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The effects of phosphonates, the heavily-used antiscalants in reverse osmosis systems, on microalgae are controversial, although they are harmless to most aquatic organisms. Herein, we assessed the inhibitory effects of etidronic acid (HEDP) and diethylenetriamine penta(methylene phosphonic acid) (DTPMP) on algal growth and revealed the mechanisms involved in both intrinsic toxicity and complexation. The phosphonates showed weak influences on Scenedesmus sp. LX1 in the first 4 d of cultivation. In contrast, a significant growth inhibition was observed subsequently with half maximal effective concentrations of 57.6 and 35.7 mg/L for HEDP and DTPMP, respectively, at 10 d. The phosphonates had little effect on cellular energy transfer and oxidative stress, quantified by adenosine triphosphate level and superoxide dismutase activity, respectively, demonstrating weak intrinsic toxicities to algal cells. Phosphonates blocked the algal assimilation of iron ions through complexation. Severe iron deficiency limited photosynthetic activity and caused chlorophyll decline, resulting in a functional loss of the photosystem followed by complete algal growth inhibition at the late cultivation stage. Our findings point to a potential ecological impact wherein harmful algal blooms are induced by the natural degradation of phosphonates due to the release of both iron and phosphate ions that stimulate algal regrowth after disinhibition., Competing Interests: Declaration of Competing Interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the submitted work titled “Assessment and Mechanisms of Microalgae Growth Inhibition by Phosphonates: Effects of Intrinsic Toxicity and Complexation”., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

As 1,2-benzisothiazol-3(2H)-one (BIT) has been widely used in high concentrations for microbial growth control in many domestic and industrial processes, its potential eco-risk should be assessed. This study investigated the interaction between BIT and microalgae in aquatic environment as the mechanism of BIT lethal effect on microalgae was unclear and whether microalgae could efficiently remove BIT was unknown. It was found that Chlorella vulgaris could be killed by high concentrations of BIT, and this lethal effect was strongly enhanced when exposed to light. Inhibition of photosystem II electron transport followed by a decrease in cellular chlorophyll led to serious damage to algal photosynthesis. The excess accumulation of reactive oxygen species caused by the photosynthetic damage under light further increased the oxidative damage and promoted cell death. Under dark condition, however, the algae could tolerate higher BIT concentrations. BIT could be efficiently removed when the growth of Scenedesmus sp. LX1 was not completely inhibited. With an initial concentration of 4.5 mg/L, over 99% of BIT was removed during 168 hour cultivation. Microalgal biodegradation was the primary reason for this removal, and the contributions of BIT hydrolytic/photolytic degradation, microalgal growth, photosynthesis and sorption were negligibly small. These results pointed to the potential application of microalgae for efficient BIT removal from wastewater., (Copyright © 2019. Published by Elsevier B.V.)

Isothiazolinones, such as 1,2-benzisothiazol-3(2H)-one (BIT), are widely used as biocides for bacterial growth control in many domestic and industrial processes. Despite their advantages as biocides, they are highly toxic and pose a potential risk to the environment. This study investigated the inhibition process and detoxification mechanism involved in microalgal survival and growth recovery after BIT poisoning. BIT could seriously inhibit the growth of Scenedesmus sp. LX1, Chlorella sp. HQ, and Chlamydomonas reinhardtii with half maximal effective concentrations at 72 h (72h-EC 50 ) of 1.70, 0.41, and 1.16 mg/L, respectively. The primary inhibition mechanism was the BIT-induced damage to microalgal photosynthetic systems. However, the inhibited strains could recover when their growth was not completely inhibited. The influence of this inhibiting effect on subsequent algal regrowth was negligible or weak. BIT consumption was the primary reason for their recovery. Notably, algae did not die even if their growth was completely inhibited. If the BIT concentration did not exceed a certain high level, then the inhibited algae could recover their growth relatively well. Microalgal generation of reduced glutathione (GSH) and the oxygen radical scavenging enzymes, superoxide dismutase (SOD) and catalase (CAT), played a key role in detoxification against BIT poisoning., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Heterotrophic cultivation of Chlorella pyrenoidosa based on straw substrate was proposed as a promising approach in this research. The straw pre-treated by ammonium sulfite method was enzymatically hydrolyzed for medium preparation. The highest intrinsic growth rate of C. pyrenoidosa reached to 0.097h -1 in hydrolysate medium, which was quicker than that in glucose medium. Rising nitrogen concentration could significantly increase protein content and decrease lipid content in biomass, meanwhile fatty acids composition kept stable. The highest protein and lipid content in microalgal biomass reached to 62% and 32% under nitrogen excessive and deficient conditions, respectively. Over 40% of amino acids and fatty acids in biomass belonged to essential amino acids (EAA) and essential fatty acids (EFA), which were qualified for high-value uses. This research revealed the rapid biomass accumulation property of C. pyrenoidosa in straw hydrolysate medium and the effectiveness of nitrogen regulation to biomass composition at heterotrophic condition., (Copyright © 2017. Published by Elsevier Ltd.)

Reuse of secondary municipal effluent from wastewater treatment plants in water bodies could effectively alleviate freshwater resource shortage. However, excessive nutrients must be efficiently removed to prevent eutrophication. Compared with other means of advanced wastewater treatment, microalgae-based processes display overwhelming advantages including efficient and simultaneous N and P removal, no requirement of additional chemicals, O 2 generation, CO 2 mitigation, and potential value-added products from harvested biomass. One particular challenge of microalgae-based advanced municipal wastewater treatment compared to treatment of other types of wastewater is that concentrations of nutrients and N:P ratios in secondary municipal effluent are much lower and imbalanced. Therefore, there should be comprehensive considerations on nutrient removal from this specific type of effluent. Removal of nutrients and organic substances, and other environmental benefits of microalgae-based advanced municipal wastewater treatment systems were summarized. Among the existing studies on microalgal advanced nutrient removal, much information on major parameters is absent, rendering performances between studies not really comparable. Mechanisms of microalgae-based nitrogen and phosphorus removal were respectively analyzed to better understand advanced nutrient removal from municipal secondary effluent. Factors influencing microalgae-based nutrient removal were divided into intrinsic, environmental, and operational categories; several factors were identified in each category, and their influences on microalgal nutrient removal were discussed. A multiplicative kinetic model was integrated to estimate microalgal growth-related nutrient removal based majorly on environmental and intrinsic factors. Limitations and prospects of future full-scale microalgae-based advanced municipal wastewater treatment were also suggested. The manuscript could offer much valuable information for future studies on microalgae-based advanced wastewater treatment and water reuse.

While reverse osmosis (RO) is a promising technology for wastewater reclamation, RO concentrate (ROC) treatment and disposal are important issues to consider. Conventional chemical and physical treatment methods for ROC present certain limitations, such as relatively low nitrogen and phosphorus removal efficiencies as well as the requirement of an extra process for hardness removal. This study proposes a novel biological approach for simultaneous removal of nitrogen, phosphorus, and calcium (Ca(2+)) and magnesium (Mg(2+)) ions from the ROC of municipal wastewater treatment plants by microalgal cultivation and algal biomass production. Two microalgae strains, Chlorella sp. ZTY4 and Scenedesmus sp. LX1, were used for batch cultivation of 14-16 days. Both strains grew well in ROC with average biomass production of 318.7 mg/L and lipid contents up to 30.6%, and nitrogen and phosphorus could be effectively removed with efficiencies of up to 89.8% and 92.7%, respectively. Approximately 55.9%-83.7% Ca(2+) could be removed from the system using the cultured strains. Mg(2+) removal began when Ca(2+) precipitation ceased, and the removal efficiency of the ion could reach up to 56.0%. The most decisive factor influencing Ca(2+) and Mg(2+) removal was chemical precipitation with increases in pH caused by algal growth. The results of this study provide a new biological approach for removing nitrogen, phosphorous, and hardness from ROC. The results suggest that microalgal cultivation presents new opportunities for applying an algal process to ROC treatment. The proposed approach serves dual purposes of nutrient and hardness reduction and production of lipid rich micro-algal biomass., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

A novel suspended-solid phase photobioreactor (ssPBR) was proposed in this paper to solve the problem of microalgal expensive and complex harvest system for biomass/biofuel production. In this ssPBR, solid carriers were added and kept suspended by aeration. Part of microalgae could attach and grow on the carriers. By catching carriers, microalgae could easily be separated from liquid phase. Three kinds of Carriers A, B, C made of cotton, mohair and linen, respectively, were used in this study. Compared with the reactor without carriers, the biomass production in each ssPBR was increased by adding these three kinds of carriers at a dosage of 2g/L, and the maximum increments of biomass were 2.2×10(5) (10.3%), 7.8×10(4) (3.9%) and 4.4×10(5) (20.5%)cells/mL, respectively. By increasing the dosage of Carriers-C to 4g/L, the maximum increment of microalgal biomass could reach up to about 30% in the ssPBR compared with control group., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Cultivation of microalgae for biomass production is a promising way to dispose of wastewater and recover nutrients simultaneously. The properties of nutrient removal and biomass production in domestic wastewater of a newly isolated microalga Scenedesmus sp. ZTY1 were investigated in this study. Scenedesmus sp. ZTY1, which was isolated from a wastewater treatment plant in Beijing, grew well in both the primary and secondary effluents of a wastewater treatment plant during the 21-day cultivation, with a maximal algal density of 3.6 × 10(6) and 1.9 × 10(6) cells · mL(-1), respectively. The total phosphorus concentrations in both effluents could be efficiently removed by over 97% after the cultivation. A high removal rate (over 90%) of total nitrogen (TN) was also observed. After cultivation in primary effluent for 21 days, the lipid content of Scenedesmus sp. ZTY1 in dry weight had reached about 32.2%. The lipid and triacylglycerol (TAG) production of Scenedesmus sp. ZTY1 was increased significantly with the extension of cultivation time. The TAG production of Scenedesmus sp. ZTY1 increased from 32 mg L(-1) at 21 d to 148 mg L(-1) at 45 d in primary effluent. All the experiments were carried out in non-sterilized domestic wastewater and Scenedesmus sp. ZTY1 showed good adaptability to the domestic wastewater environment.

Cultivating microalgae heterotrophically could overcome the light dependency, and thus increase the yield of microalgal lipid per unit area. In this study, three newly isolated strains of microalgae (Scenedesmus sp. ZTY2, Scenedesmus sp. ZTY3 and Chlorella sp. ZTY4) from a domestic wastewater treatment plant were heterotrophically cultivated in domestic wastewater with no illumination. During the cultivation, the algal densities of Scenedesmus species and Chlorella species were increased by 203.0% and 60.5% comparing with the initial densities, respectively. After 11-day cultivation, the lipid contents of Scenedesmus sp. ZTY2, Scenedesmus sp. ZTY3 and Chlorella sp. ZTY4 reached 69.1%, 55.3% and 79.2%, respectively. The DOC removal efficiencies of these three strains were 63.4%, 52.9% and 64.4%, and the biomass yield were 1.65, 1.98 and 2.31mg biomass/mg DOC, respectively. This is the first report about the heterotrophic cultivation of microalgae strains for domestic wastewater treatment and lipid production under dark condition., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Microalgal growth is the key to the coupled system of wastewater treatment and microalgal biomass production. In this study, Monod model, Droop model and Steele model were incorporated to obtain an integrated growth model describing the combined effects of nitrogen, phosphorus and light intensity on the growth rate of Scenedesmus sp. LX1. The model parameters were obtained via fitting experimental data to these classical models. Furthermore, the biomass production of Scenedesmus sp. LX1 in open pond under nutrient level of secondary effluent was analyzed based on the integrated model, predicting a maximal microalgal biomass production rate about 20 g m(-2) d(-1). In order to optimize the biomass production of open pond the microalgal biomass concentration, light intensity on the surface of open pond, total depth of culture medium and hydraulic retention time should be 500 g m(-3), 16,000 lx, 0.2 m and 5.2 d in the conditions of this study, respectively., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Graphical abstract Highlights • Straw lignocellulose pretreated by ammonium sulfite was hydrolyzed as substrate. • Growth kinetics of Chlorella pyrenoidosa in straw hydrolysate was clarified. • Hydrolysate stimulated growth at low concentration and inhibited at high. • Han-Levenspiel model was well fit to explain the growth in hydrolysate medium. Abstract As the current bioconversion of lignocellulose mainly towards on bioenergy, a bioproduct with higher value would be promising to improve the profit output from the process. Straw lignocellulose, hydrothermally pretreated by ammonium sulfite, was used as the substrate to produce high-value microalgal biomass in this study. Medium based on the lignocellulose hydrolysate were prepared to cultivate four locally isolated microalgal strains, Chlorella and Scenedesmus species. All of the microalgal strains well adapted to the lignocellulose hydrolysate. Among the strains, C. ZTY4 had the highest growth rate in hydrolysate-based medium with a generation time of only 10.2 h, which was much shorter than that in glucose medium (16.7 h) and former reports. The polyunsaturated fatty acids (PUFA) content of C. ZTY4 reached to 66.4% in total fatty acids of C. ZTY4, which was well qualified for high-value applications. C. ZTY4 was identified as Chlorella pyrenoidosa by 18S rDNA sequencing. This research proposed a bioconversion approach from straw lignocellulose to high-value microalgal biomass and screened C. ZTY4 as a qualified strain. [ABSTRACT FROM AUTHOR]

Methylisothiazolinone (MIT) has been widely used to control bacterial growth in reverse osmosis (RO) systems. However, MIT's toxicity on microalgae should be determined because residual MIT is concentrated into RO concentrate (ROC) and might have a severe impact on microalgae-based ROC treatment. This study investigated the tolerance of Scenedesmus sp. LX1 to MIT and revealed the mechanism of algal growth inhibition and toxicity resistance. Scenedesmus sp. LX1 was inhibited by MIT with a half-maximal effective concentration at 72 h (72 h-EC 50 ) of 1.00 mg/L, but the strain recovered from the inhibition when its growth was not completely inhibited. It was observed that this inhibition's effect on subsequent growth was weak, and the removal of MIT was the primary reason for the recovery. Properly increasing the initial algal density significantly shortened the adaptation time for accelerated recovery in a MIT-containing culture. Photosynthesis damage by MIT was one of the primary reasons for growth inhibition, but microalgal cell respiration and adenosine triphosphate (ATP) synthesis were not completely inhibited, and the algae were still alive even when growth was completely inhibited, which was notably different from observations made with bacteria and fungi. The algae synthesized more chlorophyll, antioxidant enzymes of superoxide dismutase (SOD) and catalase (CAT), and small molecules, such as reduced glutathione (GSH), to resist MIT poisoning. The microalgae-based process could treat the MIT-containing ROC, since MIT was added for only several hours a week in municipal wastewater reclamation RO processes, and the MIT average concentration was considerably lower than the maximum concentration that algae could tolerate. [ABSTRACT FROM AUTHOR]

In many microalgal cultivation systems, microalgae co-exist with bacteria, while little is known about the characteristics of their symbiotic relationships. In this study, twenty-six microalgae growth-promoting bacteria were isolated from a culture system of Scenedesmus sp. LX1 cultivated in the secondary effluent from domestic wastewater by using the high-throughput multiple well plate screening method. Ten strains were found to produce and secrete indole acetic acid (IAA), promoting the growth of microalgae. Meanwhile, the microalgae might have secreted signal substances to induce IAA production in bacteria, which was amplified in the tryptophan abundant environment. This indicates that bacteria may mainly promote the growth of the co-existing microalgae through secreting IAA, and microalgae would selectively enhance IAA secretion in turn. Microalgae cultured with microalgal growth-promoting bacteria would be a new potential strategy for improving large-scale microalgal cultivation in an economic and environmentally-friendly way. [ABSTRACT FROM AUTHOR]

Microalgae cultivation is a promising candidate for autotrophic sequestrating CO 2 transformation into renewable bio-products. However, potential of microalgae has not been fully explored due to the bottlenecks of biomass harvesting during large-scale microalgae cultivation. A novel capillary-driven photobioreactor (CPBR) is proposed in this study for attached microalgae cultivation obtaining high biomass productivity and overcoming the biomass harvesting limitation in traditional suspended cultures. Performances on microalgal growth, cellular composition, nutrients uptake from water, and corresponding light utilization efficiencies were investigated and emphasized. During 12 days of system operation, attached microalgal biomass on the carriers were in the range of 83.6 g/m 2 -footprint area of 121.5 g/m 2 -footprint area, corresponding to light utilization efficiency of 3.3% to 4.8%. With proper carriers packing density of 16% to 32%, obtained biomass productivities of CPBR were approximately 10 g/m 2 /d (footprint area). Biomass composition analysis revealed that lipid and carbohydrates contents in attached microalgal biomass were higher than in suspended microalgal biomass with same cultivating conditions, proving the potential of microalgae cultivated in CPBR as feedstock of biofuels. [ABSTRACT FROM AUTHOR]

Microalgae-based bioenergy has been long considered as a promising substitute to replace the fossil fuels at large-scale. After the decades of research, several bottlenecks remain to be overcome to make the large-scale production of microalgal bioenergy commercially viable. In this review, the bottlenecks that restrict the commercial production of microalgal bioenergy are summarized as “high cost” and “low efficiency” and then identified in detailed categories. Several solutions which aim to solve the bottlenecks are presented and discussed correspondingly. The solutions presented in this review including cultivating in wastewater, recycling water and nutrients, utilizing the whole biomass, selecting indigenous strains, cultivating mixed-species, supplementing low-cost organic substrates, mixing vertically in reactor, optimizing metabolic pathway and attaching the cells on suspended-solids. Furthermore, a novel system integrated with the solutions, which is combined with the wastewater treatment system and characterized as the mixed-species cultivation of wild-selected mixotrophic microalgal strains in suspended-solid phase bioreactor based on low-cost substrate, is proposed to promote the scale-up of the microalgal cultivation for bioenergy production. [ABSTRACT FROM AUTHOR]

• 6-benzylaminopurine affects algae in a dose-dependent manner. • Low dosage of 6-benzylaminopurine increased microalgal biomass accumulation. • Low dosage of 6-benzylaminopurine increased the yield of fatty acid methyl esters. • 6-benzylaminopurine enhanced microalgal photosynthetic activity. • Microalgal metabolic activity was enhanced by lower dosage 6-benzylaminopurine. Microalgae have great potential for use in the production of valuable products. However, the low productivity of microalgal cultures is a bottleneck that restricts their large-scale application. In this study, the effects of an artificial, synthetic plant growth-promoting substance, 6-benzylaminopurine (6-BA), on the growth characteristics and physiology of Scenedesmus sp. LX1 was investigated. The results showed that 6-BA affected microalgae growth and physiology in a dose-dependent manner; low doses promoted growth while high doses inhibited growth. At the optimum dose (0.1 mg L−1), the maximum algal density and the maximum population growth rate were increased by 20% and 52%, respectively, compared with the control. Meanwhile, the yield of fatty acid methyl esters (FAME) also increased, and their compositions were changed, which resulted in an increase in monounsaturated fatty acid content at lower doses. The rates of nitrogen and phosphorus uptake were also enhanced at low doses. The measurement of photosynthetic activity in the microalgae indicated that lower doses of 6-BA enhanced activity by increasing the amounts of chlorophyll, which promoted the growth of microalgae. Metabolic activity analysis showed that lower doses of 6-BA enhanced dehydrogenase activity and ATP accumulation in microalgal cells. Overall, this study shows that 6-BA has a high potential for use in the development efficient microalgal culture systems. [ABSTRACT FROM AUTHOR]

Microalgae are potential candidates for the production of valuable products, such as renewable biodiesel, health products and pigments. However, low biomass productivity has restricted their large-scale applications. In this study, the effects of two auxins (one natural type of indole-3-acetic acid (IAA) and the other synthetic type of 2,4-dichlorophenoxyacetic acid (2,4-D)) on the growth and fatty acid methyl esters (FAMEs) production of a freshwater microalgae Scenedesmus sp. LX1 were investigated. Both auxins showed a “low dosage-promotion and high dosage-inhibition” effect on the growth and FAMEs accumulation. The optimum dosage of IAA and 2,4-D were 1 mg L −1 and 0.1 mg L −1 , respectively. Moreover, the IAA could increase the monounsaturated fatty acid content. The auxins may promote the growth by enhancing the photosynthetic activity through increasing chlorophyll contents. Therefore, auxin significantly enhanced microalgal growth and FAMEs accumulation, and has a potential for application in developing efficient microalgal cultivation. [ABSTRACT FROM AUTHOR]

Nowadays, microalgae-based bioenergy has been commonly considered as a potential substitute of the non-renewable traditional energy resources with increasing attention. Nevertheless, microalgal cells will secrete/release some Soluble Algal Products (SAPs) into the culture medium during microalgal biomass cultivation. Studies on the influences, especially the negative ones, of SAPs in large-scale cultivation of microalgae for biomass/bioenergy production remain limited. The negative effects, properties, production and treatments of SAPs were therefore investigated and summarized in this paper. It was found that SAPs may inhibit microalgal growth, raise the cost of microalgal recovery and lower the performance of the downstream discharged water treatment process after microalgal cultivation, among others. The production of SAPs could be as high as 70 mg/L in terms of dissolved organic carbon (DOC), causing substantial environmental issues. The chemical properties of SAPs significantly varied among different microalgal species, growth phases and nutrient concentrations. The distribution of molecular weight of SAPs varied from less than 1 kDa to more than 100 kDa, meanwhile the hydrophilic/hydrophobic property of SAPs showed a consistency. All studies showed that more than 60% of SAPs are hydrophilic. After comparing three different SAPs treatment methods, it was concluded that biodegradation could be a feasible and economical process to remove SAPs based on the properties of SAPs. The mechanisms of SAPs formation and the mitigation of their undesirable effects are still not clear. Therefore, a systematic and deep study on those topics is urgently needed. [ABSTRACT FROM AUTHOR]

Abstract: Using wastewater as resource for microalgal cultivation was seriously considered as a promising approach for sustainable biomass and lipid production. The proper selection of microalgal species is the foundation and key point to achieve this objective. This paper reviewed the recent status of microalgal cultivation in wastewater, including the characteristics of microalgal species used in recent studies, the performance of different microalgal species in different types of wastewater, the commonly-used isolation methods of microalgal species adaptable to the growth in wastewater, and the evaluation criteria of microalgal species. It was found that microalgal biomass and lipid production in wastewater were comparable to those in artificial culture medium, although most of the data was obtained in sterilized wastewater. Among all microalgal species involved in this review, Botryococcus braunii, Chlorella pyrenoidosa and Chlamydomonas reinhardtii showed superior performance in certain studies. However, no microalgal species has been demonstrated to meet all the requirements for large-scale biomass production in wastewater. Thus, the efforts on microalgal species isolation and characterization should still be promoted. On the basis of all the information, this review explored the limitations of recent studies and future research needs on this topic. [Copyright &y& Elsevier]