Effect of light intensity and phosphate concentration on the storage and utilization of phosphate in <i>Chlorella vulgaris</i>

Jae Hee Huh; Su-Min Kwon; Kyu-Hyun Park; Eun Hea Jho; Sun-Jin Hwang

doi:10.4491/eer.2025.050

Environ Eng Res > Volume 30(6); 2025 > Article

Huh, Kwon, Park, Jho, and Hwang: Effect of light intensity and phosphate concentration on the storage and utilization of phosphate in Chlorella vulgaris

Research

Environmental Engineering Research 2025; 30(6): 250050.

Published online: March 5, 2025

DOI: https://doi.org/10.4491/eer.2025.050

Effect of light intensity and phosphate concentration on the storage and utilization of phosphate in Chlorella vulgaris

Jae Hee Huh^1,^*, Su-Min Kwon^1,^*, Kyu-Hyun Park¹, Eun Hea Jho², Sun-Jin Hwang^1,^†

¹College of Engineering, Department of Environmental Science and Engineering, Kyung Hee University, Yongin 17104, Republic of Korea

²Department of Agricultural and Biological Chemistry, Chonnam National University, Gwangju 61186, Republic of Korea

^†Corresponding author: E-mail: sjhwang@khu.ac.kr, Tel: +82-31-201-2497, ORCID: 0000-0003-3865-4811

* These authors contributed equally to this work

Received January 23, 2025 Revised March 2, 2025 Accepted March 4, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This study aims to analyze the effects of light intensity and phosphate concentration on the phosphorus metabolism of microalgae, focusing on the balance between acid soluble phosphate (ASP) and acid insoluble phosphate (AISP). From the changes in the storage and utilization of ASP and AISP under varying light conditions and external phosphorus concentrations, a conceptual model for the intracellular phosphorus metabolism of Chlorella vulgaris was proposed. Changes in photosynthesis and respiration during the utilization of ASP and AISP stored within microalgal cells revealed that higher light intensities led to greater use of ASP in photosynthetic metabolism. The increased utilization of ASP also activated photosynthetic metabolism. In contrast, more AISP was stored at lower light intensities, and the stored AISP was utilized for photosynthesis and ATP production following the depletion of external phosphorus. Furthermore, AISP utilization following external phosphorus depletion was higher under low light conditions, with increased AISP consumption correlating with reduced cellular respiration. These findings indicate that light intensity and external phosphorus concentration significantly influence the storage forms of ASP and AISP in microalgae, thereby affecting their metabolic activity.

Keywords: Acid insoluble phosphate (AISP), Acid soluble phosphate (ASP), Chlorella vulgaris, Light intensity, Photosynthesis and Respiration (P&R) analysis

Graphical Abstract

Keywords: Acid insoluble phosphate (AISP), Acid soluble phosphate (ASP), Chlorella vulgaris, Light intensity, Photosynthesis and Respiration (P&R) analysis

1. Introduction

Recent research has increasingly focused on using microalgae in biological wastewater treatment processes. Microalgae exhibit superior phosphorus removal efficiency compared to conventional bacterial-based treatment methods and provide economic advantages through their photosynthetic nature, eliminating the need for separate aeration devices. Microalgae grow by photosynthetic metabolism, using light energy to produce organic compounds necessary for cellular components and energy storage. Light energy is critical to their growth and activity. Microalgae utilize phosphorus for energy metabolism and for synthesizing cellular components such as DNA, nucleic acids, and phospholipids, and phosphorus is utilized in the form of ATP for biosynthesis of biomolecules in this process [1]. Additionally, phosphorus is vital for microalgal growth, contributing significantly to ATP production via phosphorylation pathways during photosynthesis and cellular respiration [2].

Although recent studies have experimentally demonstrated the wastewater treatment capabilities of microalgae under specific light conditions and nutrient concentrations, most have focused on the removal of carbon and nitrogen. Research on the mechanisms of wastewater treatment by microalgae is still limited. Specifically, there is a lack of research focusing on the intracellular storage and utilization of phosphorus in photosynthesis and cellular respiration by microalgae [3–5]. While several studies have explored changes in intracellular phosphorus content under various algal culture conditions [4, 6], there remains a significant gap in research that quantitatively evaluates the characteristics of phosphorus utilization from a metabolic perspective.

Fig. 1 presents a schematic representation of the forms of stored phosphorus and their utilization patterns. Microalgae typically store ingested phosphorus within the cytoplasm or vacuoles in the form of polyphosphate (poly-P). The storage form of poly-P varies significantly, predominantly existing as either ASP or AISP, depending on the light conditions and phosphorus concentrations [7].

The intracellular storage and utilization of ASP predominantly depend on photosynthesis, meaning ASP is metabolized when light and external phosphorus are abundant. In contrast, unlike ASP, which is stored through photosynthesis, AISP is stored and utilized mainly based on external phosphorus concentration [8]. AISP is primarily utilized when external phosphorus concentrations are low [9] and serves as a phosphorus source during RNA synthesis under conditions of external phosphorus scarcity and absence of light.

When microalgae internalize ingested phosphorus, the storage form varies with light intensity and external phosphorus concentrations, which in turn influences its utilization in photosynthesis and cellular respiration metabolism [10]. Therefore, understanding the mechanisms of phosphorus storage and utilization in microalgae under varying light conditions is essential when researching phosphorus removal using microalgae. This study aims to evaluate the phosphorus storage characteristics and utilization patterns in photosynthesis and cellular respiration metabolism of Chlorella vulgaris, focusing on ASP and AISP, in response to changes in light intensity and phosphorus concentrations. Specifically, the study aims to evaluate changes in the quantity of phosphorus utilized from intracellular stores under varying light intensities, and to quantitatively analyze the effects on photosynthesis and cellular respiration during the utilization of stored ASP and AISP. Furthermore, by measuring photosynthesis and cellular respiration, the study seeks to identify variations in the metabolic activity of C. vulgaris under varying light and phosphorus conditions.

2. Materials and Methods

2.1. Microalgae Cultivation

In this study, the species C. vulgaris, selected as the target microalgae, was obtained from the Korean Collection for Type Culture (KCTC). The BG-11 medium was utilized for the cultivation of C. vulgaris, and its composition is presented in Table 1.

Chlorella vulgaris was inoculated into BG-11 medium and cultured with increasing volumes of medium from 50 to 500 mL. The subcultures were carried out in a photobioreactor (HB-305M, Hanbeak CO, KOREA) maintaining a temperature of 25±1°C and a light intensity of 100 photosynthetic photon flux density (PPFD).

2.2. Pre-cultivation Conditions

In this experiment, C. vulgaris was subject to a 6-d pre-cultivation period to acclimatize the algae to the experimental conditions. A critical phenomenon in phosphorus assimilation in microalgae is the overcompensation mechanism, whereby algae exposed to phosphorus limitation initially exhibit an increase rate of phosphorus uptake upon subsequent exposure to phosphorus. During phosphorus deficiency, microalgae metabolize the phosphorus stored within their cells, leading to a depletion of intracellular phosphorus. Upon re-exposure to phosphorus, these algae compensate by absorbing an excess of phosphorus to restore their internal phosphorus balance [11]. In this study, light intensity and phosphorus concentration were used as variables to investigate the characteristics of phosphorus storage in microalgae and its subsequent effects on photosynthesis and respiration (microalgal cellular respiration). To mitigate the potential for overcompensation, C. vulgaris was cultured under sufficient phosphorus conditions during the pre-cultivation period. Furthermore, the microalgae were exposed to optimal light intensity at 150 PPFD, with the pH stabilized using 0.5 N H₂SO₄ throughout the experimental setup.

2.3. Experimental Conditions for Phosphorus Storage and Utilization in Microalgae

Following the pre-cultivation phase, C. vulgaris was subjected to centrifugation and subsequent washing before being utilized in the main experiment. The experiment was conducted over a 13-day period using a fed-batch system with a reactor capacity of 2 L. The initial inoculation density was set uniformly at 0.3 OD for both conditions, and pH was maintained at a constant level using 0.5 N H₂SO₄ for pH adjustment. The detailed experimental conditions are presented in Table 2.

2.4. Extraction and Analysis Methods for ASP and AISP

The storage form of poly-P in microalgae is influenced by both light conditions and external phosphorus concentrations. The storage and utilization of ASP, which is readily soluble in acids, are predominantly governed by photosynthesis [12]. It is well-documented that ASP is metabolized in conditions of abundance light and external phosphorus. In contrast, AISP, or acid-insoluble polyphosphate, dissolves more readily under alkaline conditions. The extraction methods for each form, as described by Aitchison and Butt [13] are outlined below.

To determine the ASP content, microalgae samples were centrifuged, and the supernatant was discarded. The resulting pellet was resuspended in 20 mL of distilled water, followed by a second centrifugation, which was repeated twice. The remaining microalgae sample was subsequently treated with 10 mL of 10% trichloroacetic acid, suspended, and incubated at −20°C for 10 min before undergoing centrifugation; this procedure was repeated twice. The first and second supernatants were combined to yield a total volume of 20 mL. The pH of the combined supernatant was neutralized using a 3 N KOH solution. The ASP content was then quantified using a Total Phosphorus (T-P) kit, and the concentration was normalized to biomass (OD) to calculate the content value.

For AISP extraction, the residual microalgae sample, following ASP extraction, was treated with 15 mL of 2 N KOH and incubated at room temperature for 10 min; this procedure was repeated twice. Following centrifugation, the first and second supernatants were combined to yield a total volume of 30 mL. The pH of the combined supernatant was neutralized using a 3 N HCl solution. The AISP content was quantified using the same T-P kit as used for the ASP measurement. The concentration of AISP was then normalized to biomass (OD) to calculate the content value.

2.5. Device and Calculation for Measuring the P&R Rate

A Photosynthesis and Respiration(P&R) analyzer as depicted in Fig. 2, specifically designed to measure the photosynthesis and respiration rates of C. vulgaris was used in this study. The P&R analyzer features LEDs that provide approximately 150 PPFD of light within its main body, which is enclosed in a black box-shaped exterior serving to block external light under dark conditions. The reaction vessel inserted into the main body of the analyzer has dimensions of 90 mm(diameter) by 93 mm(height), with a working volume of 430 mL. The dissolved oxygen (DO) meter integrated into the device continuously records the changes in DO associated with the photosynthetic and respiratory processes of the microalgae. An electrode connected to the DO meter was directly inserted into the sample to measure the variations in DO.

The DO decreases over time in dark conditions, while changes in DO occur due to the difference between the oxygen produced by photosynthesis and the oxygen consumed by respiration in light conditions. The change in DO per unit time, indicative of microalgal activity, was measured using the P&R analyzer for 15 min each in both the unlit dark conditions and under 150 PPFD of LED lighting in light conditions. The change in DO in dark conditions was used to calculate the microalgal respiration rate, while the change in DO under light conditions was used to calculate the net photosynthesis rate. Additionally, the sum of the DO changes in both dark and light conditions was calculated as the total photosynthesis. The P&R rates based on changes in DO per unit time was calculated using the equations shown in Table 3 [14].

3. Results and Discussion

3.1. Changes in ASP and AISP Content Based on Phosphorus Concentration and Light Intensity

Phosphorus uptake in microalgae is influenced by environmental factors such as phosphorus concentration and light intensity, with absorbed phosphorus being stored internally in the form of poly-P [11]. The stored poly-P is partitioned into ASP and AISP, depending on the light irradiation conditions and phosphorus concentrations [4]. This study compares the changes in the content of ASP and AISP in response to variations in light intensity and phosphorus concentration.

3.1.1. Changes in ASP content

The effect of light intensity and phosphorus concentration on the ASP content in C. vulgaris is illustrated in Fig. 3(a). From a water treatment perspective, the removal of phosphate phosphorus was more efficient under high light intensity conditions. No significant difference in ASP content was observed between the 40 PPFD and 200 PPFD conditions (Fig. 3(a)). Additionally, consistent with the pattern of phosphorus consumption in the medium, the ASP content decreased over time. According to Powell et al. [4], high light intensity enhances the storage and utilization of ASP in photosynthetic metabolism. It was anticipated that internal ASP storage at 200 PPFD would exceed that at 40 PPFD, proportional to the phosphorus uptake in the medium. However, contrary to these expectations, no significant difference in ASP content was observed. This may be attributed to the rapid utilization of stored ASP in photosynthetic metabolism, particularly at higher light intensities [15]. It is therefore hypothesized that at 200 PPFD, the amount of ASP utilized in photosynthetic metabolism was comparable to the amount stored, and this hypothesis is further supported by the results on photosynthetic rates in relation to ASP utilization presented below.

3.1.2. Changes in AISP content

AISP differs from ASP in its storage and utilization patterns within the microalgal cells; whereas ASP is stored in the cytoplasm and is immediately utilized in photosynthesis and metabolism, AISP is stored in the vacuole and only metabolized when external phosphorus becomes depleted [16]. The effect of light intensity and phosphorus concentration on the AISP content in C. vulgaris is presented in Fig. 3(b). The AISP content at 40 PPFD was approximately 1.7 times greater than that at 200 PPFD. Both the 40 PPFD and 200 PPFD conditions exhibited an increasing trend in AISP content until phosphorus depletion, followed by a decrease in stored AISP once phosphorus was exhausted. This pattern suggests that the stored AISP was metabolized in the absence of available external phosphorus sources. According to Wu et al. [17], lower light intensity increases the storage of phosphorus as AISP, whereas higher light intensity promotes the storage and utilization of ASP, as shown in Fig. 3(a). The results presented in Fig. 3(b) indicated that the AISP content before phosphorus depletion was higher under 40 PPFD, which aligns with previous research expectations. However, contrary to the assumption that higher light intensity would necessitate more AISP for active metabolism, the utilization of stored AISP was found to be greater under the 40 PPFD condition than under 200 PPFD. This observation is further elucidated in the subsequent discussion on the respiration rate data related to AISP utilization.

3.2. Photosynthetic and Respiration Rates Associated with the Utilization of ASP and AISP

The photosynthetic rates were measured using a P&R analyzer to assess the utilization of ASP in photosynthetic metabolism under varying light conditions. Due to the concurrent storage and utilization of ASP in photosynthesis in microalgae [18], it proved difficult to precisely quantify the amount of ASP utilized in photosynthetic metabolism based solely on changes in content. Higher light intensities are known to enhance both the storage and utilization of ASP in photosynthetic metabolism, promoting the hypothesis that increased ASP utilization would be linked to an enhancement in photosynthetic activity. A comparative evaluation was conducted by measuring the photosynthetic rates with the P&R analyzer. This methodology facilitated the analysis of ASP utilization and its corresponding effects on photosynthetic metabolism in microalgae under different light conditions, with the results presented in Fig. 4.

3.2.1. Photosynthetic rate associated with ASP storage and utilization

The utilization of ASP and its associated photosynthetic rate in microalgae, under varying light intensities and phosphorus concentrations, were quantified and presented in Fig. 4. ASP utilization was notably high during the first 2 days for both the 40 PPFD and 200 PPFD conditions. However, the ASP utilization at 200 PPFD was approximately 1.4 times higher than that at 40 PPFD. Similarly, the photosynthetic rates were highest during the initial 0–2 days, with the rate at 200 PPFD being approximately 1.7 times higher than at 40 PPFD. This confirms the correlation presented in Fig. 3 between the utilization of stored ASP in photosynthetic metabolism and photosynthetic rate, suggesting a direct relationship between higher ASP utilization and increased photosynthetic activity. Moreover, the photosynthetic rate at 200 PPFD gradually decreased following phosphorus depletion, eventually aligning with the rate at 40 PPFD. This observed trend indicates that, as the phosphorus in the medium was depleted, the amount of stored ASP diminished, leading to a subsequent reduction in photosynthetic rate.

3.2.2. Changes in respiration rate associated with AISP storage and utilization

Microalgae convert stored AISP into ATP when external phosphorus sources are depleted [19]. It is hypothesized that a higher amount of stored AISP leads to an increased availability of ATP. To explore this relationship between AISP metabolism and cellular respiration, cellular respiration was measured using a P&R analyzer, specifically to assess ATP production mechanisms. AISP utilization at 40 PPFD was found to be approximately 1.9 times higher than at 200 PPFD. However, the trend in respiration rates reveals that the respiration rate at 40 PPFD was lower than at 200 PPFD, suggesting an inverse relationship between AISP utilization and respiration rate. Additionally, a marked increase in respiration rate was observed under the 200 PPFD condition, starting at the point of phosphorus depletion in the medium. This phenomenon can be explained by the metabolic products of microalgal cellular respiration. During cellular respiration, microalgae synthesize ATP in their mitochondria, which is necessary for photosynthesis [20], and use the synthesized ATP to produce glucose during the photosynthetic process. Furthermore, when external phosphorus sources are depleted, stored AISP is mobilized to support ATP synthesis. Consequently, it is inferred that the lower AISP utilization at 200 PPFD resulted in an increased respiration rate to facilitate ATP synthesis for continued metabolism. Based on these experimental results and related theories, a conceptual diagram depicting the storage and metabolic utilization mechanisms of ASP and AISP in microalgae, as influenced by light intensity and phosphorus concentration, is presented in Fig. 5.

4. Conclusion

This study evaluated the phosphorus removal efficiency of C. vulgaris under varying light intensity and phosphorus concentration conditions, with a focus on water treatment applications. Additionally, the characteristics of phosphorus storage and utilization within the microalgae were analyzed in terms of ASP and AISP. Changes in ASP and AISP contents, along with photosynthesis and respiration rates, were examined to elucidate the phosphorus metabolism mechanisms of the microalgae. Phosphorus removal efficiency was higher under high light conditions (200 PPFD) compared to low light intensity (40 PPFD). However, the internal phosphorus storage and utilization characteristics of the microalgae revealed no significant difference in ASP storage across the light conditions. This finding is attributed to the simultaneous increase in ASP storage and its metabolic utilization under higher light intensities. In contrast, significant differences were observed in AISP storage; storage and metabolic utilization rates of AISP were approximately 1.9 times higher under low light conditions than under high light conditions.

Correspondingly, under high light conditions, ASP utilization predominated, resulting in a photosynthesis rate approximately 1.7 times higher than under low light. This indicates a proportional relationship between ASP utilization and photosynthetic activity. Conversely, higher AISP utilization correlated with lower respiration rates, suggesting an inverse relationship. This phenomenon implies that in conditions of limited AISP utilization, increased respiration was required to compensate for insufficient ATP, particularly through the utilization of stored AISP when external phosphorus sources were depleted.

This study provides critical insights into the phosphorus removal efficiency and the internal phosphorus storage and utilization mechanisms of C. vulgaris under different light and phosphorus conditions. These findings aim to inform the development of efficient phosphorus removal strategies that optimize light conditions and phosphorus concentrations in influent water for advanced wastewater treatment using microalgae. Furthermore, the proposal of cost-effective water treatment conditions is anticipated to stimulate further research in this field, thereby contributing to the advancement and refinement of microalgae-based wastewater treatment systems.

Notes

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2024-00350563). This research is (partially) funded by the BK21 FOUR program of Graduate School, Kyung Hee University (GS-1-JO-NON-20240394).

Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

J.H.H. (MS student) and S.-M.K. (MS student) engaged in experiments, data curation, and writing the manuscript. E.H.J. (Professor) and K.-H.P. (MS student) reviewed and edited the manuscript. S.-J.H. (Professor) supervised the experiments and revised the manuscript.

References

1. Dyhrman ST. Nutrients and their acquisition: phosphorus physiology in microalgae. The physiology of microalgae. 2016;155–183.

2. Lin S, Litaker RW, Sunda WG. Phosphorus physiological ecology and molecular mechanisms in marine phytoplankton. J. Phycol. 2016;52(1)10–36. https://doi.org/10.1111/jpy.12365

3. Martin GJ, Hill DR, Olmstead IL, et al. Lipid profile remodeling in response to nitrogen deprivation in the microalgae Chlorella sp.(Trebouxiophyceae) and Nannochloropsis sp.(Eustigmatop hyceae). PLoS One. 2014;9(8)e103389. https://doi.org/10.1371/journal.pone.0103389

4. Powell N, Shilton A, Chisti Y, Pratt S. Towards a luxury uptake process via microalgae–defining the polyphosphate dynamics. Water Res. 2009;43(17)4207–4213. https://doi.org/10.1016/j.watres.2009.06.011

5. Cembella AD, Antia NJ, Harrison PJ. The utilization of inorganic and organic phosphorous compounds as nutrients by eukaryotic microalgae: A multidisciplinary perspective: Part I. Crit. Rev. Microbiol. 1982;10(4)317–391. https://doi.org/10.3109/10408418209113567

6. Wang X, Wang X, Hui K, et al. Highly effective polyphosphate synthesis, phosphate removal, and concentration using engineered environmental bacteria based on a simple solo medium-copy plasmid strategy. Environ. Sci. Technol. 2018;52(1)214–222. https://doi.org/10.1021/acs.est.7b04532

7. Solovchenko A, Verschoor AM, Jablonowski ND, Nedbal L. Phosphorus from wastewater to crops: An alternative path involving microalgae. Biotechnol. Adv. 2016;34(5)550–564. https://doi.org/10.1016/j.biotechadv.2016.01.002

8. Whitton R, Ometto F, Pidou M, Jarvis P, Villa R, Jefferson B. Microalgae for municipal wastewater nutrient remediation: mechanisms, reactors and outlook for tertiary treatment. Environ. Technol. Rev. 2015;4(1)133–148. https://doi.org/10.1080/21622515.2015.1105308

9. John EH, Flynn KJ. Modelling phosphate transport and assimilation in microalgae; how much complexity is warranted? Ecol. Modell. 2000;125(2–3)145–157. https://doi.org/10.1016/S0304-3800(99)00178-7

10. Sforza E, Calvaruso C, La Rocca N, Bertucco A. Luxury uptake of phosphorus in Nannochloropsis salina: Effect of P concentration and light on P uptake in batch and continuous cultures. Biochem. Eng. J. 2018;134:69–79. https://doi.org/10.1016/j.bej.2018.03.008

11. Eixler S, Karsten U, Selig U. Phosphorus storage in Chlorella vulgaris (Trebouxiophyceae, Chlorophyta) cells and its dependence on phosphate supply. Phycologia. 2006;45(1)53–60. https://doi.org/10.2216/04-79.1

12. Brown N, Shilton A. Luxury uptake of phosphorus by microalgae in waste stabilisation ponds: current understanding and future direction. Rev. Environ. Sci. Bio/Technol. 2014;13:321–328. https://doi.org/10.1007/s11157-014-9337-3

13. Aitchison P, Butt V. The relation between the synthesis of inorganic polyphosphate and phosphate uptake by Chlorella vulgaris. J. Exp. Bot. 1973;24(3)497–510. https://doi.org/10.1093/jxb/24.3.497

14. Huh J-H, Sim T-S, Hwang S-J. Photosynthesis and Respiration (P&R) analyzer analysis optimization for microalgal activity evaluation. J. Korean Soc. Water Environ. 2021;37(6)449–457. https://doi.org/10.15681/KSWE.2021.37.6.449

15. Su Y. Revisiting carbon, nitrogen, and phosphorus metabolisms in microalgae for wastewater treatment. Sci. Total Environ. 2021;762:144590. https://doi.org/10.1016/j.scitotenv.2020.144590

16. Shebanova A, Ismagulova T, Solovchenko A, et al. Versatility of the green microalga cell vacuole function as revealed by analytical transmission electron microscopy. Protoplasma. 2017;254:1323–1340. https://doi.org/10.1007/s00709-016-1024-5

17. Wu Q, Guo L, Li X, Wang Y. Effect of phosphorus concentration and light/dark condition on phosphorus uptake and distribution with microalgae. Bioresour. Technol. 2021;340:125745. https://doi.org/10.1016/j.biortech.2021.125745

18. Liu J, Wu Y, Wu C, et al. Advanced nutrient removal from surface water by a consortium of attached microalgae and bacteria: a review. Bioresour. Technol. 2017;241:1127–1137. https://doi.org/10.1016/j.biortech.2017.06.054

19. Xie L, Jakob U. Inorganic polyphosphate, a multifunctional polyanionic protein scaffold. J. Biol. Chem. 2019;294(6)2180–2190. https://doi.org/10.1074/jbc.REV118.002808

20. Markou G, Vandamme D, Muylaert K. Microalgal and cyanobacterial cultivation: The supply of nutrients. Water Res. 2014;65:186–202. https://doi.org/10.1016/j.watres.2014.07.025

Fig. 1

Phosphorus uptake and storage mechanisms in microalgae.

Fig. 2

Photosynthesis and Respiration(P&R) analyzer used in this study. (①LED controller, ②Photometer, ③Main reactor box on a magnetic stirrer, ④DO meter, ⑤DO probe).

Fig. 3

Characteristics of ASP(a) and AISP(b) storage and utilization in C. vulgaris in response to changes in light intensity and phosphorus concentration. (The point at which phosphorus in the medium was depleted is indicated by a dashed line).

Fig. 4

Changes in ASP(a) and AISP(b) utilization and photosynthetic rate in C. vulgaris according to light intensity and phosphorus concentration. (Dashed lines indicate the point of phosphorus depletion for each condition).

Fig. 5

Mechanism of phosphorus storage and utilization as ASP(a) and AISP(b) in microalgae under high / low light intensity.

Table 1

Composition of BG-11 medium used for cultivation of Chlorella vulgaris.

Composition	Concentration (mg/L)
NaNO₃	1,500
K₂HPO₄	40
MgSO₄·7H₂O	75
CaCl₂·2H₂O	36
Citric Acid	6
Ferric Ammonium Citrate	6
EDTA	1
Na₂CO₃	20
H₃BO₃	2.86
MnCl₂·4H₂O	1.81
ZnSO₄·7H₂O	0.22
Na₂MoO₄·2H₂O	0.39
CuSO₄·5H₂O	0.079
Co(NO₃)₂·6H2O	0.049

Table 2

Experimental conditions for evaluating phosphorus storage and utilization characteristics of microalgae.

Reactor	Batch reactor with working volume 2 L	Medium	Modified BG-11 medium NaHCO₃, NaNO₃, K₂HPO₄ were modified as following C-source : NaHCO₃ 500 mg-C/L N-source : NaNO₃ 50 mg-N/L P-source : K₂HPO₄ 7 mg-P/L
pH	7.5 ± 0.5 by pH controller
Algae species	Chlorella vulgaris
Seeding conc.	0.3 OD	Additional C, N dosage	Additional carbon (C) and nitrogen (N) supply through pulse injection.
Light intensity	40 PPFD, 200 PPFD	Additional C, N dosage

Table 3

Equations for evaluating photosynthesis and respiration rates.

Respiration rate evaluation
Equation for evaluating respiration rate per microalgal OD	$\frac{R e s p i r a t i o n a m o u n t s (O_{2} - m g)}{L \times t_{1} (h r)} \times \frac{1}{O D (660 n m)} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/L/OD/hr)

Equation for evaluating respiration rate per microalgal MLSS	$\frac{R e s p i r a t i o n a m o u n t s (O_{2} - m g)}{L \times t_{1} (h r)} \times \frac{L}{m g - M L S S} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/mg-MLSS/hr)

Net-photosynthesis rate evaluation

Equation for evaluating net-photosynthesis rate per microalgal Chl-a	$\frac{N e t p h o t o s y n t h e s i s a m o u n t s (O_{2} - m g)}{L \times t_{2} (h r)} \times \frac{L}{m g - C h l - a} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/mg-Chl-a/hr)

Equation for evaluating net-photosynthesis rate per microalgal OD	$\frac{N e t p h o t o s y n t h e s i s a m o u n t s (O_{2} - m g)}{L \times t_{2} (h r)} \times \frac{L}{O D} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/L/OD/hr)

Equation for evaluating net-photosynthesis rate per microalgal MLSS	$\frac{N e t p h o t o s y n t h e s i s a m o u n t s (O_{2} - m g)}{L \times t_{2} (h r)} \times \frac{L}{m g - M L S S} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/mg-MLSS/hr)

Total-photosynthesis rate evaluation

Equation for evaluating total photosynthesis rate per microalgal Chl-a	$\frac{T o t a l p h o t o s y n t h e s i s a m o u n t s (O_{2} - m g)}{L \times t_{2} (h r)} \times \frac{L}{m g - C h l - a} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/ mg -Chl-a/hr)

Equation for evaluating total photosynthesis rate per microalgal OD	$\frac{T o t a l p h o t o s y n t h e s i s a m o u n t s (O_{2} - m g)}{L \times t_{2} (h r)} \times \frac{L}{O D} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/L/OD/hr)

Equation for evaluating total photosynthesis rate per microalgal MLSS	$\frac{T o t a l p h o t o s y n t h e s i s a m o u n t s (O_{2} - m g)}{L \times t_{2} (h r)} \times \frac{L}{m g - M L S S} \times \frac{1000 μ m o l}{32 m g}$
(μmol-O₂/mg-MLSS/hr)

Respiration amounts : DO decreases during the Dark period (mg-O₂/L)

Net-photosynthesis amounts : DO increases during the Light period (mg-O₂/L)

Total-photosynthesis amounts : Sum of both Respiration and Net-photosynthesis amounts (mg-O₂/L)

t₁ : Dark condition period (hr) t₂ : Light condition period (hr)