Introduction

Phthalate esters (PAEs) are a family of industrial chemicals used as softeners, adhesives or solvents by a variety of industries. They are mainly used in the polymer industry as plasticizers in PVC and to a lesser extent in the non-polymer industry for different consumer products (sealants, paints, printing inks, cosmetics, coatings of different products such as cars, coils, cables or fabrics, etc.) (OSPAR Commission 2006). Because phthalates are external plasticizers which soften the resins without reacting chemically with them, they tend to migrate slowly out of the plastic, either into the air by volatilization or into water or other solvents by dissolution (Laughlin et al. 1978). In China, the highest level of PAEs in rivers and lakes was 263.8 μg l−1, which was almost the same as those from the other rivers and lakes in the world (Tian et al. 2003; Sha et al. 2007).

Di-n-butyl phthalate (DBP), butyl benzyl phthalate (BBP) and di-2-ethylhexyl phthalate (DEHP) are three members of PAEs, and identified as priority controlled hazardous substances by the United States Environmental Protection Agency (USEPA) (Jin 1990). In recent years, they have been shown to have potential for endocrine disrupting effects on vertebrates and humans (OSPAR Commission 2006). Although some information on the acute and chronic toxicities of these three PAEs to aquatic microorganisms, algae, invertebrates and fish is available (Stamples et al. 1997; Huang et al. 1998, 1999; Ministry of Environment, Government of Japan 2002, 2004; Bradlee and Thomas 2003; Norman et al. 2007), relatively little is known about their chronic toxicities on rotifers (Zhao et al. 2007).

Being a natural food link between the primary producers (algae) and zooplanktivorous fish, rotifers are important to maintain an ecological balance in aquatic ecosystems (Nogrady et al. 1993). So, there is an increasing trend in the use of rotifers as bioassay organisms for aquatic ecotoxicological studies in recent 20 years (Snell and Janssen 1995; Snell and Joaquim-Justo 2007). Among all the reported studies, analysis of the survival and reproduction, integrated by life-table calculations, can provide information on the intrinsic rate of population increase, net reproductive rate, generation time and life expectancy at hatching under a given set of environmental conditions, and be an important theoretical basis to measure the effects of chronic exposure of populations to toxicants (Ferrando et al. 1993).

The main purpose of the present study is to assess the effects of three phthalate esters, DBP, BBP and DEHP, on survival and reproduction of the freshwater rotifer B. calyciflorus using life-table demographic techniques, and to screen out sensitive endpoints which could be used to monitor the ecological effects of sublethal concentrations of the three phthalate esters.

Materials and methods

The rotifer B. calyciflorus used in this experiment was obtained by hatching resting eggs collected from sediments of Lake Jinghu and then clonally culturing under controlled laboratory conditions. Stock rotifer cultures were kept at 25 ± 1°C in natural light and daily fed on 2.0 × 106 cells ml−1 of Scenedesmus obliquus. For mass cultures as well as for experiments, reconstituted hardwater (EPA medium) (USEPA 1985) was used as the medium. Algae were grown in a semi-continuous culture using HB-4 medium (Li et al. 1959) renewed daily at 20%. Algae in exponential growth were centrifuged and then resuspended in EPA medium.

DBP, BBP and DEHP (standard grade, ≥97%, Sigma-Aldrich, Germany) were used as the toxicants. Stock solutions were prepared by dissolving DBP, BBP and DEHP in 100% acetone, then diluted to the desired concentrations using EPA medium, respectively.

Based on the concentrations of PAEs in water from all the studied rivers and lakes in China (Tian et al. 2003; Sha et al. 2007) and the results of preliminary tests, seven toxicant concentrations (0.005, 0.05, 0.5, 5, 50, 500, 5,000 μg l−1), a blank control (EPA medium) and a solvent control [containing 0.5% (v/v) acetone which was the same as that in the toxicant at 5,000 μg l−1] were selected for the present study, each consisting of four replicates of 10 rotifers. Before experiment commenced, rotifers with amictic eggs were randomly removed from the stock cultures and placed into a glass dish containing 10 ml of EPA medium with 2.0 × 106 cells ml−1 of S. obliquus which is optimal for survival and reproduction of the rotifers (Xi et al. 2001). After 3 h, ten neonates for each replicate were collected and transferred into a 6-ml glass cup containing 5 ml of test solution with 2.0 × 106 cells ml−1 of S. obliquus. Thereafter, the number of neonates produced and the number of original test individuals alive were recorded and then neonates were removed every 8 h (Xi and Hu 2003; Huang et al. 2007; Zha et al. 2007). The original rotifers alive were transferred into freshly prepared test solution every 24 h. The life-table experiments were conducted in darkness (expect when the rotifers were observed under a light microscope) (Janssen et al. 1994; Marcial et al. 2005) at 25 ± 1°C until each individual of every cohort died.

Based on the data collected, age-specific survivorship (l x ) and age-specific fecundity (m x ) were constructed for each cohort (replicate) using conventional life-table techniques (Poole 1974). Intrinsic rate of population increase (r m ), net reproductive rate (R 0), generation time (T) and life expectancy at hatching (e 0) were calculated according to Krebs (1985) and Lotka (1913).

One-way analysis of variance (ANOVA), with the concentration of DBP, BBP or DEHP as the independent variable, and each of the life-table demographic parameters as the dependent variable, followed by Dunnett’s test was conducted for pair-wise comparisons of each concentration of test chemicals to the solvent control (Zar 1999).

Results

DBP and BBP both had a lethal effect at 5,000 μg l−1 before the rotifers reached sexual maturity. When compared to the solvent control, DBP at 500 μg l−1 increased significantly the age-specific fecundity of rotifers, but none of the three phthalate esters at the other concentrations significantly affected the age-specific fecundity. The age-specific survival of the rotifers exposed to all the three phthalates was similar to the solvent control (Fig. 1).

Fig. 1
figure 1

Age-specific survivorship (filled square) and age-specific fecundity (open triangle) of B. calyciflorus exposed to different concentrations (μg l−1) of di-n-butyl phthalate (DBP), butyl benzyl phthalate (BBP) and di-2-ethylhexyl phthalate (DEHP)

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Compared to the blank control, the solvent (acetone at 0.5%) did not significantly affect any of the life-table demographic parameters of the rotifers.

DBP significantly influenced all the life-table demographic parameters of B. calyciflorus (Table 2). Compared to the solvent control, DBP at 500 μg l−1 significantly increased the net reproductive rate, and prolonged the generation time and the life expectancy at hatching of the rotifers, but DBP at 50 μg l−1 markedly decreased the intrinsic rate of population increase of the rotifers (Table 1).

Table 1 Effects of different concentrations (μg l− 1) of di-n-butyl phthalate (DBP), butyl benzyl phthalate (BBP) and di-2-ethylhexyl phthalate (DEHP) on net reproductive rate (R 0), intrinsic rate of population increase (r m ), generation time (T) and life expectancy at hatching (e 0) of B. calyciflorus
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BBP also significantly affected all the life-table demographic parameters of B. calyciflorus (Table 2). Compared to the solvent control, BBP at 500 μg l−1 markedly increased the net reproductive rate, and prolonged the generation time and the life expectancy at hatching of rotifers; BBP at 50 and 500 μg l−1 significantly increased the intrinsic rate of population increase of the rotifers (Table 1).

Table 2 Results of analysis of variance (one-way ANOVA) performed for different life-table demographic variables of B. calyciflorus exposed to di-n-butyl phthalate (DBP), butyl benzyl phthalate (BBP) and di-2-ethylhexyl phthalate (DEHP)
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DEHP did not markedly affect the net reproductive rate, the intrinsic rate of population increase, the generation time nor the life expectancy at hatching of B. calyciflorus (Tables 1, 2).

Discussion

As a consequence of chronic toxicant stress, a reduction in net reproductive rate was observed in B. patulus, B. calyciflorus, B. plicatilis and D. magna (Rao and Sarma 1986; Fernandez-Casalderrey et al. 1991, 1993; Janssen et al. 1994; Huang et al. 1999; Xi and Hu 2003; Chu et al. 2005; Xu et al. 2005). However, sublethal concentrations of aldrin, chlordecone and DBP increased the net reproductive rates of B. calyciflorus and D. magna (Huang et al. 1999, 2007; Zha et al. 2007). Similarly, in the present study, DBP and BBP both at 500 μg l−1 increased the net reproductive rate of B. calyciflorus. The above stated results indicated that sublethal concentrations of pollutants with endocrine disrupting activity for vertebrates might have an intriguing effect on the reproduction of the rotifers, but higher concentrations of them might have a toxic effect. It is worthy of further research as to how sublethal concentrations of DBP, BBP and DEHP stimulate the reproduction of the rotifers.

The population growth of rotifers decreased under the stress of all the tested pollutants with estrogenic activity for vertebrates (Rao and Sarma 1986, 1990; Fernandez-Casalderrey et al. 1991, 1993; Ferrando et al. 1993; Janssen et al. 1994; Snell and Carmona 1995; Gallardo et al. 1997; Preston et al. 2000; Preston and Snell 2001; Radix et al. 2002; Mariraz-Perez et al. 2004; Xi and Feng 2004; Marcial et al. 2005; Chu et al. 2005), except micromolar concentrations of gamma-aminobutyric acid, growth hormone, human chorionic gonadotropin and 5-hydroxytryptamine, which increased the population growth of B. plicatilis, aldrin and DBP, which increased the intrinsic rate of population increase of B. calyciflorus and D. magna (Gallardo et al. 1997; Huang et al. 1999, 2007), and juvenile hormone, 20-hydroxyecdysone and tri-iodothyronine, which had no effect on population growth (Gallardo et al. 1997). In the present study, BBP at 50 and 500 μg l−1 increased the intrinsic rate of population increase of the rotifers, but the reverse was also true for DBP at 50 μg l−1. DEHP at 0.005–5,000 μg l−1 did not affect the intrinsic rate of population increase.

Sublethal concentrations of aldrin increased the life expectancy at hatching of B. calyciflorus (Huang et al. 2007), and glyphosate, aldrin and chlordecone prolonged the mean lifespan of B. calyciflorus (Chu et al. 2005; Huang et al. 2007; Zha et al. 2007). Identical to the effects of those pollutants, in the present study DBP and BBP both at 500 μg l−1 prolonged the life expectancy at hatching of the rotifers, but DEHP at 0.005–5,000 μg l−1 did not affect it. The above stated results indicate that sublethal concentrations of pollutants with endocrine disrupting activity might be beneficial to not only the reproduction but also the survival of rotifers.

Several studies have found that sublethal concentrations of aldrin and chlordecone prolonged the generation time of rotifers, but Cu, lindane, thiophanate-methyl and deltamethrin shortened the generation time of rotifers (Janssen et al. 1994; Xi and Hu 2003; Xu et al. 2005; Huang et al. 2007; Zha et al. 2007). In the present study, DBP and BBP, both at 500 μg l−1, prolonged the generation time of B. calyciflorus, but DEHP at 0.005–5,000 μg l−1 did not affect it.

Similar to the non-significant dose–response relationship between the concentration of aldrin as well as chlordecone and each of all the life-table demographic parameters of B. calyciflorus (Huang et al. 2007; Zha et al. 2007), any significant dose–response relationship between the concentration of DBP, BBP as well as DEHP and each of the life-table demographic parameters of B. calyciflorus was not observed in the present study, which might be attributed to 10-fold pollutant concentration interval.

The lifespan and the net reproductive rate of B. calyciflorus differed with the rotifer clones. At 25°C and under toxicant free conditions, the B. calyciflorus clone which has been used in our laboratory for ecotoxicologial studies had a lifespan of 89–148 h, a life expectancy at hatching of 88–126 h and a net reproductive rate of 11–22 (Huang et al. 2007; Zha et al. 2007), and that studied by Araujo and McNair (2007) had a lifespan of 12–14 days and a net reproductive rate of 17–19. Considering the significantly intriguing effect of BBP at 500 μg l−1 on the population growth of B. calyciflorus (Zhao et al. 2007), another rotifer clone with a shorter lifespan and a lower net reproductive rate was used in the present study to detect the possibly intriguing effects of the three PAEs on its survival and reproduction. The present results showed that, although the B. calyciflorus clone used in the present study had a shorter life expectancy at hatching of 64 h and a lower net reproductive rate of 3.19 (Table 1), it could be used to sensitively detect the intriguing effects of DBP and BBP on survival and reproduction of rotifers.

Compared with the conventional test animals such as D. magna and Pimephales promelas (fathead minnow), rotifers had comparable sensitivities to most compounds, but no single species was consistently the most sensitive to all compounds (Snell et al. 1991a, b; Persoone et al. 1993, Persoone and Janssen 1993). As far as the intriguing effect of DBP on the reproduction of zooplankton was concerned, B. calyciflorus had the same sensitivity to DBP as D. magna, because DBP at 0.5 mg l−1 stimulated the net reproductive rate of both B. calyciflorus and D. magna (Huang et al. 1998, 1999; the present study).

The 48-h LC50 values of DBP and BBP were 3.0 and 1.0–4.7 mg l−1 for D. magna, respectively (Mayer and Ellersieck 1986; Barera and Adams 1981). The 10-day LC50 values of DBP and BBP were 0.63 and 0.46 mg l−1 for Hyalella azteca; and 2.48 and 1.23 mg l−1 for Lumbriculus variegates, respectively (Call et al. 2001). DBP and DEHP were less toxic to a crustacean, a green alga, and a bacterium than BBP (Jonsson and Baun 2003). Similarly, the comparison among the effects of the three phthalate esters on the survivorships of B. calyciflorus obtained in the present study showed that the toxicity of BBP was the highest, and that of DEHP was the lowest.

Conclusion

All the life-table demographic parameters of B. calyciflorus were markedly affected by DBP and BBP, but not affected by DEHP. The net reproductive rate which represents the output of reproduction was more affected than all the other parameters which represent population growth, development or survival of the rotifers. Among all the parameters, the intrinsic rate of population increase was the most sensitive to DBP and BBP. Sublethal concentrations of DBP and BBP had intriguing effects on survival and reproduction of the rotifers, but the levels of PAEs in water from all the studied rivers and lakes in the world did not affect the population growth of the rotifers.