelty topic to achieve energy-efficient practical application of microalgae. This review found that EPS showed positive effect in non-gas uptake, extracellular electron transfer, toxicity resistance and heterotrophic symbiosis, but negative impact in gas transfer and light utilization during microalgae cultivation. For biomass harvesting, EPS favored biomass flocculation and large-size cell self-flocculation, but unfavored small size microalgae self-flocculation, membrane filtration, charge neutralization and biomass dewatering. During bioproducts extraction, EPS exhibited positive impact in extractant uptake, but the opposite effect in cellular membrane permeability and cell rupture. Future research on microalgal EPS were also identified, which offer suggestions for comprehensive understanding of microalgal EPS roles in various scenarios.

Dialysis bag-microalgae photobioreactor: Novel strategy for enhanced bioresource production and wastewater purification

Cultivating microalgae in wastewater offers various advantages, but it still faces limitations such as bacteria and other impurities in wastewater affecting the growth and purity of microalgae, difficulty in microalgae harvesting, and extracellular products of microalgae affecting effluent quality. In this study, a novel dialysis bag-microalgae photobioreactor (Db-PBR) was developed to achieve wastewater purification and purer bioresource recovery by culturing microalgae in a dialysis bag. The dialysis bag in the Db-PBR effectively captured the microalgae cells and promoted their lipid accumulation, leading to higher biomass (1.53 times of the control) and lipid production (2.50 times of the control). During the stable operation stage of Db-PBR, the average soluble microbial products (SMP) content outside the dialysis bag was 25.83 mg L-1, which was significantly lower than that inside the dialysis bag (185.63 mg L-1), indicating that the dialysis bag effectively intercepted the SMP secreted by microalgae. As a result, the concentration of dissolved organic carbon (DOC) in Db-PBR effluent was significantly lower than that of traditional photobioreactor. Furthermore, benefiting from the dialysis bag in the reactor effectively intercepted the microorganisms in wastewater, significantly improving the purity of the cultured microalgae biomass, which is beneficial for the development of high-value microalgae products.

Effects of disturbance modes and carbon sources on the physiological traits and nutrient removal performance of microalgae (S. obliquus) for treating low C/N ratio wastewater

Wastewater treatment with microalgae is an ecologically sustainable process. In this study, the growth characteristics, nutrient removal, and spectral changes of dissolved organic matter (DOM) in microalgae bioreactors were investigated for treating low C/N ratio wastewater under different disturbance modes (agitation and aeration) and carbon sources (sucrose and humic acid). The results showed that the biomass and chlorophyll-a contents of Scenedesmus obliquus in the aeration condition (725.32-811.16 × 104 cells mL-1, 1.58-1.69 mg L-1) were higher than those in the agitation condition (426.06-465.14 × 104 cells mL-1, 1.48-1.61 mg L-1). The better removal of nutrients (TN, 29.62-36.39 mg L-1, TP, 1.84-2.30 mg L-1) by microalgae in sucrose-containing wastewater under agitation conditions occurred on the second day, with removal efficiencies of 21.33-30.67% and 44.84-58.51%, respectively; while it was on the fifth day both in sucrose and humic acid-containing wastewater under aeration conditions (TN, 19.56-31.20 mg L-1, TP, 0.26-0.30 mg L-1), with removal efficiencies of 13.92-46.75% and 88.36-90.50%, respectively. The wastewater DOM primarily consisted of humic-like substances under agitation and aeration conditions characterized by high levels of aromaticity, molecular weight and humification. Furthermore, the aromatization and humification properties of DOM in humic acid wastewater were higher than those in sucrose wastewater, which was corresponding with the lower removal and availability of pollutants by algae. Microalgae showed good biomass accumulation and nutrients removal at incubation time of 2 days (agitation condition) and 5 days (aeration condition), respectively. Consequently, a technical reference is provided for the microalgae coupled with other treatment processes.

Unexpected growth promotion of Chlorella sacchrarophila triggered by herbicides DCMU

The ecotoxicological effects of herbicide contamination on the autotrophic growth of microalgae in aquatic environments have been major concerns. However, little is known about the influence of herbicides on the mixotrophic growth of microalgae. This study investigated the ecotoxicological effect of 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU) on the mixotrophic growth of Chlorella sacchrarophila FACHB 4. Results showed that C. sacchrarophila in mixotrophy was more resistant to DCMU than in photoautotrophy. Moreover, a low content of DCMU (20-80 μg·L^-1) promoted the mixotrophic growth of C. sacchrarophila, and the promotion effect was obviously enhanced with the increase in light intensity. The chlorophyll content and glucose absorption rate of C. sacchrarophila were found to increase after incubation with DCMU for 24 h. Transcriptome analyses revealed that the mechanism of DCMU to promote the mixotrophic growth of C. sacchrarophila was probably through accelerating glucose uptake and utilization, which was accomplished by reducing photodamage and increasing the chlorophyll content of C. sacchrarophila. This study not only revealed an unexpected bloom of mixotrophic microalgae triggered by herbicides, but it also shed new light on an effective and low-cost strategy to improve the microalgae productivity for utilization.