1 INTRODUCTION

The genus Phaeocystis were globally distributed and extensive blooms were reported mostly along the European and Chinese coast (Schoemann et al., 2005). Ten Phaeocystis species were identified morphologically and molecularly including P. globosa, P. antarctica, P. poucheti, P. cordata, P. brucei, P. jahnii, P. amoeboidea, P. giraudii, P. scrobiculata, and P. sphaeroides (Shen et al., 2018; Wang et al., 2021), among which, P. globosa (He et al., 1999) and P. poucheti (Hansen et al., 2004) were reported toxic. P. globosa giant colony were first reported in coastal China since 1997 (Chen et al., 1999), Viet Nam since 2002 (Qi et al., 2004; Hai et al., 2010), and the Arabian Sea (Madhupratap et al., 2000), causing massive fish death (Liu et al., 2015), mussel mortalities (Peperzak and Poelman, 2008), power plant closure (Gong et al., 2018; Liu and Zhou, 2018), and tourism closure (Nejstgaard et al., 2007; Blauw et al., 2010). The estimated economic loss of P. globosa reached ca. 35 million US dollars in South China Sea in 1997 (Qi et al., 2004), and ca. 0.65 million US dollars in Phan Ri Bay of Vietnam in 2002 (Doan et al., 2003).

The potential effects of Phaeocystis blooms include the production of hemolytic substance (He et al., 1999), dimethylsulfoniopropionate (DMSP), reactive oxygen species (ROS), and other polyunsaturated fatty acids (PUFA) (Van Leeuwe and Stefels, 2007; Sheehan and Petrou, 2020). Blooming of P. globosa can transform DMSP into dimethyl sulfide (DMS) with the production of ~0.5 Tmol/a (Mohapatra et al., 2013). Polyunsaturated aldehydes (PUAs), converted from PUFA with the presence of ROS, may lead to the blood apoptosis of sea urchin or human cytotoxicity (Pohnert, 2005). Hemolytic activity of P. globosa posed the most threat to the environment and fisheries (Yang et al., 2009; Liu et al., 2010). In addition, the giant colony of P. globosa, clogging the cooling system of power plant (Kang et al., 2020; Xu et al., 2020) or forming mucilage to make gelatinous foam (Karlson et al., 2021) were recently reported as the major ecological impact in Asian waters. Meanwhile, P. globosa blooms have influenced the viscosity change of seawater and leaded to the variation of marine ecological environment (Peperzak and Gäbler-Schwarz, 2012; Sheik et al., 2014). In terms of hemolytic toxins, the known components are mainly glycolipids, unsaturated fatty acids, fatty acid amides and porphyrins (Hiraga et al., 2008; Henrikson et al., 2010; Bertin et al., 2012). Hemolytic substance from one strain of P. globosa was similar with the structure of digitonin (He et al., 1999). Those hemolysins function similarly as the detergent solubilization, digtonin or Triton X-100, which caused acute toxicities to aquatic animals, i.e., Artemia salina, Brachionus plicatilis, and other zooplanktons (Yang et al., 2009). In additon, the synergetic effect of hemolytic toxins and ROS may show a critical role in fish killing during the blooms (Twiner and Trick, 2000; Ling and Trick, 2010).

Now the question becomes that what would be the driver(s) of the toxicity of P. globosa. Previous studies indicated that light (Moisan et al., 2006; Guo et al., 2007; Masotti et al., 2010), temperature (Selleslagh and Amara, 2008; Cao et al., 2015), and algicidal compounds (Yang et al., 2015; Zhang et al., 2017) could impact the hemolytic activity of P. globosa. The highest level of hemolytic activity was observed at 25 ℃ with the irradiance of 100 μmol/(m²·s) and the N: P ratio of 16:1 (Cao et., 2015). With the increasing salinity (19–29), temperature (15–25 ℃), and irradiance (14–38 μmol/(m²·s), the production of CH₃Cl and CH₃Br increased accordingly (Yan et al., 2019). Superfluous ROS of P. globosa can be triggered by the algicidal compounds, such as cyclo-(Pro-Gly), prodigiosin and combination of urocanic acid, N-acetylhistamine, and L-histidine (Zhuang et al., 2018). The production of DMSP in P. globosa was found associated with low temperature (20 ℃), high salinity (40) (Shen et al., 2011), acidic pH (pH=5) (Mohapatra et al., 2014), low ratio of N: P, and high concentrations of iron (Zhu et al., 2013).

Therefore, how would it possible that the hemolytic compounds of P. globosa be driven by the external stresses? In this case, a serious experiment was designed to investigate the response of photosynthetic system and hemolytic activity of P. globosa under the external stresses of light, temperature, iron, and algicidal compounds.

2 MATERIAL AND METHOD 2.1 Algae and culture condition

The strain of P. globosa was isolated from the coast of Shantou, South China Sea in 1997 and obtained from Research Center of Harmful Algae and Marine Biology, Jinan University, China. All cultures were incubated in f/2-Si medium (salinity of 28 and pH 8) under a light: dark (L: D) cycle of 12 h: 12 h at 25 ℃ with an irradiance of 90 μmol/(m²·s).

Cell number was estimated using a hemacytometer every two days and growth rate was calculated by the difference in cell number during the exponential growth using the equation given by Guillard (1975).

where N₂ and N₁ are the cell numbers at time t₂ and t_1.

2.2 Physical environment stress

The stress experiments were carried out at three temperatures (16, 22, and 28 ℃), five light intensities (30, 60, 100, 180, and 270 μmol/(m²·s)) and three different FeCl₃ concentrations (0, 10^-5, and 10^-7 μmol/L). Artificial seawater was used instead of natural seawater (Harrison et al., 1980) for two generations to preculture the algae in f/2-Si medium. The exponential growth phase of P. globosa was inoculated into fresh f/2-Si medium to ensure the same initial density and cultured under designed conditions as shown in Table 1. The position of bottles was changed every day to reduce the effects of uneven lighting or temperature. All treatments were conducted in triplicate. Subsamples were collected every two days to determine cell density, photosynthetic fluorescence, and hemolytic activity.

2.3 Photoperiod experiments

A stock culture of P. globosa was grown in a f/2-Si medium at 25 ℃ under 90 μmol/(m²·s) of cool-white fluorescent illumination on a 12-h: 12-h L: D cycle (light period 09:00–21:00). Subsamples were collected every four hours to determine cell density, photosynthetic fluorescence and hemolytic activity. The experiment last for 24 h.

2.4 Photosynthetic electron inhibitors stress

Diuron, atrazine, dibromothymoquinone (DBMIB), and paraquat were chosen as four photosynthetic electron inhibitors. Certain amount of these inhibitors, 0.1, 0.15, 0.1, and 4.5 mg/L for diuron, atrazine, DBMIB, and paraquat, respectively, were added into 100 mL of exponential growth culture of P. globosa. Then samples were mixed and placed under 25 ℃ with an irradiance of 100 μmol/(m²·s). Photosynthetic efficiency and hemolytic activity were detected after one hour.

2.5 Data analysis 2.5.1 Photosynthetic fluorescence

Photosynthetic activity of samples was determined by measuring in-vivo chlorophyll fluorescence of photosystem II (PS II) using a Phyto-pulse amplitude modulation (PAM) (Walz, Effeltrich, Germany). Subsamples were taken and treated in darkness at 25 ℃ for 5 min each. The temperature experiment was conducted in darkness under the corresponding temperature conditions. The photosynthetic efficiency (F_v/F_m), the relative electron transfer rate (ETR), and quantum yield of photosystem II (Yield) were determined by Phyto-PAM.

2.5.2 Hemolytic activity assay

The hemolytic activity of P. globosa was evaluated by rabbit erythrocytes described in Eschbach et al. (2001) and Ling and Trick (2010). The rabbit erythrocytes were taken from the auricular veins of New Zealand rabbits and washed twice in the erythrocyte lysis assay (ELA) buffer. For the hemolytic activity, algae subsamples were placed into 10-mL tubes and centrifuged at 6 000×g for 10 min. Then the pallets were re-suspended in 400-μL ELA buffer and stored at -20 ℃. Before detecting the hemolytic activity, the samples were crushed in an ice bath by an ultrasonic crusher under the conditions of 10% power (650 W), 50 s (pulse on 2 s, pulse off 1 s) to obtain the hemolytic toxin extract. One hundred and fifty microlitre of P. globosa hemolysin extract and 150 μL of 0.6% rabbit erythrocytes were mixed in a 1.5-mL centrifuge tube (As). After centrifugation of the same volume of hemolysin buffer and rabbit erythrocytes, the absorbance of the supernatant was measured to eliminate the background value of erythrocytes (Aa). The absorbance of the supernatant was measured as the negative control (Ab) after centrifugation of the same volume of hemolysin extract and erythrocytes buffer. The positive control was the absorbance of the supernatant of completely lysed rabbit erythrocytes (Ac). All samples were well-mixed and placed under 25 ℃ with an irradiance of 100 μmol/(m²·s) for 5 h, then centrifuged at 3 000 r/min for 10 min. The supernatant was used to measure the absorbance at 414 nm in Microplate Reader (Biotek Synergy HT, USA).

The hemolytic activity was calculated according to the following formula (Ling and Trick, 2010):

where As is samples, Aa is algae background value, Ab is the background of erythrocytes (negative control), and Ac is the lysed rabbit erythrocytes (positive control).

The half effect concentration (EC₅₀) is the concentration of algae cells that lyse half of the rabbit's red blood cells within 5 h. The EC₅₀ was determined as the reaction algae density for subsequent experiments. The algae were gradually diluted into seven concentrations, 1.0, 2.0, 4.0, 8.0, 20.0, 40.0, and 80.0×10⁵ cells/mL. The EC₅₀ of P. globosa was then determined as 1×10⁶ cells/mL.

2.5.3 Pigment analysis

The pigment samples were extracted every two days under light experiment as described in Buffan-Dubau and Carman (2000) and Zapata and Garrido (1991). Fifty milliliter of subsamples were filtered through Waterman GF/F glass fiber filters (0.7-μm nominal pore size, 25-mm diameter). Then the filter membranes were cut into pieces and incubated in 3 mL of 95% methanol (v: v). Then the samples were centrifuged at 4 000×g for 5 min. The supernatant was filtered into the chromatographic bottle for pigment detection. Pigment were determined using high performance liquid chromatography (HPLC)(Agilent 1200) with C8 column (4.6 mm×150 mm, 3.5 μm) and UV-detector as described in Zapata et al. (2000). The mobile phase used in this study were eluent A: Milli-Q water, eluent B: acetonitrile, eluent C: methanol-acetonitrile-acetone (volume ratio: 1:1:3), eluent D: methanol-acetonitrile-acetic acid pyridine (2:1:1). Elution time was 40 min and flow rate was 1 mL/min.

For each analysis, 30-μL standard pigment solution, 90-μL methanol, and 40-mL Milli-Q water were mixed as standard working solution. All treatments were performed under low light. Standard pigments were obtained from DHI Inc. (Denmark)

2.5.4 Statistical analysis

One-way ANOVA and correlation analysis were used to test the differences of photosynthetic fluorescence value, hemolytic activity, growth, and pigment under different conditions. All statistical analysis were conducted using SPSS 19.0 software. Origin 8.0 and Sigmaplot 12.5 were used for graphic rendering. Significant differences were determined using simple t-test (P < 0.05).

3 RESULT 3.1 Effect of temperature

Low temperature inhibited the growth of P. globosa. The maximum growth rate (μ_max) of P. globosa increased with the increasing temperature, reaching 0.21, 0.33, and 0.44 (n=3) at 16, 22, and 28 ℃, respectively (Table 1). The exponential growth lasted 10 days in higher temperature, whereas 8 days in low temperature, resulting to the highest cell concentration of 1.70×10⁶ and 1.89×10⁶ cells/mL in 22 ℃ and 28 ℃ (Fig. 1a).

The temperature stress on photosynthetic system II (PSII) of P. globosa were also pronounced (Fig. 1b). Significant low values of F_v/F_m (circle), Yield (bar) and ETR (square) cells were observed in 16 ℃ (Fig. 1b), indicating the temperature inhibition of photosynthetic activity of P. globosa. Photosynthetic efficiency (F_v/F_m) of P. globosa kept the maximum level of 0.64±0.03 (n=18) on average during the entire growth at 28 ℃ (Fig. 1b), on the other hand, dramatically (One Way ANOVA, P < 0.05) decreased F_v/F_m from 0.63 at exponential growth to 0.47 when cell aged at 22 ℃, and from 0.57±0.01 to 0.37±0.01 at 16 ℃ (Fig. 1b). Similar trends were observed in the effective quantum yield of PSII (Yield) and relative electron transport rate (ETR). The maximum Yield and ETR of P. globosa were 0.58±0.01 and 34±0.6 at exponential growth of P. globosa at 28 ℃. The colder (16 ℃) and older (day 11) cells of P. globosa were under great stress, with Yield of 0.30 and ETR of 14.33, respectively.

The response of hemolytic activity of P. globosa was significant as the varied temperatures (Fig. 1c–d) and cell ages (Fig. 1c). The cells cultured under low temperature, had low hemolytic activity during the entire growth phase. In contrast, hemolytic activity of P. globosa significantly increased (One Way ANOVA, P < 0.05) at day 4 in 22 ℃ and 28 ℃, which varied from 23% to 51% and 63% on average, respectively (Fig. 1c). In general, hemolytic activity of P. globosa increased with increasing temperature (Fig. 1d). To evaluate the response of hemolytic activity to the photosynthetic system of P. globosa under the temperature treatments, the correlations between hemolytic activity and F_v/F_m, Yield, and ETR were analyzed in Fig. 1e. The clear positive correlations were found between hemolytic activity (HA) and F_v/F_m, Yield and ETR, with the R² of 0.45, 0.43, and 0.44, respectively.

3.2 Effect of iron

Iron significantly affected the physiological and toxinological characteristics of P. globosa. Growth response of P. globosa under the exposure of iron are presented in Fig. 2a. P. globosa did not grow without iron, but with small amount of iron, i.e., 10^-7 μmol/L, cells growth rapidly, reaching 0.49/d (Fig. 2a). And the maximum growth rate was found in 10^-5 μmol/L of FeCl₃ with 0.62/d.

As for the response of F_v/F_m, Yield, and ETR, healthier cells of P. globosa, higher values of PSII activity, were observed under iron sufficient (10^-5 μmol/L) condition (Fig. 2b), however, even the growth was not limited under iron low condition (Fig. 2a), but all photosynthetic activities were declined from 0.64 to 0.38 (F_v/F_m), 0.40 to 0.25 (Yield), and 26.33 to 15.67 (ETR) at day 4. The F_v/F_m, Yield, and ETR values of P. globosa reached 0.09, 0.03, and 1.67 under most stressful condition when cell aged (Fig. 2b).

Iron also had a great impact on hemolytic toxin production of P. globosa. Lowest hemolytic activity was detected under iron free condition through all growth stage, with an average of 18% (n=15) per 1 million cells. On the other hand, hemolytic compounds were produced when iron was added. Higher hemolytic activity with around 43% was detected in iron sufficient cultural condition (Fig. 2c & d).

Linear relationship of photosynthetic and hemolytic activity was found in Fig. 2e. Similar as the temperature treatments, significant positive relationship between F_v/F_m (R²=0.45), Yield (R²=0.43) and ETR (R²=0.44) and hemolytic activity were also found.

3.3 Effect of irradiance

Light intensities of 30, 60, 100, 180, and 270 μmol/(m²·s) were used in the present study. In Fig. 3a, the apparent growth was found in light treatments of over 60 μmol/(m²·s). The growth rates varied but not significant, with around 0.45/d on average. The highest growth rate, 0.68/d, was found at light of 100 μmol/(m²·s). In contrast, P.globosa grew much slower, ~0.13/d at the lowest irradiance (30 μmol/(m²·s)). Interestingly, the exponential growth last 9 days in the highest two light treatments(I₁₈₀, I₂₇₀), however, 11 days in the middle light treatments (I₁₀₀ and I₆₀), resulting to the significant difference of maximum cell concentrations (180 000 vs. 200 000 cells/mL).

Photosynthetic activities of P. globosa, F_v/F_m, Yield, and ETR are shown in Fig. 3b. The two highest light treatments, I₁₈₀ and I₂₇₀, stressed the photosynthetic activity of P. globosa as the evidence of significant low values of F_v/F_m, Yield, and ETR (Fig. 3b). Light of 100 μmol/(m²·s) was sufficient for F_v/F_m (no declined through the entire growth), but shrunk after day 5 in both Yield and ETR. Low light, I₃₀ and I₆₀, did not stress the PSII of P. globosa, but no significant growth was found in I₃₀ (Fig. 3a).

Hemolytic activities of P. globosa under variable light treatments were shown in Fig. 3c & d. Significant high amount (One Way ANOVA, P < 0.05) of hemolytic toxins were produced under the low light treatment, 30 μmol/(m²·s), with an average of 72% through the entire growth, followed by 60, 100, 180, and 270 μmol/(m²·s) (Fig. 3d). Hemolytic activities reached 32%, 24%, 20%, and 13% under the light of 60, 100, 180, and 270 μmol/(m²·s), respectively. The hemolytic activity decreased with the increasing irradiance during the whole exponential period (Fig. 3c). Linear relationship between hemolytic and photosynthetic activity showed the significant positive response of F_v/F_m (R²=0.48), Yield (R²=0.54), and ETR (R²=0.59) (Fig. 3e).

Pigment contents varied greatly with light and growth phase (Fig. 4). Pigments of chlorophyll c3 (Chl c3), chlorophyll c2 (Chl c2), 19′-But-fucoxanthin (But-fuco), fucoxanthin (Fuco), 19-Hex-fucoxanthin (Hex-fuco), diadinoxanthin (Diad), and chlorophyll a (Chl a) were detected in P. globosa. Chl c3, Chl c2, and Fuco were the dominant pigments in P. globosa. Cellular Chl c3 and Fuco of P. globosa, ranging from 78% to 31%, decreased with the increasing light intensity. Photopigments were further investigated with the relationship of hemolytic activity by principal component analysis (PCA) (Fig. 5). The scores of the first two principal components (PC1 and PC2) reached 49.9% and 24.5% for P. globosa. The hemolytic activity was apparent in highly positive PC1 space and appeared quite correlated with the pigments of Chl c2, Chl c3, and Fuco.

3.4 Daily cycle variation

Photosynthetic and hemolytic activity of P. globosa were investigated in a daily cycle (Fig. 6). The variations of F_v/F_m, Yield, and ETR of P. globosa are S-shaped. After entering the light time, the photosynthetic transfer efficiency gradually increased, and reached the highest value at 7-h light exposure, then gradually decreased. The lowest F_v/F_m were found after 3-h dark cycle. Similarly, hemolytic activity was higher during the day time, and lower during the night time. The maximum hemolytic activity of 74%±0.48% was obtained from 16:00 pm to 20:00 pm and the minimum of 64%±3.5% from 8:00 am to 16:00 pm and from 0:00 pm to 8:00 am.

3.5 Effect of photosynthetic electron inhibitors

Four inhibitors of diuron, atrazine, DBMIB, and paraquat inhibited the photosynthetic activity of P. globosa, but had no effect on hemolytic activity (Fig. 7). F_v/F_m decreased from 0.58 to 0.18 (diuron), 0.43 (atrazine), 0.5 (DBMIB), and 0.55 (paraquat), respectively. Similar trends were observed in Yield and ETR of P. globosa. Diuron was the most effective inhibitors on PSII of P. globosa (Oneway ANOVA; P < 0.05).

4 DISCUSSION

Phaeocystis globosa has attracted much attention and research due to the fundamental role and harmful algal bloom effects in many regions of the world (Blauw et al., 2010; Buchan et al., 2014). The temperature was one of the triggers that can affect the hemolytic activity of P. globosa in many aquatic system (Cao et al., 2015; Kang et al., 2020). Cells of P. globosa in an optimum temperature condition, i.e., 24 and 27 ℃ (Xu et al., 2017) or 27–30 ℃ (Guo et al., 2007), produce more hemolytic toxins into the water system (Guo et al., 2007). Similar results were also found in our study (Fig. 1). A higher hemolytic activity was observed in the faster growing rate of P. globosa and the healthier cells (higher values of PSII activity).

Iron is an important nutrient in marine phytoplankton (Lill, 2009), and would be a limiting factor in the metabolism of phytoplankton during algal blooms (Larson et al., 2018). Furthermore, metal oxides are photocatalysts that facilitate photocatalysis by absorbing solar photons over a broad spectral range, ultimately enhancing the absorption of visible light (Zhou et al., 2018; Wan et al., 2019). As for P. globosa, trace amount of iron, i.e., 10^-7 μmol/L, can trigger the growth, as well as the active PSII and hemolytic activity (Fig. 2). Iron sufficient would contributed the growth of P. globosa (Slagter et al., 2016) and enhanced the hemolytic activity (Liu et al., 2006; Jiang et al., 2012). The stress of iron did not inhibit the growth, PSII and toxic activity in P. globosa, indicating the production of hemolytic compounds may associate with the growth and PSII activity.

Light was reported as the trigger of hemolytic compounds in many harmful algal species (Cao et al., 2015; de Q Mendes et al., 2017; Wang et al., 2019). The saturation light for one isolate of P. globosa was determined as 60 μmol/(m²·s) and the photoinhibition point was 230 μmol/(m²·s) (Xu et al., 2017). Lower light, i.e., 80 μmol/(m²·s), inhibited the growth rate of P. globosa isolated from North Sea, but had no effect on photosynthetic activity (Hoogstraten et al., 2012). Light intensities from 40 to 150 μmol/(m²·s) promoted P. globosa isolated from the East China Sea to growth faster (Liu et al., 2011). In our study, the growth rate of P. globosa did not change dramatically within the light of 60 to 270 μmol/(m²·s). However, the exponential duration under higher light intensities (I₁₈₀ and I₂₇₀) were inhibited to 9 days compared to 11 days under the relatively low light condition. Another evidence of P. globosa preferred low light is that the value of F_v/F_m in low light (25 μmol/(m²·s)) was 10%–20% higher than that in high-light intensity (Maat et al., 2016). As shown in Fig. 3b, light of 60– 100 μmol/(m²·s) was the most PSII active condition for P. globosa in the present study. The continuous light at daytime may inhibit the growth of P. globosa, which may due to the circadian rhythm pattern of DNA synthesis and cell division (Wang et al., 2014).

The hemolytic activity of P. globosa varied significantly under high and low light conditions (Cao et al., 2015). This finding can be seen in the light and daily cycle experiments in the present study. The significant high amount of hemolytic activity under light of 30 μmol/(m²·s) (Fig. 3c–d), and down regulated hemolytic activity at dark cycle (Fig. 6) indicated that light was essential for P. globosa to produce hemolytic toxins and a lower light promoted the production of the toxins.

Therefore, how could the hemolytic toxin production associate with the PSII activity of P. globosa? The significantly positive correlation between hemolytic and photosynthetic activity of P. globosa under temperature, light, iron, and daily cycle treatments made us to further consider whether the hemolytic compounds would be photopigments associated compounds or related with the electron transfer chain? Thereafter, pigments as well as electron inhibitors experiments were conducted.

The main pigments of P. globosa include Chl c3, magnesium-divinyl-pheo-porphyrin a5 monomethyl ester (Mg DVP), Chl c2, But-Fuco, Fuco, Hex-Fuco, Diad, Chl a, and Caroteniods (Caro) (Zapata et al., 2004). Chl c2, and c3 also are light-harvesting pigments (Bollivar, 2006), and part of the Fuco-chl a/c protein complex. In addition, Chl c3, Fuco, and Chl c2 are particularly sensitive to low light conditions. But-fuco is often described a light-harvesting role (Van Leeuwe et al., 2014). As we know, pigment composition of P. antarctica varied with light and iron conditions (Seoane et al., 2009; Van Leeuwe et al., 2014). Here the increased Fuco were confirmed at low light (Fig. 4f). Under the high light, P. globosa cells responded by reducing their content of light harvesting pigments, i.e., Chl c2, Chl c3, Fuco (Fig. 4d, e, f) and by increasing the presence of photo-protective xanthophylls diadinoxanthin (Fig. 4c). The results of PCA on hemolytic activity and pigments (Fig. 5) suggested that the production of hemolytic compounds might associate with the light-harvesting pigments.

However, the inhibitors limited the PSII activity of P. globosa, but no effect on hemolytic activity. DBMIB can block the PSII to PSI by the cytochrome b₆f complex in activation of the kinase (Mao et al., 2002; Roberts et al., 2004; Trebst, 2007). Atrazine may cause the controversial response to different organisms (Brain et al., 2012). Trace amount of DBMIB resulted to significant inhibition of PSII photochemistry (Belatik et al., 2013); however, no response of hemolytic activity may indicate not the electron transfer chains in PSII, but other biosynthetic pathway of hemolytic activity may involve in P. globosa.

Stress, such as low temperature, high light, iron deplete, dark, and electron inhibitors, showed significant impact on the growth and photosynthetic activity of P. globosa, while, the external stresses do not correspond to the hemolytic activity of P. globosa. The heathier P. globose is, the higher hemolytic activity is. Stress could not be the driver of hemolytic activity of P. globosa.

5 CONCLUSION

The current study focuses on regulation of external stressors, such as light, temperature, iron, and photosynthetic electron inhibitors, on photosynthetic system of P. globosa and their hemolytic activity. Light was essential for P. globosa to produce hemolytic toxins. However, high light intensity (i.e., > 100 μmol/(m²·s)) inhibited the hemolytic activity, together with the lower temperature (16 ℃) and iron-limited stress. Meanwhile, photosynthetic activity (F_v/F_m, Yield, and ETR) and hemolytic activity of P. globosa were positively correlated. However, the hemolytic activity of P. globosa was not affected by photosynthetic electron inhibitors, which blocked the photosynthetic activity significantly. Therefore, it could be concluded that hemolytic toxin production of P. globosa were associated with photosynthetic process but not with the electron transfer chain. The present study provides important information for understanding the formation mechanisms of hemolytic compounds of P. globosa.

6 DATA AVAILABILITY STATEMENT

All data generated and/or analyzed during this study are contained within this article.