Elsevier

Chemosphere

Volume 150, May 2016, Pages 285-293
Chemosphere

Effects of artificial sweeteners on metal bioconcentration and toxicity on a green algae Scenedesmus obliquus

https://doi.org/10.1016/j.chemosphere.2016.02.043Get rights and content

Highlights

  • Artificial sweeteners are frequently detected in aquatic environments and can affect the fate and toxicity of heavy metals.

  • ACE enhanced the growth of S. obliquus and the bioconcentration of Cd2+, while the impacts of SUC were not significant.

  • The toxic effect of Cd2+ and Cu2+ to S. obliquus decreased with the presence of ACE and SUC, respectively.

  • Pulse-amplitude-modulated parameters of the algae disclosed the mechanisms involved in metal toxicity at subcellular levels.

Abstract

The ecotoxicity of heavy metals depends much on their speciation, which is influenced by other co-existing substances having chelating capacity. In the present study, the toxic effects of Cd2+ and Cu2+ on a green algae Scenedesmus obliquus were examined in the presence of two artificial sweeteners (ASs), acesulfame (ACE) and sucralose (SUC) by comparing the cell specific growth rate μ and pulse-amplitude-modulated (PAM) parameters (maximal photosystem II photochemical efficiency Fv/Fm, actual photochemical efficiency Yield, and non-photochemical quenching NPQ) of the algae over a 96-h period. Simultaneously, the bioconcentration of the metals by the algal cells in the presence of the ASs was measured. The presence of ACE enhanced the growth of S. obliquus and promoted the bioconcentration of Cd2+ in S. obliquus, while the impacts of SUC were not significant. Meanwhile, EC50 values of Cd2+ on the growth of S. obliquus increased from 0.42 mg/L to 0.54 mg/L and 0.48 mg/L with the addition of 1.0 mg/L ACE and SUC, respectively. As for Cu2+, EC50 values increased from 0.13 mg/L to 0.17 mg/L and 0.15 mg/L with the addition of 1.0 mg/L ACE and SUC, respectively. In summary, the two ASs reduced the toxicity of the metals on the algae, with ACE showing greater effect than SUC. Although not as sensitive as the cell specific growth rate, PAM parameters could disclose the mechanisms involved in metal toxicity at subcellular levels. This study provides the first evidence for the possible impact of ASs on the ecotoxicity of heavy metals.

Introduction

Heavy metals are one class of the most common contaminants in aquatic environments. Substantially high concentrations of cadmium (Cd), copper (Cu), lead and zinc, which are among the thirteen priority metal/metalloid pollutants categorized by US EPA, have been found in lakes, rivers, estuaries and coastal areas (Wang et al. 2012). The toxicity and bioconcentration of heavy metals on microalgae are important aspects in assessing their ecological risk since algae are the primary producers in aquatic ecosystems. Moreover, the heavy metal absorbed by algae may be transferred to other organisms through food webs (Wang et al., 2012, Cheng et al., 2013) and eventually cause serious effects at higher trophic levels (Barakat, 2011). Chlorophyll fluorescence has been recognized as a rapid, non-intrusive technique to monitor photosynthetic apparatus prior to the appearance of visible damage of algae, as well as to analyze their protective responses (Suresh Kumar et al., 2014). Apart from its utility in determining the physiological status of photosynthesizers in the natural environment, chlorophyll fluorescence-based methods are applied in ecophysiological and toxicological studies to examine the effect of environmental changes and pollutants on algae. A number of studies have employed pulse-amplitude-modulated (PAM) fluorometry to explore metal effects on algal photosynthesis, which endows a deep insight into the impact on the process of photosynthesis (Ou-Yang et al., 2013). PAM chlorophyll fluorescence can reflect the interactions of photosystem II (PSII), membrane degradation and photosynthetic electron transport efficiency. The maximal photosystem II photochemical efficiency (Fv/Fm) is a measurement of the light energy transfer in dark-adapted samples or the photochemical quantum yield of open PSII centers, where the actual photochemical efficiency (Yield) is measured in the light and shows a quantitative relationship with the quantum yield of CO2-assimilation under laboratory conditions where linear electron flow is dominant (Fryer et al., 1998). The non-photochemical quenching (NPQ) depicts the relative increase in the sum of the rate constants of the non-photochemical deactivation processes (fluorescence emission, heat dissipation and spillover of excitation energy from PSII to PSI) relative to the dark-adapted state (Suresh Kumar et al., 2014).

It is well known that the bioconcentration potential and toxicity of a heavy metal are governed not only by its total concentration but also by its chemical speciation, which is influenced by environmental conditions and the presence of co-existing substances. Chelating agents, including natural organic matters and synthetized chelating agents like EDTA are the most important co-existing substances that affect the speciation of heavy metals (Leštan et al., 2008). In addition, organic pollutants with polar groups with chelating capacity can also influence the speciation and toxicity of heavy metals. Recently, related study has drawn much attention since most of “emerging chemicals” are polar compounds. For instance, the antibiotics oxytetracycline and ciprofloxacin were reported to be able to bind metals via multiple coordination sites and the complex compounds showed higher toxicity to Vibrio fischeri using luminescence inhibition test (Zhang et al., 2012). Chen et al. found that the toxicity of Cu2+ to an aquatic unicellular alga Scenedesmus obliquus could be mediated by a herbicide imazethapyr (Chen et al., 2013).

In recent years, artificial sweeteners (ASs) have drawn growing attention due to their high production and potential adverse effects on non-target organisms (Lange et al., 2012). ASs are compounds containing few or no calories and are widely used all over the world as sugar substitutes in substantial quantities in foods, drinks, drugs, and hygiene products (Scheurer et al., 2009). The wide usage of products containing ASs makes these compounds frequently detected in the environment. Lubick (2008) gave the first report in the world on the existence of ASs, which attracted the attention of other scientists soon afterwards. Buerge et al. (2009) found the presence of acesulfame (ACE) in 65% of the groundwater samples investigated and up to 4.7 μg/L of sucralose (SUC) in the groundwater in Switzerland. In a subsequent more extensive investigation throughout Europe, ACE and SUC were extensively (>88%) detected in wastewater treatment plant effluents in European countries with concentrations up to 2.5 mg/L and 12.9 μg/L, respectively (Loos et al., 2013). Meanwhile, Mawhinney et al. (2011) detected SUC in wastewater, surface water and groundwater samples from United States, in concentrations ranging between 0.8 and 1.8 μg/L, < 0.05–1.8 μg/L and 0.6–2.4 μg/L, respectively. Our group for the first time reported the existence of ASs in the environment of China, and a total concentration of 3.5 ng/L to 1.3 mg/L of the seven common ASs were detected in the aquatic environment of Tianjin, China, with ACE, saccharin (SAC), and cyclamate being the most dominant (Gan et al., 2013a). Comprehensive toxicological tests had been conducted on the ASs before their applications and they appear to be nontoxic to human health within the regulated concentrations (Chattopadhyay et al., 2011). Though studies about ecotoxicity of ASs on aquatic organisms are rather scarce (Karstadt, 2010), it has been generally recognized that ASs have low toxicity on aquatic organisms either (Stolte et al., 2013). SUC was proposed to have the potential to affect aquatic ecosystems because it mimics sucrose, a biochemical that is linked to various biological functions, including photosynthesis and ‘‘feeding cues in zooplankton’’ (Kessler, 2009). In plants, SUC was shown to interfere with sucrose uptake in sugar cane by disrupting the substrate-binding site of the sucrose uptake gene, ShSUT1 (Reinders et al., 2006). However, SUC did not inhibit plant cotelydon sucrose uptake, nor did it affect frond number, wet weight, or growth rate in aquatic plant, Lemna gibba (Soh et al., 2011). There is only one report about the effect of ASs on algae. Stolte et al. (2013) examined the 24 h toxicity of ACE and SUC towards green algae Scenedesmus vacuolatus, and the No Observed Effect Concentration (NOEC) was given as 1000 mg/L and the Lowest Observed Effect Concentration (LOEC) as > 1000 mg/L, which are two to four orders of magnitude higher than their reported environmental concentrations.

Even though the toxicity of the ASs themselves is low, their increasing concentration in the aquatic environment may affect the fate of other co-existing contaminants and need to be further studied. In fact, some ASs, such as SAC and aspartame (ASP) are polyfunctional ligands widely studied in coordination chemistry (Haider et al., 1985, Yilmaz et al., 2001, Çakir et al., 2003). The imino hydrogen in the molecule of SAC is acidic and the molecule can be easily converted into the corresponding nitranion (the so-called saccharinate anion) and offers different coordination sites to metallic centers, i.e., one N, one O (carbonylic) and two O (sulfonic) atoms. Using these donor atoms, the anion can form either N- or O-monodentate or bidentate (N, O) coordination, as well as more complex polymeric species with the participation of all possible donor atoms (Haider et al., 1985). However, whether the interactions between ASs and heavy metals can affect the fate and toxicity of heavy metals in the environment has never been reported.

Hence, the objective of the present study was to evaluate the potential effects of two most frequently detected ASs, ACE and SUC, on the bioconcentration and toxicity of heavy metals. Toward this objective, the bioconcentration and toxicity of two common heavy metal ions, Cd2+ and Cu2+ on a green algae S. obliquus with and without the ASs were studied. The two metal ions were selected due to their extensive detection in the environment and also due to their different chelating patterns. Attempts were made to explore the mechanisms by examining the changes in algal growth, the chlorophyll fluorescence of algal cells and the concentration and distribution of the heavy metals in the algae.

Section snippets

Chemicals and algal culture

The standards for ACE and SUC were procured from Sigma–Aldrich (St. Louis, MO, USA). Sucralose-d6 and acesulfame-d4, used as internal standards, were obtained from TRC (North York, ON, Canada). The ion pair reagent tris(hydroxymethyl)aminomethane (TRIS) was obtained from Sigma–Aldrich (St. Louis, MO, USA). The standard solutions of Cd and Cu (1000 mg/L ± 2% in 1% HNO3) were purchased from O2si (Charleston, SC, USA). All other solvents and reagents were of HPLC or analytical grade.

The freshwater

Metal bioconcentration and subcellular fractionation

The initial concentrations ([M]0) of Cd2+ and Cu2+ determined using samples taken immediately after spiking are listed in Table S2. The measured values were not significantly different from the nominal values (p > 0.05), and the difference between the single metal group with those in mixture groups in the presence of ASs was less than 9%. Nevertheless, the measured [M0] in each group was used as the reference value in calculation of variations to calibrate the error during spiking so as to get

Conclusions

Effects of artificial sweeteners on metal bioconcentration and toxicity to a green alga S. obliquus were studied for the first time. The presence of ACE contributed to the growth of S. obliquus and promoted the uptake of Cd2+ in the algal cells and further reduced the toxic effect. The SUC alone didn't have any significant influence on S. obliquus growth (p > 0.05), whereas it can lower the impact of metal ions to S. obliquus either, though not as strong as ACE. Although not as sensitive as the

Acknowledgment

This study was supported by Natural Science Foundation of China (No. 41225014), National University Student Innovation Program (No. 201310055022) and Ministry of Education of China as an innovative research team project (No. IRT13024).

References (43)

  • A.-J. Miao et al.

    Cadmium toxicity to two marine phytoplankton under different nutrient conditions

    Aquat. Toxicol.

    (2006)
  • F. Monnet et al.

    Relationship between Psii activity, CO2 fixation, and Zn, Mn and Mg contents of Lolium perenne under zinc stress

    J. Plant Physiology

    (2001)
  • M. Pérez-Rama et al.

    Cadmium removal by living cells of the marine microalga Tetraselmis Suecica

    Bioresour. Technol.

    (2002)
  • S. Stolte et al.

    Ecotoxicity of artificial sweeteners and stevioside

    Environ. Int.

    (2013)
  • K. Suresh Kumar et al.

    Algal Photosynthetic responses to toxic metals and herbicides assessed by chlorophyll a fluorescence

    Ecotoxicol. Environ. Saf.

    (2014)
  • N. Wang et al.

    Effects of microcystin-Lr on the metal bioaccumulation and toxicity in Chlamydomonas reinhardtii

    Water Res.

    (2012)
  • V.T. Yilmaz et al.

    Saccharin complexes of Co (ii), Ni (ii), Cu (ii), Zn (ii), Cd (ii) and Hg (ii) with ethanolamine and diethanolamine: synthesis, spectroscopic and thermal characteristics. Crystal structures of [Zn(Ea)2(Sac)2] and [Cu2(Μ-Dea)2(Sac)2]

    Polyhedron

    (2001)
  • Y. Zhang et al.

    Insights into aquatic toxicities of the antibiotics oxytetracycline and ciprofloxacin in the presence of metal: complexation versus mixture

    Environ. Pollut.

    (2012)
  • I.J. Buerge et al.

    Ubiquitous occurrence of the artificial sweetener acesulfame in the aquatic environment: an ideal chemical marker of domestic wastewater in groundwater

    Environ. Sci. Technol.

    (2009)
  • S. Chattopadhyay et al.

    Artificial sweeteners–a review

    J. Food Sci. Technol.

    (2011)
  • M.J. Fryer et al.

    Relationship between Co2 assimilation, photosynthetic electron transport, and active O2 metabolism in leaves of maize in the field during periods of low temperature

    Plant Physiol.

    (1998)
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