Synergistic effects of nano-sized titanium dioxide and zinc on the photosynthetic capacity and survival of Anabaena sp.

Anabaena sp. was used to examine the toxicity of exposure to a nano-TiO2 suspension, Zn2+ solution, and mixtures of nano-TiO2 and Zn2+ suspensions. Typical chlorophyll fluorescence parameters, including effective quantum yield, photosynthetic efficiency and maximal electron transport rate, were measured by a pulse-amplitude modulated fluorometer. Nano-TiO2 particles exhibited no significant toxicity at concentrations lower than 10.0 mg/L. The 96 h concentration for the 50% maximal effect (EC50) of Zn2+ alone to Anabaena sp. was 0.38 ± 0.004 mg/L. The presence of nano-TiO2 at low concentrations (<1.0 mg/L) significantly enhanced the toxicity of Zn2+ and consequently reduced the EC50 value to 0.29 ± 0.003 mg/L. However, the toxicity of the Zn2+/TiO2 system decreased with increasing nano-TiO2 concentration because of the substantial adsorption of Zn2+ by nano-TiO2. The toxicity curve of the Zn2+/TiO2 system as a function of incremental nano-TiO2 concentrations was parabolic. The toxicity significantly increased at the initial stage, reached its maximum, and then decreased with increasing nano-TiO2 concentration. Hydrodynamic sizes, concentration of nano-TiO2 and Zn2+ loaded nano-TiO2 were the main parameters for synergistic toxicity.

1. Introduction

Heavy metals are discharged into aquatic ecosystems from various industries, such as the textile, mining, electroplating, and metallurgical industries. Heavy metals are persistent environmental contaminants that cannot be destroyed or degraded [1]. Heavy metals pose a serious threat to human beings and aquatic ecosystems because of their persistent toxicity, bioaccumulation, and biomagnifications through the food chain. Algae, a class of organisms forming the basic nourishment for the food chain, are commonly used as model organisms to study the toxicity of heavy metals [2]. Recent studies have focused on the interaction between heavy metals and different aquatic conditions, such as temperature, irradiance, pH, ethylenediaminetetraacetic acid (EDTA), anions, and nutrients in algae [3,4].

The expansion of nanotechnology has resulted in subsequent increased release of nanoparticles (NPs) into aquatic environments during the cycle of manufacturing, transportation, consumption, and disposal [5]. Among these NPs, nano-sized titanium dioxide (nano-TiO2) is one of the most popular engineered nanomaterials increasingly being incorporated into various consumer products. The negative environmental effects of NPs have drawn significant attention in recent years [6-8]. Numerous studies have focused on the inhibitory effects of titanium dioxide, zinc oxide, copper oxide, silicon oxide, and alumina NPs in algae [9-12]. Scholars have obtained different results regarding the toxicity mechanism of oxide NPs to algae, such as the contribution of dissolved metal ions from NPs or the agglomerates of NPs onto algae [10-13].

Heavy metals including Zn, Cd, Pb, Ni, Cu, and Co have adverse effects on the growth, cell division, photosynthesis, and destruction of primary metabolites in algae [14-18]. The toxicity of heavy metals is usually a function of free heavy metal ions because these species are generally the most bioavailable ones [4,19]. This toxicity is likely associated with glutathione redox cycle, reactive oxygen species production, and phytohormone production [18,20,21]. The individual toxicity of NPs and heavy metals to algae has been widely investigated [10-13,16-19]. Studies on the synergistic effects of these two categories on algae are limited and controversial. The toxicity of heavy metals to green algae is eliminated in the presence of TiO2 NPs with high surface area [22,23]. However, the inhibition of green algae at the same heavy metal concentration is not notably affected by adding various sizes of TiO2 NPs [24].

To further explore the mechanism of the synergistic toxicity of NPs and heavy metals to algae, we investigate how TiO2 NPS influence the bioavailability of heavy metal zinc (Zn). Zn is an essential component of various enzymes for algae, particularly those in photosynthetic electron transport. At elevated concentrations, Zn is toxic with its most toxic form Zn2+[25]. Algae and cyanobacteria are abundant in aquatic ecosystems and envisaged as an ideal model to study any adverse effects of released NPs [26]. The cyanobacterium Anabaena sp. is used as a model to study the toxicity of nano-TiO2 and Zn2+. Current use of algae in the study of NPs toxic effects on photosynthesis seems to be a convenient method [27]. The change of photosynthetic activity affects the photosynthetic process and cellular growth, which may be indicated by fluorescence emission. Fluorescence measurements thus serve as an important indicator to provide information of NPs interaction with photosynthesis and toxic effects on the physiological state of algae [28].

The objective of this study is to determine the synergistic toxicity of nano-TiO2 and Zn2+ on Anabaena sp. using a pulse-amplitude modulated (PAM) fluorometer, a rapid and efficient tool for in vivo studies of photosynthetic activity. The correlation between algal cell growth and photosynthetic fluorescence parameters of soluble Zn2+ alone and nano-TiO2 on Anabaena sp. is also investigated to provide background information for toxicity comparisons.

2. Results and Discussion

2.1. Characterization and Sedimentation of Nano-TiO2 in Culture Medium

In this study, the nominal diameter of commercial nano-TiO2 ranged from 40 to 50 nm. As shown in the dynamic light scatting (DLS) results and Scanning electron microscope (SEM) images in Figure 1a, the average diameter of the NPs suspended in BG11 culture medium dramatically increased to approximately 450 to 650 nm. The aggregation tendency of the NPs was ascribed to the relatively low zeta potential (−7.8 mV). This rapid formation of nano-TiO2 aggregates was also observed in previous studies [29,30], indicating that algae and other living organisms were exposed to nano-TiO2 beyond their original nanoscale particle size in environment systems. At the same time, aggregated nano-TiO2 were much more toxic than their bulk counterpart [31]. The nano-TiO2 attached on the surface of the algal cells and the direct contact is demonstrated clearly by the SEM images in Figure 1b. So, the hydrodynamic size and the adsorption of nano-TiO2 on algae affected their toxicity.

Figure 1

SEM images of (a) nano-TiO2; (b) algae in the presence of 1.0 mg/L nano-TiO2.

2.2. Sorption of Zn2+ onto Nano-TiO2

The interactions of Zn with nano-TiO2 were determined by examining the sorption equilibrium. In the equilibrium isotherm experiment, a correlation between Zn2+ adsorbed on the nano-TiO2 (qe, mg/g) and the non-adsorbed Zn2+ concentration (Ce, mg/L) in the culture medium was determined. Figure 2 shows the sorption density of nano-TiO2 as a function of initial Zn2+ concentration up to 10.0 mg/L. Nano-TiO2 could adsorb Zn2+ from the culture medium. With the initial Zn2+ concentration increased from 3.0 to 10.0 mg/L, Zn2+ adsorption approached the saturation point. The Langmuir isotherm was used to fit these adsorption data using Matlab.

Figure 2

Adsorption isotherms of Zn2+ on nano-TiO2 in the culture medium; pH = 7.0; temperature = 298 K.

The calculated adsorption capacity qmax was approximately 11.38 mg/g, and the parameter for the b = 6.92. A good correlation is shown in Figure 2 suggesting a monolayer adsorption of Zn2+ on nano-TiO2. Therefore, nano-TiO2 adsorption had an important impact on the Zn2+ concentration in the culture medium.

2.3. Toxicity of Nano-TiO2

The toxicity of nano-TiO2 to algae was reported by other researchers [11,22,23]. However, these results were not significantly comparable because of the different sources and properties of NPs. The inhibition of Anabaena sp. at different nano-TiO2 concentrations from 1.0 to 50.0 mg/L is shown in Figure 3. After 96 h of exposure, the inhibition was observed at nano-TiO2 concentrations more than 10.0 mg/L to algae. The changes in the content of chlorophyll-a and the photochemical transformation of energy were observed. The difference in the toxicity of nano-TiO2 could be related to particle size, crystal form, and test method. Large aggregates of TiO2 nanoparticles entrapped algal cells (Figure 1b), which reduced the light available to the algal cells and inhibited their growth [32,33]. Moreover, nutrients adsorbed by nano-TiO2 in culture medium would contribute to the toxicity [34].

Figure 3

Toxic effect of nano-TiO2 on the inhibition of Anabaena sp. at 96 h. (a) Biomass of algae at different initial level of nano-TiO2; (b) Chlorophyll-a concentration at different level of nano-TiO2; (c) Photosynthetic yield of algae at different initial level of nano-TiO2.

2.4. Toxicity of Zn2+ in the Absence and Presence of Nano-TiO2

Figure 4 shows the inhibition of Anabaena sp. at different Zn2+ concentrations after 96 h. No significant inhibition was observed at Zn2+ concentrations below 0.3 mg/L, whereas the biomass of Anabaena sp. notably decreased with increasing Zn2+ concentration from 0.5 to 1.0 mg/L. The 96 h growth process of Anabaena sp. with increasing Zn2+ concentration from 0 to 1.0 mg/L is shown in Figure 5. The exposure of Anabaena sp. to Zn2+ resulted in a clear difference in cell number between the control and experimental samples. Higher initial Zn2+ concentrations reduced cell density significantly. Growth inhibition was essentially proportional to Zn2+ concentration. However, at the lowest Zn2+ concentration considered (0.1 mg/L), an increase in the growth of Anabaena sp. was actually observed. The 96 h EC50 value for Anabaena sp. growth was calculated to be 0.38 ± 0.004 mg/L. This finding was in accordance with the results of a previous study on the exposure of Micractinium pusillum to Zn [35]. When Zn2+ was in high concentration, Anabaena sp. created physiological stress leading to generation of free radicals. Stress in turn induced the production of reactive oxygen species (ROS). The ROS could rapidly attack all types of biomolecules such as nucleic acids, protein, lipids, and amino acids, leading to irreparable metabolic dysfunction and algae death [36]. Results from H2DCF-DA dye test using microplate reader showed that the intracellular ROS was raised in the algal cells with different initial Zn2+ concentration in Figure 4. When the initial Zn2+ concentration was higher than 0.7 mg/L, the intracellular ROS entered in the medium with the algal cells rupture.

Figure 4

Inhibition of Anabaena sp. growth and relative ROS rate at different initial concentrations of Zn2+.

Figure 5

The growth process of Anabaena sp. at different initial concentrations of Zn2+.

The synergistic toxic effect of Zn2+ and nano-TiO2 was examined using a fixed concentration of nanoparticles; the nano-TiO2 particles alone were not toxic at low concentrations from 1.0 to 10.0 mg/L. Figure 6 shows the effect of Zn2+ on the 96 h growth process of Anabaena sp. in the presence of nano-TiO2. The nanoparticles significantly impacted the toxicity of Zn2+. At high concentration such as 10.0 mg/L, the toxicity of Zn2+ was reduced and the EC50 value of Zn2+ was 0.49 ± 0.001 mg/L. A high nano-TiO2 concentration could effectively reduce the soluble Zn2+ by adsorbing Zn2+ on NPS in Table S1. At same time, as shown in Figure S2, nano-TiO2 at high concentrations easily settled to the bottom of the reactor, so the soluble Zn2+ concentration around algae was low and the toxicity was reduced. At low concentration such as 1.0 mg/L, the toxicity of Zn2+ was enhanced and the EC50 value of Zn2+ with 1.0 mg/L nano-TiO2 was about 0.29 ± 0.005 mg/L. The results indicate that Zn2+ toxicity was significantly enhanced by 1.0 mg/L nano-TiO2 in the culture medium. However, the low concentration of nano-TiO2 reduced the soluble Zn2+ concentration, as shown in Table S1. Soluble Zn2+ and adsorbed Zn2+ were believed to contribute to the overall toxic effect on algae. The direct adherence of nano-TiO2 resulted in a high localized concentration on the algal surface, which could be due to high levels of free Zn2+[37]. Nano-TiO2 at low concentrations was relatively stable in the culture medium. Nano-TiO2 easily attached on the surface of the algal cells, which limited their mobility. The adsorbed Zn2+ had direct contact with the algae. The synergistic toxic effect of Zn2+ and nano-TiO2 was attributed to the concentration of nano-TiO2 and the free Zn2+.

Figure 6

Toxic effect of Zn2+ on the inhibition of Anabaena sp. with the fixed nano-TiO2 at 96 h.

2.5. Toxicity of Nano-TiO2 in the Presence of Zn2+

Although Zn2+ at concentrations below 0.3 mg/L showed no significant toxic effects on Anabaena sp., a synergistic effect might occur if nano-TiO2 was also present in this system. The toxicity of nano-TiO2 in the presence of constant concentrations of Zn2+ was examined. Figure 7 shows the nano-TiO2 toxicity result after 96 h of exposure at 0.3 mg/L Zn2+. It could be seen that the toxicity of nano-TiO2 in the presence of Zn2+ was significantly different from that of nano-TiO2 alone. With increasing nano-TiO2 concentration, the inhibition of Anabaena sp. increased at the initial stage and then decreased afterwards. The same trend was observed in the photochemical transformation of energy and in the chlorophyll content of Anabaena sp. When the added nano-TiO2 was more than 1.0 mg/L, the overall toxicity decreased. This could have been caused by the adsorption of Zn2+ onto the nano-TiO2, which significantly reduced the soluble Zn2+ concentration with high concentration of nano-TiO2. Increased nano-TiO2 enhanced aggregation, resulting in a lower suspended concentration. Thus, the overall toxicity could also be decreased by reduced uptake of nano-TiO2 by algae [37]. The addition of nano-TiO2 enhanced Zn2+ toxicity, with the maximum enhancement observed at 1.0 mg/L nano-TiO2. This result is consistent with the results shown in Figure 6. By contrast, nano-TiO2 was non-toxic at concentrations less than 1.0 mg/L, and the contribution of bare nano-TiO2 to algal toxicity was neglected. The soluble Zn2+ concentration decreased in the presence of nano-TiO2, however, the overall toxicity significantly increased. First, the decrease in residual Zn2+ concentration reduced the toxic effect. This result was similar to the scenario for both heavy metals and other carriers [38]. Second, the adsorbed Zn2+ on nano-TiO2 contributed to toxicity once nano-TiO2 was taken up by algae. The addition of nano-TiO2 increased the total uptake of Zn2+-loaded nanoparticles, and the mortality increased accordingly.

Figure 7

Toxic effect of Nano-TiO2 on the inhibition of Anabaena sp. with the fixed Zn2+ at 96 h. (a) Biomass of algae at different initial level of nano-TiO2; (b) Chlorophyll-a concentration at different level of nano-TiO2; (c) Photosynthetic yield of algae at different initial level of nano-TiO2.

NPs in aquatic systems produced potential risks, not only from nano-particles, but also from their ability to accumulate and enhance the toxicity of these background contaminants. Nano-TiO2 alone at low concentrations (<10.0 mg/L) did not cause significant inhibitory effects. Thus, its fate and potential aquatic effects could be easily overlooked. However, low-concentration nano-TiO2 served as Zn2+ carriers and increased the total Zn2+ uptake by algae. Moreover, the concentrations of the NPs in the water body were always at the microgram level. The biomagnifications of NPs from lower trophic aquatic organisms to higher ones strengthened this risk [6]. Therefore, the synergistic effects of the background toxic substances with released NPs could be more serious than the effects of NP alone.

3. Experimental Section

3.1. Culture of Anabaena sp

Samples of Anabaena sp. were obtained from the Institute of Wuhan Hydrobiology (China). The composition of BG11 culture medium is listed in Table S2. NaNO3, K2HPO4, MgSO4·7H2O, CaCl2·2H2O, citric acid, ferric ammonium citrate, EDTANa2, Na2CO3, H3BO3, MnCl2·4H2O, Na2MoO4, CuSO4·5H2O, Co(NO3)2, and ZnSO4·7H2O were purchased from Sinopharm Medicine. The deionized water (DI) used to prepare reagents and culture medium was purified by Millipore reverse osmosis. The initial pH of the medium was adjusted to 7.0 using 0.01 M HCl or NaOH solution. The algae were produced by cultivation in a constant-temperature incubator at 25 ± 1 °C. The illumination intensity in the incubator was 4000 Lux with a light-dark cycle of 12 h:12 h. The stock culture of Anabaena sp. were shaken three to four times a day, and their growth curves were recorded to ensure that the algae used in the test were in the logarithmic growth phase. The concentrations of experimental samples were measured using a spectrophotometer. The optical density (OD) values were in linear relation to algal concentration. The OD at 680 nm of the algal culture was 0.11 to 0.12, which corresponded to an algal concentration of 2.32 × 109 cells/L.

3.2. Characterization and Behavior of Nano-TiO2 in the Medium

Nano-TiO2 particles (rutile form) 40 to 50 nm in diameter were purchased from Zhejiang Hongsheng Material Technology Co., China. The suspensions (1000 mg/L) were placed in an ultra-sound water bath (100 W, 40 kHz) for 30 min before being diluted to different exposure concentrations. Zeta potentials and particle sizes of nano-TiO2 were measured by a dynamic light scatting (DLS) size analyzer (Zetasizer Nano-ZS, Malvern, UK). Scanning electron microscope (SEM) images were taken using a JEOL SM4800 SEM. Suspensions of 1.0 and 10.0 mg/L nano-TiO2 were prepared by dilution in the culture medium. At 10 min intervals, the absorbance of nano-TiO2 suspension was measured using a UV-vis spectrophotometer. The settling behavior of the NPs was investigated by the reduction of absorbance over 600 min.

3.3. Sorption of Zn2+ on Nano-TiO2

The interactions of Zn2+ with nano-TiO2 were studied by performing the traditional batch sorption experiment. A stock solution of Zn2+ was prepared by dissolving ZnSO4 into DI water. The solution was diluted into 125 mL flasks to serial concentrations of 0.1, 0.2, 0.4, 0.5, 1.0, 2.0, 3.0, 5.0, and 10.0 mg/L with 50 mL culture medium. The pH of the Zn2+ solutions was adjusted to 7.0 ± 0.1 using 0.01 M HCl or NaOH. The nano-TiO2 suspensions were diluted to a concentration of 10.0 ± 0.1 mg/L in each flask. The mixed suspensions were then shaken to achieve sorption equilibrium within 10 h. The mixed suspensions were centrifuged at 5000 rpm for 10 min. The supernatants were collected and again centrifuged at 5000 rpm for 10 min [9]. Zinc concentrations in the supernatants were measured by inductively coupled plasma atomic emission spectroscopy (ICP-optima 2001DV, Perkin-Elmer, Waltham, MA, USA).

3.4. Toxicity Tests

In the toxicity tests, the algal growth results were obtained by the difference between the final and initial algae densities and chlorophyll fluorescence parameters. Growth of Anabaena sp. density was monitored daily for 96 h and assessed by initial and final OD value at 680 nm. All chlorophyll fluorescence parameters were determined using a Phyto-PAM fluorometer (Pyhto-PAM, Walz, Germany). Phyto-PAM is a four-wavelength chlorophyll fluorometer used to assess the chlorophyll content and photosynthetic activity of planktonic algae. The variables Chl-a fluorescence (Fv) and maximal fluorescence (FM) were measured. Photosystem II activity was determined using the ΔF mode (F, fluorescence yield = FM − Fv). The yield (Y, photochemical transformed energy) was calculated as Y = Fv/FM. In recent years, fluorescence parameters based on fluorescence yield have been proposed to be a useful tool for the toxic evaluation of pollutants [28].

The toxicity experiments were carried out using 50 mL cultures grown in 125 mL flasks. The Anabaena sp. solutions with a series of Zn2+ concentrations were cultured and observed in an incubator. The final and initial algae densities and chlorophyll fluorescence parameters were used to examine Zn2+ toxicity. The Anabaena sp. solutions with a series of nano-TiO2 concentrations were tested following the same methods in Zn2+ toxicity tests. To investigate the synergistic effects of Zn2+ and nano-TiO2, two sets of experiments were studied. The first set of experiments studied the toxic effects of Zn2+ with fixed nano-TiO2 concentrations. The second set of experiments examined the toxic effects of nano-TiO2 with fixed Zn2+ concentrations. After the toxicity test, the mixed suspensions were centrifuged and the supernatants were collected. Zinc concentrations in the supernatants were measured by ICP.

ROS production was measured by using the cell permeable indicator 2′,7′-dichlorodihydrofluorescein diacetate (H2DCF-DA) [39]. The specific method of operation is that 1.0 mL algal cells grown for 72 h were centrifuged at 10,000 rpm for 10 min, after which the supernatant was discarded, washed with phosphate buffer solution twice, followed immediately by the addition of 10 μM H2DCF-DA to the cell pellet. Next they were incubated in a water bath at 37 °C for 2 h in the dark, and washed with PBS again. The fluorescence intensity of algae cells was measured by a fluorescence microplate reader (Synergy™, Bio-Tek, Richmond, CA, USA) at excitation/emission wavelengths of 488/525 nm. Changes in ROS levels as compared to the control were evaluated using relative ROS level.

3.5. Statistical Analysis

The effective concentrations causing 50% inhibition in algal growth (EC50) were calculated and statistical significance was considered at the p < 0.05 level. Differences in growth rates between the control and experimental samples were demonstrated using a comparison of means test for each test concentration. Algal toxicity tests with Anabaena sp. were performed in triplicate. Data were presented as the average values of three parallel detections.

4. Conclusions

The mortality of Anabaena sp. was mostly a result of Zn2+ uptake. At a fixed nano-TiO2 concentration, the mortality was also dependent on Zn2+ concentration. However, at a fixed Zn2+ concentration, the addition of nano-TiO2 had a dual effect on Anabaena sp. At low nano-TiO2 concentrations, the mortality increased with increasing nano-TiO2. When the nano-TiO2 concentration reached a certain value, the amount of Zn2+ dissolved and adsorbed by algae sharply decreased. High nano-TiO2 concentrations reduced aggregation, which decreased the mortality of Anabaena sp. with increasing nano-TiO2. The results revealed that photosynthetic parameters were useful in predicting the synergistic toxicity profiles of NPs and heavy metals.