Luxury uptake of phosphorus in Nannochloropsis salina : Effect of P concentration and light on P uptake in batch and continuous cultures

Getting Grip on Phosphorus: Potential of Microalgae as a Vehicle for Sustainable Usage of This Macronutrient

Phosphorus (P) is an important and irreplaceable macronutrient. It is central to energy and information storage and exchange in living cells. P is an element with a "broken geochemical cycle" since it lacks abundant volatile compounds capable of closing the P cycle. P fertilizers are critical for global food security, but the reserves of minable P are scarce and non-evenly distributed between countries of the world. Accordingly, the risks of global crisis due to limited access to P reserves are expected to be graver than those entailed by competition for fossil hydrocarbons. Paradoxically, despite the scarcity and value of P reserves, its usage is extremely inefficient: the current waste rate reaches 80% giving rise to a plethora of unwanted consequences such as eutrophication leading to harmful algal blooms. Microalgal biotechnology is a promising solution to tackle this challenge. The proposed review briefly presents the relevant aspects of microalgal P metabolism such as cell P reserve composition and turnover, and the regulation of P uptake kinetics for maximization of P uptake efficiency with a focus on novel knowledge. The multifaceted role of polyPhosphates, the largest cell depot for P, is discussed with emphasis on the P toxicity mediated by short-chain polyPhosphates. Opportunities and hurdles of P bioremoval via P uptake from waste streams with microalgal cultures, either suspended or immobilized, are discussed. Possible avenues of P-rich microalgal biomass such as biofertilizer production or extraction of valuable polyPhosphates and other bioproducts are considered. The review concludes with a comprehensive assessment of the current potential of microalgal biotechnology for ensuring the sustainable usage of phosphorus.

Continuous microalgae cultivation for wastewater treatment - Development of a process strategy during day and night

Conventional wastewater treatment (WWT) is not able to recycle nutrients from the wastewater (WW) directly. Microalgae integrate the valuable nutrients nitrogen and phosphorus within their biomass very efficiently, making them predestined for an application in WWT. Nevertheless, microalgae-based processes are driven by natural sunlight as energy source, making a continuous process mode during day and night difficult. The aim of this study was therefore to investigate metabolic activities of the continuously cultivated microalgae Chlorella vulgaris at light and dark periods (16 h,8 h) with focus on nutrient uptake during night from a synthetic WW. Varying the dilution rate D (D = 0.0-1.0 d-1 in 0.1 d-1-steps) causes different limitations for algae growth. Nutrient limitations at low D's cause maximum accumulation of intracellular storage components (sum of carbohydrates and lipids) of ~70 % of dry biomass, starch is converted to lipids at the absence of light. From middle to high D's, the growth rate is determined by light limitation, reducing the intracellular storage components to ~20 % of dry biomass. Complete nutrient uptake is measurable up to D = 0.5 d-1, marking the maximum operating point for wastewater purification. At that point, cells are characterised by high protein (up to 57%DBM) and pigment (up to 6.9%DBM) quotas. During the night, the build-up of proteins at the degradation of intracellular storage components is furthermore visible. Applying the concept of active biomass (cells without storage components), a constant cellular protein (~68%ABM) and nitrogen quota (11.94%ABM) was revealed. A nitrogen spiking experiment clearly showed nitrogen uptake and proliferation during the night period. Based on the experimental data, a window of operation for a continuous WWT process was designed, allowing the hypothesis that continuous WWT using microalgae during day and night operation is possible without the supplementation of artificial light. This revealed the system's capacity to treat WW throughout 24 h applying cell recycling and storage of carbohydrate-rich biomass. At the end of the night, protein-rich biomass is available for further valorisation.

Nannochloropsis as an Emerging Algal Chassis for Light-Driven Synthesis of Lipids and High-Value Products

In light of the escalating global energy crisis, microalgae have emerged as highly promising producers of biofuel and high-value products. Among these microalgae, Nannochloropsis has received significant attention due to its capacity to generate not only triacylglycerol (TAG) but also eicosapentaenoic acid (EPA) and valuable carotenoids. Recent advancements in genetic tools and the field of synthetic biology have revolutionized Nannochloropsis into a powerful biofactory. This comprehensive review provides an initial overview of the current state of cultivation and utilization of the Nannochloropsis genus. Subsequently, our review examines the metabolic pathways governing lipids and carotenoids, emphasizing strategies to enhance oil production and optimize carbon flux redirection toward target products. Additionally, we summarize the utilization of advanced genetic manipulation techniques in Nannochloropsis. Together, the insights presented in this review highlight the immense potential of Nannochloropsis as a valuable model for biofuels and synthetic biology. By effectively integrating genetic tools and metabolic engineering, the realization of this potential becomes increasingly feasible.

Lighting the way to sustainable development: Physiological response and light control strategy in microalgae-based wastewater treatment under illumination

The Sustainable Development Goals link pollutant control with carbon dioxide reduction. Toward the goal of pollutant and carbon reduction, microalgae-based wastewater treatment (MBWT), which can simultaneously remove pollutants and convert carbon dioxide into biomass with value-added metabolites, has attracted considerable attention. The photosynthetic organism microalgae and the photobioreactor are the functional body and the operational carrier of the MBWT system, respectively; thus, light conditions profoundly influence its performance. Therefore, this review takes the general rules of how light influences the performance of MBWT systems as a starting point to elaborate the light-influenced mechanisms in microalgae and the light control strategies for photobioreactors from the inside out. Wavelength, light intensity and photoperiod solely or interactively affect biomass accumulation, pollutant removal, and value-added metabolite production in MBWT. Physiological processes, including photosynthesis, photooxidative damage, light-regulated gene expression, and nutrient uptake, essentially explain the performance influence of MBWT and are instructive for specific microalgal strain improvement strategies. In addition, light causes unique reactions in MBWT systems as it interacts with components such as photooxidative damage enhancers present in types of wastewater. In order to provide guidance for photobioreactor design and light control in a large-scale MBWT system, wavelength transformation, light transmission, light source distribution, and light-dark cycle should be considered in addition to adjusting the light source characteristics. Finally, based on current research vacancies and challenges, future research orientation should focus on the improvement of microalgae and photobioreactor, as well as the integration of both.

Predicting phosphorus accumulation and proposing conditions needed for an algal-based phosphorus uptake process

Algal-based waste stabilisation ponds (WSP) are a common wastewater treatment system for small communities but have poor phosphorus removal. Under certain conditions algae in WSPs will perform 'luxury uptake' increasing their phosphorus content to over 3% (gP/gSS) by storing polyphosphate. For the first time in the literature this paper presents a systematic study which determines the conditions needed to maximise phosphorus accumulation within WSP biomass taking into account the interactions between key variables. The key variables of temperature, phosphorus concentration, light intensity, mixing intensity, organic load, and pH were evaluated in 40 batch factorial experiments using a WSP algal culture. All six variables examined had significant main effects or interactions on the phosphorus content of the biomass. These were incorporated into a regression equation which was successfully validated against independent data sets from the literature. The conditions required to maximise the phosphorus content of the biomass were predicted for both summer (high light and high temperature) and winter (low light and low temperature) scenarios. The required conditions were revealed to be high phosphorus concentration, high mixing intensity, no supplementary CO2 addition, and low organic load. Interestingly, these conditions were consistent for both summer and winter suggesting that year-round treatment is possible. Practical methods of achieving these conditions were proposed. While further work will be needed to evaluate the effect of growth and potential influence of algal species, the findings presented provide a vital step towards developing a new phosphorus removal treatment process based on an enhanced understanding of environmental biotechnology.

LASSO Regression with Multiple Imputations for the Selection of Key Variables Affecting the Fatty Acid Profile of Nannochloropsis oculata

The marine microalga Nannochloropsis oculata has garnered significant interest as a potential source of lipids, both for biofuel and nutrition, containing significant amounts of C16:0, C16:1, and C20:5, n-3 (EPA) fatty acids (FA). Growth parameters such as temperature, pH, light intensity, and nutrient availability play a crucial role in the fatty acid profile of microalgae, with N. oculata being no exception. This study aims to identify key variables for the FA profile of N. oculata grown autotrophically. To that end, the most relevant literature data were gathered and combined with our previous work as well as with novel experimental data, with 121 observations in total. The examined variables were the percentages of C14:0, C16:0, C16:1, C18:1, C18:2, and C20:5, n-3 in total FAs, their respective ratios to C16:0, and the respective content of biomass in those fatty acids in terms of ash free dry weight. Many potential predictor variables were collected, while dummy variables were introduced to account for bias in the measured variables originating from different authors as well as for other parameters. The method of multiple imputations was chosen to handle missing data, with limits based on the literature and model-based estimation, such as using the software PHREEQC and residual modelling for the estimation of pH. To eliminate unimportant predictor variables, LASSO (Least Absolute Shrinkage and Selection Operator) regression analysis with a novel definition of optimal lambda was employed. LASSO regression identified the most relevant predictors while minimizing the risk of overfitting the model. Subsequently, stepwise linear regression with interaction terms was used to further study the effects of the selected predictors. After two rounds of regression, sparse refined models were acquired, and their coefficients were evaluated based on significance. Our analysis confirms well-known effects, such as that of temperature, and it uncovers novel unreported effects of aeration, calcium, magnesium, and manganese. Of special interest is the negative effect of aeration on polyunsaturated fatty acids (PUFAs), which is possibly related to the enzymatic kinetics of fatty acid desaturation under increased oxygen concentration. These findings contribute to the optimization of the fatty acid profile of N. oculata for different purposes, such as production of, high in PUFAs, food or feed, or production of, high in saturated and monounsaturated FA methyl esters (FAME), biofuels.

Effect of phosphorus limitation on Se uptake efficiency in the microalga Nannochloropsis oceanica

Microalgae are considered an efficient accumulator and promising source of Se for feed additive purposes. This study aimed at investigating, for the first time, the effect of phosphorus limitation on Se accumulation and uptake efficiency in N.oceanica. A range of phosphorus concentrations (0-2470 µM) were tested in either the presence or absence of sodium selenite (0, 5, 30 µM). Se accumulation was increased up to 16-fold and Se uptake efficiency was increased up to 3.6-fold under phosphorus growth-limiting concentrations. N.oceanica was then cultivated in a 1.8L flat-panel photobioreactor in batch operation under two phosphorus growth-limiting concentrations (250 and 750 µM) where the accumulation of Se in the microalgal biomass, as well as its presence in the spent medium were analysed. This study is the first to investigate the effect of phosphorus limitation for increasing Se accumulation in microalgae, and to prevent the release of Se in wastewater.

Microalgae growth in ultra-thin steady-state continuous photobioreactors: assessing self-shading effects

To disclose the net effect of light on microalgal growth in photobioreactors, self-shading and mixing-induced light–dark cycles must be minimized and discerned from the transient phenomena of acclimation. In this work, we performed experiments of continuous microalgal cultivation in small-scale photobioreactors with different thicknesses (from 2 to 35 mm): working at a steady state allowed us to describe the effect of light after acclimation, while the geometry of the reactor was adjusted to find the threshold light path that can discriminate different phenomena. Experiments showed an increased inhibition under smaller culture light paths, suggesting a strong shading effect at thicknesses higher than 8 mm where mixing-induced light–dark cycles may occur. A Haldane-like model was applied and kinetic parameters retrieved, showing possible issues in the scalability of experimental results at different light paths if mixing-induced light–dark cycles are not considered. To further highlight the influence of mixing cycles, we proposed an analogy between small-scale operations with continuous light and PBR operations with pulsed light, with the computation of characteristic parameters from pulsed-light microalgae growth mathematical modeling.

Dissolved organic phosphorus bioremediation from food-waste centrate using microalgae

Dissolved organic phosphorus (DOP) accounts for a substantial proportion of the total phosphorus remaining in the wastewater discharge and remains a concern for the receiving environment. This study assessed the potential of wastewater microalgae for the bioremediation of DOP from anaerobically digested food-waste centrate. For high DOP to low DIP ratio, the microalgal consortia was able to remove over 98% of DOP and 95% of total dissolved phosphorus. However, under a 1:1 ratio of DOP to DIP, the microalgal consortia was only able to remove 5% of the organic phosphorus and 76% of total dissolved phosphorus. All five main microalgal species were capable of producing alkaline phosphatase to some degree, the enzyme responsible for hydrolysing the phosphorus. For the dominant species Desmodesmus communis, total phosphatase activity reduced from 46.0 ± 2.3 mmol L^-1 h^-1 in axenic cultures to only 6.3 ± 0.7 mmol L^-1 h^-1 in presence of its microbiome. This resulted in a reduction in biomass from 209 ± 13 g m^-3 to 73 ± 5 g m^-3. For Tetradesmus dimorphus, alkaline phosphatase increased from 6.5 ± 0.3 mmol L^-1 h^-1 in the axenic culture to 169.8 ± 40.1 mmol L^-1 h^-1 in presence of both its microbiome and centrate-sourced bacteria but had little impact on biomass production. DOP removal rates across all five species, in all treatments ranged from 17 to 91%. With the exception of D. communis, the nutrient removal efficiency of DOP per unit biomass suggested luxury uptake of phosphorus into the microalgal cell. For wastewaters with low inorganic and moderate to high organic phosphorus microalgal-based wastewater treatment systems may offer a cost-effective mechanism for the removal and recovery of dissolved organic phosphorus from wastewater. Further research on refining organic phosphorus bioremediation in a range of wastewater types, particularly at pilot and full-scale, is needed.

Granular indigenous microalgal-bacterial consortium for wastewater treatment: Establishment strategy, functional microorganism, nutrient removal, and influencing factor

Granular indigenous microalgal-bacterial consortium (G-IMBC) system integrates the advantages of the MBC and granular activated sludge technologies, also with superior microalgal wastewater adaptation capacity. In this review, the concept of IMBC was firstly described, followed by its establishment and acclimation strategies. Characteristics and advantages of G-IMBC system compared to other IMBC systems (i.e., attached and floc IMBC systems) were then introduced. Moreover, the involved functional microorganisms and their interactions, as well as nutrient removal mechanisms were systematically and critically reviewed. Finally, the influencing factors including wastewater characteristics and operation factors were discussed. This study aims to provide a comprehensive up-to-date summary of the G-IMBC system for sustainable wastewater treatment.

Microalgae Cultivation on Nutrient Rich Digestate: The Importance of Strain and Digestate Tailoring under PH Control

The bioremediation of digestate using microalgae presents a solution to the current eutrophication issue in Northwest Europe, where the use of digestate as soil fertiliser is limited, thus resulting in an excess of digestate. Ammonium is the main nutrient of interest in digestate for microalgal cultivation, and improving its availability and consequent uptake is crucial for optimal bioremediation. This work aimed to determine the influence of pH on ammonium availability in cultures of two green microalgae, additionally screened for their growth performances on three digestates produced from different feedstocks, demonstrating the importance of tailoring a microalgal strain and digestate for bioremediation purposes. Results showed that an acidic pH of 6–6.5 resulted in a better ammonium availability in the digestate media, translated into better growth yields for both S. obliquus (GR: 0.099 ± 0.001 day−1; DW: 0.23 ± 0.02 g L−1) and C. vulgaris (GR: 0.09 ± 0.001 day−1; DW: 0.49 ± 0.012 g L−1). This result was especially true when considering larger-scale applications where ammonium loss via evaporation should be avoided. The results also demonstrated that digestates from different feedstocks resulted in different growth yields and biomass composition, especially fatty acids, for which, a digestate produced from pig manure resulted in acid contents of 6.94 ± 0.033% DW and 4.91 ± 0.3% DW in S. obliquus and C. vulgaris, respectively. Finally, this work demonstrated that the acclimation of microalgae to novel nutrient sources should be carefully considered, as it could convey significant advantages in terms of biomass composition, especially fatty acids and carbohydrate, for which, this study also demonstrated the importance of harvesting time.

Fluoxetine and Nutrients Removal from Aqueous Solutions by Phycoremediation

The tertiary treatment using microalgae offers an attractive alternative to the removal of low but relevant concentrations of pharmaceuticals from domestic wastewaters. The removal of fluoxetine from aqueous solutions by living and non-living (lyophilized) Chlorella vulgaris was assessed. The determination of the pH at the point of zero charge, Fourier transmittance infrared analysis, and scanning electron microscopy were performed to characterize the microalgae biomass. Kinetic and equilibrium experiments were performed. The pseudo-second-order model described the kinetics of fluoxetine. The corresponding kinetic constants indicated that biosorption was faster onto non-living biomass than onto living biomass. The equilibrium results showed that the systems followed the Langmuir isotherm model. The maximum capacity of living microalgae (1.9 ± 0.1 mg·g⁻¹) was slightly higher than the non-living microalgae (1.6 ± 0.2 mg·g⁻¹). Living Chlorella vulgaris, free and immobilized in calcium-alginate, were also used to remove fluoxetine and nutrients (nitrogen and phosphorus) from treated municipal wastewater in a batch system. In both experiments, fluoxetine was completely removed within six days. The total phosphorus (TP) and total nitrogen (TN) removal efficiencies achieved for free and immobilized cells were, null and 65.0 ± 0.1%, and 86.2 ± 0.1% and 81.8 ± 3.1, respectively.

Effect of phosphorus concentration and light/dark condition on phosphorus uptake and distribution with microalgae

In this study, the effects of P concentration and light/dark condition on the distribution of P in microalgae were tracked with Scenedesmus sp.393. Results showed that different culture conditions affected the accumulation capacity and transformation of P in intracellular polymeric substances (IPS), extracellular polymeric substances (EPS), and soluble microbial products (SMP). At low P concentration (0.70 mg P/L), inorganic phosphorus (IP) absorbed in EPS (19.40%) and organic phosphorus (OP) accumulated in IPS (70.98%) were mainly P forms in microalgae. High P concentration (>21.42 mg P/L) promoted the luxury uptake and accumulation of IP by IPS, and the conversion of IP to OP. However, the adsorption of IP by EPS was inhibited when exposed to high external P concentration. Continuous illumination promoted the microalgae growth, and dark condition stimulated the P accumulation in microalgae biomass. The results of this study could provide valuable information for P recovery with microalgae.

Analysis of the Status and Improvement of Microalgal Phosphorus Removal from Municipal Wastewater

Phosphorus, as one of the main pollutants in municipal sewage, has received increasing attention recently. Phosphorus recovery also increases the sustainable development of municipal wastewater. Since algae have the ability to effectively redirect nutrients, including phosphorus, from municipal sewage to algae biomass, municipal sewage treatments involving microalgae have piqued the interest of many researchers. The phosphorus removal depends on the potential of the microalgae to absorb, preserve, or degrade phosphorus in municipal wastewater. It is, therefore, of great interest to study the mechanisms underlying the absorption, storage, and degradation of phosphorus by microalgae to ensure the viability of this phosphorus removal process in wastewater. The objectives of this review were to summarize phosphorus metabolism in microalgae, examine key external and internal factors impacting phosphorous removal by microalgae from wastewater, and examine the status of phosphorous-metabolism-related research to improve our understanding of microalgae-based municipal wastewater treatments. In addition, the methods of recovery of microalgae after phosphorous removal were summarized to ensure the sustainability of municipal wastewater treatment. Finally, a potential approach using nanomaterials was proposed to enhance the overall phosphorous removal performance in municipal wastewater through the addition of nanoparticles such as magnesium and iron.

Revisiting carbon, nitrogen, and phosphorus metabolisms in microalgae for wastewater treatment

Threats posed to humans - including environmental pollution, water scarcity, food shortages, and resource crises drive a new concept to think about wastewater and its treatment. Wastewater is not only a waste but also a source of energy, renewable and/or non-renewable resources, including water itself. The nutrient in wastewater should not only be removed but also need to be upcycled. Microalgae based wastewater treatment has attracted considerable interests because algae have the potential to efficiently redirect nutrients from wastewater to the accumulated algal biomass. Additionally, microalgae are commercialized in human consumption and animal feed owing to their high content of essential amino and fatty acids, vitamins, and pigments. The whole process establishes a circular economy, totally relying on the ability of microalgae to uptake and store nutrients in wastewater, such as carbon (C), nitrogen (N), and phosphorus (P). It makes the study of the mechanisms underlying the uptake and storage of nutrients in microalgae of great interest. This review specifically aims to summarize C, N, and P metabolisms in microalgae for a better understanding of the microalgae-based wastewater treatment from the nutrient uptake pathway, and examine the key physiological factors or the operating conditions related to nutrient metabolisms that may affect the treatment efficiency. At last, I discuss the potential approaches to enhance the overall treatment performance by adjusting the critical parameters for C, N, and P metabolisms.

Proteomic and Transcriptomic Patterns during Lipid Remodeling in Nannochloropsis gaditana

Nutrient limited conditions are common in natural phytoplankton communities and are often used to increase the yield of lipids from industrial microalgae cultivations. Here we studied the effects of bioavailable nitrogen (N) and phosphorus (P) deprivation on the proteome and transcriptome of the oleaginous marine microalga Nannochloropsis gaditana. Turbidostat cultures were used to selectively apply either N or P deprivation, controlling for variables including the light intensity. Global (cell-wide) changes in the proteome were measured using Tandem Mass Tag (TMT) and LC-MS/MS, whilst gene transcript expression of the same samples was quantified by Illumina RNA-sequencing. We detected 3423 proteins, where 1543 and 113 proteins showed significant changes in abundance in N and P treatments, respectively. The analysis includes the global correlation between proteomic and transcriptomic data, the regulation of subcellular proteomes in different compartments, gene/protein functional groups, and metabolic pathways. The results show that triacylglycerol (TAG) accumulation under nitrogen deprivation was associated with substantial downregulation of protein synthesis and photosynthetic activity. Oil accumulation was also accompanied by a diverse set of responses including the upregulation of diacylglycerol acyltransferase (DGAT), lipase, and lipid body associated proteins. Deprivation of phosphorus had comparatively fewer, weaker effects, some of which were linked to the remodeling of respiratory metabolism.

Mobilization and Cellular Distribution of Phosphate in the Diatom Phaeodactylum tricornutum

Unicellular organisms that live in marine environments must cope with considerable fluctuations in the availability of inorganic phosphate (P_i). Here, we investigated the extracellular P_i concentration-dependent expression, as well as the intracellular or extracellular localization, of phosphatases and phosphate transporters of the diatom Phaeodactylum tricornutum. We identified P_i-regulated plasma membrane-localized, ER-localized, and secreted phosphatases, in addition to plasma membrane-localized, vacuolar membrane-localized, and plastid-surrounding membrane-localized phosphate transporters that were also regulated in a P_i concentration-dependent manner. These studies not only add further knowledge to already existing transcriptomic data, but also highlight the capacity of the diatom to distribute P_i intracellularly and to mobilize P_i from extracellular and intracellular resources.

Biotransformation of phosphorus in enhanced biological phosphorus removal sludge biochar

Biochar derived from enhanced biological phosphorus removal (EBPR) sludge could be a potential phosphorus (P) fertilizer. Soil microorganisms play a regulating role on the turnover of P in soil. When the EBPR sludge biochar is added to soil, it would inevitably interact with soil microorganisms. Thus, for the wise use of the EBPR sludge biochar, it is imperative to understand the interaction between the biochar and soil microorganisms. In this study, Pseudomonas putida (P. putida), a common soil microorganism, was applied to investigate the biotransformation of P in two EBPR sludge biochars. The results reveal that P released from biochar produced at 700 °C (E700) was more easily absorbed by P. putida than that released from biochar produced at 400 °C (E400). This is attributed to the higher polyphosphates (poly-P) content in E700 and poly-P has higher affinity to P. putida surface compared to orthophosphates. Furthermore, E400 has a negative effect on intracellular poly-P formation in P. putida, which is probably caused by the oxidative stress induced by the free radicals from E400. As intracellular poly-P plays a critical role on bacteria survival and their interaction with surrounding environment, high-temperature biochar (E700) in this case would be more suitable for soil remediation.

Phosphorus-Induced Lipid Class Alteration Revealed by Lipidomic and Transcriptomic Profiling in Oleaginous Microalga Nannochloropsis sp. PJ12

Phytoplankton are primary producers in the marine ecosystem, where phosphorus is often a limiting factor of their growth. Hence, they have evolved strategies to recycle phosphorus by replacing membrane phospholipids with phosphorus-free lipids. However, mechanisms for replacement of lipid classes remain poorly understood. To improve our understanding, we performed the lipidomic and transcriptomic profiling analyses of an oleaginous marine microalga Nannochloropsis sp. PJ12 in response to phosphorus depletion (PD) and replenishing. In this study, by using (liquid chromatography couple with tandem mass spectrometry) LC-MS/MS-based lipidomic analysis, we show that membrane phospholipid levels are significantly reduced upon PD, while phosphorus-free betaine lipid levels are increased. However, levels of phosphorus-free photosynthetic galactolipid and sulfolipid are not increased upon PD, consistent with the reduced photosynthetic activity. RNA-seq-based transcriptomic analysis indicates that enzymes involved in phospholipid recycling and phosphorus-free lipid synthesis are upregulated, supporting the lipidomic analysis. Furthermore, enzymes involved in FASII (type II fatty acid synthesis) elongation cycle upon PD are transcriptionally downregulated. EPA (eicosapentaenoic acid) level decrease upon PD is revealed by both GC-MS (gas chromatography coupled with mass spectrometry) and LC-MS/MS-based lipidomic analyses. PD-induced alteration is reversed after phosphorus replenishing. Taken together, our results suggest that the alteration of lipid classes upon environmental change of phosphorus is a result of remodeling rather than de novo synthesis in Nannochloropsis sp. PJ12.

Phosphorus Enhances Photosynthetic Storage Starch Production in a Green Microalga (Chlorophyta) Tetraselmis subcordiformis in Nitrogen Starvation Conditions

Microalgae are potential starch producers as alternatives to agricultural crops. This study disclosed the effects and mechanism of phosphorus availability exerted on storage starch production in a starch-producing microalga Tetraselmis subcordiformis in nitrogen starvation conditions. Excessive phosphorus supply facilitated starch production, which differed from the conventional cognition that phosphorus would inhibit transitory starch biosynthesis in plants. Phosphorus enhanced energy utilization efficiency for biomass and storage starch production. ADP-glucose pyrophosphorylase (AGPase), conventionally known to be critical for starch biosynthesis, was negatively correlated to storage starch biosynthesis. Excessive phosphorus supply maintained large cell volumes, enhanced activities of starch phosphorylases (SPs) along with branching enzymes and isoamylases, and increased phosphoenolpyruvate and trehalose-6-phosphate levels to alleviate the inhibition of high phosphate availability to AGPase, all of which improved starch production. This work highlighted the importance of phosphorus in the production of microalgal starch and provided further evidence for the SP-based storage starch biosynthesis pathway.