Quorum sensing molecules in activated sludge could trigger microalgae lipid synthesis

The aging of microplastics exacerbates the damage to photosynthetic performance and bioenergy production in microalgae (Chlorella pyrenoidosa)

The toxicity of microplastics (MPs) on freshwater plants has been widely studied, yet the influence of aged MPs remains largely unexplored. Herein, we investigated the influence of polyvinyl chloride (PVC) MPs, both before and after aging, at different environmentally relevant concentrations on Chlorella pyrenoidosa, a freshwater microalgae species widely recognized as a valuable biomass resource. During a 96-h period, both virgin and aged MPs hindered the growth of C. pyrenoidosa. The maximum growth inhibition rates were 32.40 % for virgin PVC at 250 mg/L and 44.72 % for aged PVC at 100 mg/L, respectively. Microalgae intracellular materials, i.e., protein and carbohydrate contents, consistently decreased after MP exposure, with more pronounced inhibition observed with aged PVC. Meanwhile, the MP aging significantly promoted the nitrogen uptake of C. pyrenoidosa, i.e., 1693.45 ± 42.29 mg/L (p < 0.01), contributing to the production of humic acid-like substances. Additionally, aged PVC induced lower chlorophyll a and Fv/Fm when compared to virgin PVC, suggesting a more serious inhibition of the photosynthesis process of microalgae. The toxicity of MPs to C. pyrenoidosa was strongly associated with intercellular oxidative stress levels. The results indicate that MP aging exacerbates the damage to photosynthetic performance and bioenergy production in microalgae, providing critical insights into the toxicity analysis of micro(nano)plastics on freshwater plants.

Enhancing strategies of photosynthetic hydrogen production from microalgae: Differences in hydrogen production between prokaryotic and eukaryotic algae

Hydrogen production through the metabolic bypass of microalgae photosynthesis is an environmentally friendly method. This review examines the genetic differences in hydrogen production between prokaryotic and eukaryotic microalgae. Additionally, the pathways for enhancing microalgae-based photosynthetic hydrogen generation are summarized. The main strategies for enhancing microalgal hydrogen production involve inhibiting the oxygen-generating process of photosynthesis and promoting the oxygen tolerance of hydrogenase. Future research is needed to explore the regulation of physiological metabolism through quorum sensing in microalgae to enhance photosynthetic hydrogen production. Moreover, effective evaluation of carbon emissions and sequestration across the entire photosynthetic hydrogen production process is crucial for determining the sustainability of microalgae-based production approaches through comprehensive lifecycle assessment. This review elucidates the prospects and challenges associated with photosynthetic hydrogen production by microalgae.

Communication mediated interaction between bacteria and microalgae advances photogranulation

Recently, photogranules composed of bacteria and microalgae for carbon-negative nitrogen removal receive extensive attention worldwide, yet which type of bacteria is helpful for rapid formation of photogranules and whether they depend on signaling communication remain elusive. Varied signaling communication was analyzed using metagenomic method among bacteria and microalgae in via of two types of experimentally verified signaling molecule from bacteria to microalgae, which include indole-3-acetic acid (IAA) and N-acyl homoserine lactones (AHLs) during the operation of photo-bioreactors. Signaling communication is helpful for the adaptability of bacteria to survive with algae. Compared with non-signaling bacteria, signaling bacteria more easily adapt to the varied conditions, evidenced by the increased abundance in the operated reactors. Signaling bacteria are easier to enter the phycosphere, and they dominate the interactions between bacteria and algae rather than non-signaling bacteria. The co-abundance groups (CAGs) with signaling bacteria possess higher abundance than that without signaling bacteria (22.27 % and 6.67 %). Importantly, signaling bacteria accessibly interact with microalgae, which possess higher degree centralities and 32.50 % of them are keystone nodes in the network, in contrast to only 18.66 % of non-signaling bacteria. Thauera carrying both IAA and AHLs synthase genes are highly enriched and positively correlated with nitrogen removal rate. Our work not only highlights the essential roles of signaling communication between microalgae and bacteria in the development of photogranules, but also enriches our understanding of microbial sociobiology.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Evaluating application of photosynthetic microbial fuel cell to exhibit efficient carbon sequestration with concomitant value-added product recovery from wastewater: A review

The emission of CO2 from industrial (24%) and different anthropogenic activities, like transportation (27%), electricity production (25%), and agriculture (11%), can lead to global warming, which in the long term can trigger substantial climate changes. In this regard, CO2 sequestration and wastewater treatment in tandem with bioenergy production through photosynthetic microbial fuel cell (PMFC) is an economical and sustainable intervention to address the problem of global warming and elevating energy demands. Therefore, this review focuses on the application of different PMFC as a bio-refinery approach to produce biofuels and power generation accompanied with the holistic treatment of wastewater. Moreover, CO2 bio-fixation and electron transfer mechanism of different photosynthetic microbiota, and factors affecting the performance of PMFC with technical feasibility and drawbacks are also elucidated in this review. Also, low-cost approaches such as utilization of bio-membrane like coconut shell, microbial growth enhancement by extracellular cell signalling mechanisms, and exploitation of genetically engineered strain towards the commercialization of PMFC are highlighted. Thus, the present review intends to guide the budding researchers in developing more cost-effective and sustainable PMFCs, which could lead towards the commercialization of this inventive technology.

Bacteria and microalgae associations in periphyton-mechanisms and biotechnological opportunities

Phototrophic and heterotrophic microorganisms coexist in complex and dynamic structures called periphyton. These structures shape the biogeochemistry and biodiversity of aquatic ecosystems. In particular, microalgae-bacteria interactions are a prominent focus of study by microbial ecologists and can provide biotechnological opportunities for numerous applications (i.e. microalgal bloom control, aquaculture, biorefinery, and wastewater bioremediation). In this review, we analyze the species dynamics (i.e. periphyton formation and factors determining the prevalence of one species over another), coexisting communities, exchange of resources, and communication mechanisms of periphytic microalgae and bacteria. We extend periphyton mathematical modelling as a tool to comprehend complex interactions. This review is expected to boost the applicability of microalgae-bacteria consortia, by drawing out knowledge from natural periphyton.

Enhancing biolipid production and self-flocculation of Chlorella vulgaris by extracellular polymeric substances from granular sludge with CO2 addition: Microscopic mechanism of microalgae-bacteria symbiosis

Microalgae-bacteria symbiotic systems were known to have great potential for simultaneous water purification and resource recovery, among them, microalgae-bacteria biofilm/granules have attracted much attention due to its excellent effluent quality and convenient biomass recovery. However, the effect of bacteria with attached-growth mode on microalgae, which has more significance for bioresource utilization, has been historically ignored. Thus, this study attempted to explore the responses of C. vulgaris to extracellular polymeric substances (EPS) extracted from aerobic granular sludge (AGS), for enhancing the understanding of microscopic mechanism of attached microalgae-bacteria symbiosis. Results showed that the performance of C. vulgaris was effectively boosted with AGS-EPS treatment at 12-16 mg TOC/L, highest biomass production (0.32±0.01 g/L), lipid accumulation (44.33±5.69%) and flocculation ability (20.83±0.21%) were achieved. These phenotypes were promoted associated with bioactive microbial metabolites in AGS-EPS (N-acyl-homoserine lactones, humic acid and tryptophan). Furthermore, the addition of CO2 triggered carbon flow into the storage of lipids in C. vulgaris, and the synergistic effect of AGS-EPS and CO2 for improving microalgal flocculation ability was disclosed. Transcriptomic analysis further revealed up-regulation of synthesis pathways for fatty acid and triacylglycerol that was triggered by AGS-EPS. And within the context of CO2 addition, AGS-EPS substantially upregulated the expression of aromatic protein encoding genes, which further enhanced the self-flocculation of C. vulgaris. These findings provide novel insights into the microscopic mechanism of microalgae-bacteria symbiosis, and bring new enlightenment to wastewater valorization and carbon-neutral operation of wastewater treatment plants based on the symbiotic biofilm/biogranules system.

Quorum Sensing as a Trigger That Improves Characteristics of Microbial Biocatalysts

Quorum sensing (QS) of various microorganisms (bacteria, fungi, microalgae) today attracts the attention of researchers mainly from the point of view of clarifying the biochemical basics of this general biological phenomenon, establishing chemical compounds that regulate it, and studying the mechanisms of its realization. Such information is primarily aimed at its use in solving environmental problems and the development of effective antimicrobial agents. This review is oriented on other aspects of the application of such knowledge; in particular, it discusses the role of QS in the elaboration of various prospective biocatalytic systems for different biotechnological processes carried out under aerobic and anaerobic conditions (synthesis of enzymes, polysaccharides, organic acids, etc.). Particular attention is paid to the biotechnological aspects of QS application and the use of biocatalysts, which have a heterogeneous microbial composition. The priorities of how to trigger a quorum response in immobilized cells to maintain their long-term productive and stable metabolic functioning are also discussed. There are several approaches that can be realized: increase in cell concentration, introduction of inductors for synthesis of QS-molecules, addition of QS-molecules, and provoking competition between the participants of heterogeneous biocatalysts, etc.).

Effect of the N-hexanoyl-L-homoserine Lactone on the Carbon Fixation Capacity of the Algae-Bacteria System

Algae-bacteria systems are used widely in wastewater treatment. N-hexanoyl-L-homoserine lactone (AHL) plays an important role in algal-bacteria communication. However, little study has been conducted on the ability of AHLs to regulate algal metabolism and the carbon fixation ability, especially in algae-bacteria system. In this study, we used the Microcystis aeruginosa + Staphylococcus ureilyticus strain as a algae-bacteria system. The results showed that 10 ng/L C₆-HSL effectively increased the chlorophyll-a (Chl-a) concentration and carbon fixation enzyme activities in the algae-bacteria group and algae group, in which Chl-a, carbonic anhydrase activity, and Rubisco enzyme increased by 40% and 21%, 56.4% and 137.65%, and 66.6% and 10.2%, respectively, in the algae-bacteria group and algae group, respectively. The carbon dioxide concentration mechanism (CCM) model showed that C₆-HSL increased the carbon fixation rate of the algae-bacteria group by increasing the CO₂ transport rate in the water and the intracellular CO₂ concentration. Furthermore, the addition of C₆-HSL promoted the synthesis and secretion of the organic matter of algae, which provided biogenic substances for bacteria in the system. This influenced the metabolic pathways and products of bacteria and finally fed back to the algae. This study provided a strategy to enhance the carbon fixation rate of algae-bacteria consortium based on quorum sensing.

Screening of microalgae species and evaluation of algal-lipid stimulation strategies for biodiesel production

Microalgae is considered an alternative source for biodiesel production producing renewable, sustainable and carbon-neutral energy. Microalgae property changes among species, which determines the efficiency of biodiesel production. Besides the lipid content evaluation, multi-principles (including high lipid productivity, high biomass yield, pollution resistance and desired fatty acid, etc.) for superior oil-producing species screening was proposed in this review and three microalgae species (Chlorella vulgaris, Scenedesmus obliquus and Mychonastes afer) with high bio-lipid producing prospect were screened out based on big data digging and analysis. The multilateral strategies for algal-lipid stimulating were also compared, among which, nutrient restriction, temperature control, heterotrophy and chemicals addition showed high potential in enhancing lipid accumulation; while electromagnetic field showed little effect. Interestingly, it was found that the lipid accumulation was more sensitive to nitrogen (N)-limitation other than phosphorus (P). Nutrient restriction, salinity stress etc. enhanced lipid accumulation by creating a stressed environment. Hence, optimum conditions (e.g. N:15-35 mg/L and P:4-16 mg/L) should be set to balance the lipid accumulation and biomass growth, and further guarantee the algal-lipid productivity. Otherwise, two-step cultivation could be applied during all the stressed stimulation. Different from lab study, effectiveness, operability and economy should be all considered for stimulation strategy selection. Nutrient restriction, temperature control and heterotrophy were highly feasible after the multidimensional evaluation.

Nitrogen recovery from wastewater by microbial assimilation - A review

The increased nitrogen (N) input with low utilization rate in artificial N management has led to massive reactive N (Nr) flows, putting the Earth in a high-risk state. It is essential to recover and recycle Nr during or after Nr removal from wastewater to reduce N input while simultaneously mitigate Nr pollution in addressing the N stress. However, mechanisms for efficient Nr recovery during or after Nr removal remain unclear. Here, the occurrence of N risk and progress in wastewater treatment in recent years as well as challenges of the current technologies for N recovery from wastewater were reviewed. Through analyzing N conversion fluxes in biogeochemical N-cycling networks, microbial N assimilation through photosynthetic and heterotrophic microorganisms was highlighted as promising alternative for synergistic N removal and recovery in wastewater treatment. In addition, the prospects and gaps of Nr recovery from wastewater through microbial assimilation are discussed.

Obtaining Bioproducts from the Studies of Signals and Interactions between Microalgae and Bacteria

The applications of microalgae biomass have been widely studied worldwide. The classical processes used in outdoor cultivations of microalgae, in closed or open photobioreactors, occur in the presence of bacteria. Understanding how communication between cells occurs through quorum sensing and evaluating co-cultures allows the production of microalgae and cyanobacteria to be positively impacted by bacteria, in order to guarantee safety and profitability in the production process. In addition, the definition of the effects that occur during an interaction, promotes insights to improve the production of biomolecules, and to develop innovative products. This review presents the interactions between microalgae and bacteria, including compounds exchanges and communication, and addresses the development of new pharmaceutical, cosmetic and food bioproducts from microalgae based on these evaluations, such as prebiotics, vegan skincare products, antimicrobial compounds, and culture media with animal free protein for producing vaccines and other biopharmaceutical products. The use of microalgae as raw biomass or in biotechnological platforms is in line with the fulfillment of the 2030 Agenda related to the Sustainable Development Goals (SDGs).

Harsh temperature induces Microcystis aeruginosa growth enhancement and water deterioration during vernalization

Cyanobacterial blooms are seasonal phenomena in eutrophic water. Cyanobacteria grow fast in the warm spring/summer while disappearing in cold autumn/winter. The temperature change induces algal vernalization. However, whether vernalization affects cyanobacterial blooms, and the regulatory signaling mechanisms are unclear. This study used Microcystis aeruginosa as the model cyanobacteria, and 4 °C and 10 °C as the low-temperature stimulation to explore the cell growth, metabolites, and signaling pathways in cyanobacteria vernalization. Low temperatures induced M. aeruginosa vernalization; the growth rate and cell density increased by 35±4% and 33±2%. Vernalization influenced peptidoglycan synthesis and cell permeability. Soluble microbial products (SMPs) in water increased by 109±5%, resulting in water deterioration. Polysaccharides were the predominant SMPs during the initial term of vernalization. Tryptophan protein-like & humic acid-like substances became the main increased SMPs in the middle-later period of vernalization. Harsh temperatures triggered quorum sensing and two-component system. Signaling sensing systems upregulated photosynthesis, glycolysis, TCA cycle, oxidative phosphorylation, and DNA replication, enhancing M. aeruginosa growth and metabolism during vernalization. This study verified that low temperature stimulates cyanobacteria growth and metabolism, and vernalization possibly aggravates cyanobacterial blooms and water deterioration. It provides new insights into the mechanism of seasonal cyanobacterial blooms and the pivotal role of signaling regulation.

Dynamic patterns of quorum sensing signals in phycospheric microbes during a marine algal bloom

In the marine environment, the interactions among various species based on chemical signals play critical roles in influencing microbial structure and function. Quorum sensing (QS), the well-known signal-dependent communication autoinducer, is an important regulator in complex microbial communities. Here, we explored the QS gene profiles of phycosphere bacteria during a microcosmic phytoplankton bloom using metagenomic sequence data. More than fifteen subtypes of QS systems and 211,980 non-redundant amino acid sequences were collected and classified for constructing a hierarchical quorum-sensing database. The abundance of the various QS subtypes varied at different bloom stages and showed a strong correlation with phycosphere microorganisms. This suggested that QS is involved in regulating the phycosphere microbial succession during an algal bloom. A neutral community model revealed that the QS functional gene community assemblies were driven by stochastic processes. Co-occurrence model analysis showed that the QS gene networks of phycospheric microbes had similar topological structure and functional composition, which is a potential cornerstone for maintaining signal communication and population stabilization among microorganisms. Overall, QS systems have a strong relationship with the development of algal blooms and participate in regulating algal-associated microbial communities as chemical signals. This research reveals the chemical and ecological behavior of algal symbiotic bacteria and expands the current understanding of microbial dynamics in marine algal blooms.

Transboundary intercellular communications between Penicillium and bacterial communities during sludge bulking: Inspirations on quenching fungal dominance

Fungal bulking is caused by the evolution toward a fungi-dominant unbalanced sludge system, which is indeed the phenomenon of fungi competing against bacterial cells. We hypothesized that the cross-kingdom intercellular communication between fungi and bacteria was internal driving force that stimulated fungal bulking. In this study, we identified three signal molecules related to Penicillium fungi bulking under low-pH stress in an activated sludge reactor, which inspired us to propose a sludge bulking prevention strategy using the quorum quenching theory. When pH dropped from 7.0 to 4.5, the abundance of Penicillium increased from 12.5% to 44.8%. However, some functional bacterial genera, such as Nitrosomonas and Sphingopyxis, were washed out from the sludge. The production of quorum-sensing (QS) molecules N-Heptanoyl-L-homoserine lactone (C7-HSL), N-Dodecanoyl-L-homoserine lactone (C12-HSL), and N-Tetradecanoyl-L-homoserine lactone (C14-HSL) was regulated with sludge bulking; especially the response of the latter two was significantly negative to Penicillium blooming (P < 0.05). To test their roles, trace commercial C12-HSL and C14-HSL were added to Penicillium culture, successfully causing 8.3% and 30.2% inhibition of mycelial formation, respectively. They also contributed to the improvement of activated sludge settleability by 6.1% and 39.7%, respectively (represented by sludge volume index). The transcriptome technique further revealed the regulation of the expression of genes in |logFC| >1, involving signal transduction, mycelium synthesis, and metabolic pathways. Our study provided an innovative strategy for controlling fungal bulking from the perspective of microbial transboundary informatics.

Trace phenolic acids simultaneously enhance degradation of chlorophenol and biofuel production by Chlorella regularis

Coupling the cultivation of microalgae with wastewater treatment is a promising technology to recover bioresources from wastewater. However, toxic pollutants in wastewater seriously inhibit the growth of microalgae and the removal of pollutants. Phenolic acids are similar to phytohormones, could potentially relieve the toxicity to microalgae and simultaneously promote pollutant degradation and lipid accumulation. Chlorella and 4-chlorophenol (4-CP) were utilized to simulate the toxic wastewater treatment, and the roles of two typical phenolic acids, such as p-hydroxybenzoic acid (p-HBA) and caffeic acid (CA), were explored. The 0.2 μM concentration of p-HBA or CA improved the specific growth rate by 7.6% by enhancing photosynthesis and DNA replication. The oxidative damage caused by 4-CP was reduced by 30.3-49.7% via the synthesis of more antioxidant enzymes and the direct scavenging of free radicals by phenolic acids. Furthermore, the 4-CP removal rate increased by 27.0%, and toxic 4-CP was degraded into non-toxic compounds. The phenolic acids did not change the 4-CP degradation pathway but accelerated its removal and detoxification by enhancing the expression of 4-CP degradation enzymes. Simultaneously, lipid production increased by 20.5-23.1% due to the upregulation of enzymes related to fatty acid and triacylglycerol synthesis. Trace phenolic acids stimulated the mitogen-activated protein kinase signaling cascade and the calcium signaling pathway to regulate the physiology of the microalgae and protect cells from toxic stress. This study provides a promising new strategy for toxic wastewater treatment and bioresource recovery.

Enhanced wastewater treatment performance by understanding the interaction between algae and bacteria based on quorum sensing

In order to further obtain sustainable wastewater treatment technology, in-depth analysis based on algal-bacterial symbiosis, quorum sensing signal molecules and algal-bacterial relationship will lay the foundation for the synergistic algal-bacterial wastewater treatment process. The methods of enhancing algae and bacteria wastewater treatment technology were systematically explored, including promoting symbiosis, reducing algicidal behavior, eliminating the interference of quorum sensing inhibitor, and developing algae and bacteria granular sludge. These findings can provide guidance for sustainable economic and environmental development, and facilitate carbon emissions reduction by using algae and bacteria synergistic wastewater treatment technology in further attempts. The future work should be carried out in the following four aspects: (1) Screening of dominant microalgae and bacteria; (2) Coordination of stable (emerging) contaminants removal; (3) Utilization of algae to produce fertilizers and feed (additives), and (4) Constructing recombinant algae and bacteria for reducing carbon emissions and obtaining high value-added products.

Microalgae tolerant of boron stress and bioresources accumulation during the boron removal process

Boron (B) industry and consuming produce large amounts of B-containing wastewater. Low tolerance of microorganisms and plants resulted in the biological removal of B was limited. Microalgae show high adaptability in adverse environments. Whether microalgae able to be utilized in B removal meanwhile produce bioresources, and the B tolerant mechanisms and regulation pathway of microalgae are unclear. In this study, the cell growth, B removal, and lipid/starch production of Chlorella regularis under different levels of B stress (0.5, 10, 25, and 50 mg/L) were examined. The mechanisms of signal perception and response were explored by transcriptome and network analysis. Microalgae tolerated 25 mg/L high B stress, cell growth showed no decline and biomass reach up to 4.5 g/L. Microalgae took in B with 3.35 mg/g and bonded them to protein and carbon components in cells, the B removal capability was higher than some special adsorbents. Microalgae produced 188.65 mg/(L∙d) lipids and 305.35 mg/(L∙d) starch. The mitogen-activated protein-kinase signaling pathway was involved in the B tolerance of microalgae and regulated B efflux, glycolysis, and lipid/starch accumulation to relieve B stress. This study provides potential biological technique for B removal in wastewater and promotes new insight into signal role in toxic pollutants biological treatment.

A comprehensive review on the use of algal-bacterial systems for wastewater treatment with emphasis on nutrient and micropollutant removal

The scarcity of water resources and environmental pollution have highlighted the need for sustainable wastewater treatment. Existing conventional treatment systems are energy-intensive and not always able to meet stringent disposal standards. Recently, algal-bacterial systems have emerged as environmentally friendly sustainable processes for wastewater treatment and resource recovery. The algal-bacterial systems work on the principle of the symbiotic relationship between algae and bacteria. This paper comprehensively discusses the most recent studies on algal-bacterial systems for wastewater treatment, factors affecting the treatment, and aspects of resource recovery from the biomass. The algal-bacterial interaction includes cell-to-cell communication, substrate exchange, and horizontal gene transfer. The quorum sensing (QS) molecules and their effects on algal-bacterial interactions are briefly discussed. The effect of the factors such as pH, temperature, C/N/P ratio, light intensity, and external aeration on the algal-bacterial systems have been discussed. An overview of the modeling aspects of algal-bacterial systems has been provided. The algal-bacterial systems have the potential for removing micropollutants because of the diverse possible interactions between algae-bacteria. The removal mechanisms of micropollutants - sorption, biodegradation, and photodegradation, have been reviewed. The harvesting methods and resource recovery aspects have been presented. The major challenges associated with algal-bacterial systems for real scale implementation and future perspectives have been discussed. Integrating wastewater treatment with the algal biorefinery concept reduces the overall waste component in a wastewater treatment system by converting the biomass into a useful product, resulting in a sustainable system that contributes to the circular bioeconomy.

Converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia: A critical review

Conventional wastewater treatment using activated sludge cannot efficiently eliminate nitrogen and phosphorus, thus engendering the risk of water eutrophication and ecosystem disruption. Fortunately, a new wastewater treatment process applying microalgae-bacteria consortia has attracted considerable interests due to its excellent performance of nutrients removal. Moreover, some bacteria facilitate the harvest of microalgal biomass through bio-flocculation. Additionally, while stimulating the functional bacteria, the improved biomass and enriched components also brighten bioenergy production from the perspective of practical applications. Thus, this review first summarizes the current development of nutrients removal and mutualistic interaction using microalgae-bacteria consortia. Then, advancements in bio-flocculation are completely described and the corresponding mechanisms are thoroughly revealed. Eventually, the recent advances of bioenergy production (i.e., biodiesel, biohydrogen, bioethanol, and bioelectricity) using microalgae-bacteria consortia are comprehensively discussed. Together, this review will provide the ongoing challenges and future developmental directions for better converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia.

Salinity-induced microalgal-based mariculture wastewater treatment combined with biodiesel production

Mariculture wastewater has drawn growing attention due to associated threats for coastal environment. However, most biological techniques exhibit unfavorable performance due to saline inhibition. Furthermore, only NaCl was used in most studies causing clumsy evaluation, undermining the potential of microalgal mariculture wastewater treatment. Herein, various concentrations of NaCl and sea salt are comprehensively examined and compared for their efficiencies of mariculture wastewater treatment and biodiesel conversion. The results indicate sea salt is a better trigger for treating wastewater (nearly 100% total nitrogen and total phosphorus removal) and producing high-quality biodiesel (330 mg/L•d). Structure equation model (SEM) further demonstrates the correlation of wastewater treatment performance and microalgal status is gradually weakened with increment of sea salt concentrations. Furthermore, metabolic analysis reveals enhanced photosynthesis might be the pivotal motivator for preferable outcomes under sea salt stimulation. This study provides new insights into microalgae-based approach integrating mariculture wastewater treatment and biodiesel production.

Responses of Alpha-linolenic acid strain (C-12) from Chlorella sp. L166 to low temperature plasma treatment

LTP treatment was applied to induce a high-content alpha-linolenic acid (ALA) strain (C-12) from Chlorella sp. L166, the ALA content of C-12 was increased by 48.37%. The mechanism of LTP induction were examined. The results showed that LTP could facilitate pH change, induce malondialdehyde (MDA) production, cause protein leakage, and destroyed the microalgae cells. The genes of C-12 encoding pyruvate dehydrogenase (E₂) were up-regulated 2.47-fold, and acetyl-CoA carboxylase (ACCase) down-regulated 0.48-fold compared to the wild type, these changed in the direction of ALA accumulation. Furthermore, the enzymes in DNA replication were significantly up regulated. Take ALA and biomass accumulation into account, LTP technology had a positive effect on ALA accumulation. Global view of metabolic variation also suggested that C-12 had an excellent adaptability to the changes of pH and peroxidation of LTP production. C-12 could be a promising ALA source of alternative materials for it do not compete with land.

Phytotoxic effect and molecular mechanism induced by nanodiamonds towards aquatic Chlorella pyrenoidosa by integrating regular and transcriptomic analyses

The growing diverse applications of nanodiamonds (NDs), especially as adsorbents and catalysts for wastewater treatment, have significantly increased their discharge and potential risk towards aquatic ecosystems. Although NDs have been certified for superior biocompatibility and lower toxicity towards numerous human cell lines, the characteristic response and underlying mechanism of aquatic microalgal response remains unclear. Here, the response of Chlorella pyrenoidosa to five concentrations of NDs was thoroughly investigated by comprehensive phenotypic and transcriptional examinations. Results indicated that higher concentration of NDs (50 mg/L) induced 75.4% growth inhibition, exacerbated oxidative stress and malformed morphology of microalgae after 48 h exposure. Meanwhile, the aggregated microalgae formed several flocs, apparently under 50 mg/L NDs. Noticeably, photosynthesis was susceptible to the NDs exposure. Although, the chlorophyll content and genes involved in photosynthesis were significantly improved by NDs, the results obtained from the photochemical parameters indicated that the excessive electrons during photosynthesis might be a pivotal reason for oxidative stress generation. Additionally, the genes included in amino acids metabolism and protein synthesis were up-regulated to alleviate the oxidative stress. Collectively, this work discloses the explicit molecular mechanisms of aquatic microalgae and provides comprehensive insights of potential aqueous environmental risk of gradually emergent NDs.

Benzoic and salicylic acid are the signaling molecules of Chlorella cells for improving cell growth

Cell-to-cell communication regulates microalgae production via signaling molecules (SMs), but few microalgal SM species are known. Here, we document two new microalgae SMs, benzoic acid (BA) and salicylic acid (SA). Initially, crude SMs were extracted from a microalgae culture in which microalgae grew on heterotrophic-enriched phosphorus nutrition. The extracted SMs enhanced Chlorella growth by ∼72%, promoted nutrient uptake, and up-regulated the mitogen-activated protein-kinase signaling cascade. Fourier transform infrared and nuclear magnetic resonance analyses identified the putative SMs was aromatic carboxylic acids. BA and SA were identified using high-resolution mass spectrometry. BA and SA addition increased cell growth by ∼75% and ∼25%; and improved ATP production by ∼35% and ∼20%. Transcriptomic analysis showed that BA and SA were biosynthesized via CoA-dependent, non-oxidative pathway. The SMs upregulated TCA-cycle enzymes, which promoted carbon assimilation and activated DNA-replicating enzyme, so that accelerated cell division. This study identified two new SMs for microalgae cell communication and provides means to identify other SMs.

Exogenous refractory protein enhances biofilm formation by altering the quorum sensing system: A potential hazard of soluble microbial proteins from WWTP effluent

Soluble microbial refractory proteins are major components of effluent from wastewater treatment plants that utilize a biological wastewater treatment process. The remaining proteins could negatively affect downstream treatment processes by altering the bacterial quorum sensing system. In this work, we elaborated the effects of exogenous refractory protein on biofilm formation. The results showed a linear relationship between biofilm formation and experimental protein concentrations at the range typically found in effluent, 0-8.0 mg/L. Micro-observation revealed that the exogenous refractory protein stimulated extracellular polysaccharide secretion to promote biofilm maturation. Extracellular polysaccharides increased by ~200% with the addition of only 2.0 mg/L protein. In addition, exogenous refractory proteins altered the quorum sensing system gene expression and polysaccharide gene expression. This work found that exogenous protein accelerated biofilm formation by influencing the quorum sensing system, thus providing new insight into the potential harm of soluble microbial refractory products.

Hormesis effects of phosphorus on the viability of Chlorella regularis cells under nitrogen limitation

BACKGROUND

Phosphorus (P) is an essential element of microalgae, which is either required for anabolism or for energy metabolism. When employing a nitrogen limitation strategy to trigger microalgal intracellular lipid accumulation, P supplementation was always simultaneously applied to compensate for the accompanied growth inhibition.

RESULTS

This study identified that P exerts hormesis effects on microalgae. Slight excess of P (≤ 45 mg L^-1) under nitrogen limitation condition stimulated the cell growth of Chlorella regularis and achieved a 10.2% biomass production increase. This also improved mitochondrial activity by 25.0% compared to control (P = 5.4 mg L^-1). The lipid productivity reached 354.38 mg (L d)^-1, which increased by 39.3% compared to control. Such an improvement was caused by the intracellularly stored polyphosphate energy pool. However, large excess of P (250 mg L^-1) inhibited the cell growth by 38.8% and mitochondrial activity decreased by 71.3%. C. regularis cells showed obvious poisoning status, such as enlarged size, plasmolysis, deformation of cell walls, and disorganization of organelles. This is probably because the over-accumulated P protonated the amide-N and disrupted membrane permeability.

CONCLUSIONS

These results provide new insight into the roles of P in microalgae lipid production: P does not always play a positive role under nitrogen limitation conditions.

Evaluation of oleaginous eustigmatophycean microalgae as potential biorefinery feedstock for the production of palmitoleic acid and biodiesel

This study aimed to evaluate the potential of six oleaginous eustigmatophytes for use as biorefinery feedstock for the co-production of palmitoleic acid (PA) and biodiesel under different initial nitrogen concentrations (INCs). Six eustigmatophytes were studied, the nitrogen deficiency strategy significantly stimulated the simultaneous hyper-accumulation of PA and lipids, and led to a desirable fatty acid profile (FAP), except in Vacuoliviride sp. and Nannochloropsis oculata. Particularly, Eustigmatos cf. polyphem exhibited great potential when supplied with 1 mM INC and yielded the highest PA (29.71% of dry weight (DW)) and lipid (72.01% of DW) contents, as their productivities increased to 96.26 and 232.79 mg/L/d, respectively. Furthermore, neutral lipids accounted for 91.82% of the total lipids and were rich in PA, and the favourable FAPs of C16-C18 (87.95%) and monounsaturated FAs (70.10%) ensured good biodiesel properties including the cetane number (55.69) and iodine value (92.81 gI₂/100 g), and all met the standard requirements.