Acute toxicity of bisphenol B
In the test of acute toxicity, comparing with the control group, the solvent control groups did not cause significant effect on the growth or survival of organisms, indicating that 0.05% DMSO was negligible in the exposure groups. Significant dose-response relationships between BPB concentrations and the inhibition rate of algal growth, the immobilization of D. magna or the mortality of Danio rerio were observed (Fig. 1). The resulting EC50 (or LC50) values with the 95% confidence intervals were listed in Table 1. For T. obliquus, the 96 h EC50 value was 12.3 mg/L. Czarny et al. (2021)) studied the toxic effect of BPB on two cyanobacteria, and found that the 7–14 d EC50 were 36.5–40.3 mg/L and 44.2–87.3 mg/L. Apparently, the green algae were more sensitive to BPB than cyanobacteria. For D. magna, the 48 h EC50 value was 3.93 mg/L. This was in good agreement with the result reported by Chen et al., who found that the 48 h EC50 value of BPB to D. magna was 5.50 mg/L (Chen et al. 2002). For Danio rerio, the 96 h LC50 value was 4.13 mg/L. Unfortunately, to our knowledge, there are no other studies on the acute toxicity of BPB to zebrafish.
Table 1
Half effective (or lethal) concentrations of BPB to three organisms in the acute toxicity.
Species | Endpoints | Effect value (mg/L) | 95% Confidence interval (mg/L) |
Tetradesmus obliquus | 96 h EC50 | 12.3 | 11.7–13.1 |
Daphnia magna | 48 h EC50 | 3.93 | 3.53–4.37 |
Danio rerio | 96 h LC50 | 4.13 | 4.07–4.19 |
According to the criteria from “the globally harmonized system of classification and labelling of chemicals (GHS)” (GHS 2019), chemicals with 10 mg/L < EC50 (or LC50) < 100 mg/L were considered as class III toxic substances; while chemicals with EC50 (or LC50) < 10 mg/L were considered as class II toxic substances. Thereout, D. magna and Danio rerio seem to be more vulnerable to BPB than T. obliquus. Obviously, there was an order of magnitude difference in EC50 (or LC50) value between T. obliquus and other two organisms, indicating that BPB posed little risk to the primary trophic organisms. Among the three species, D. magna was the most sensitive species to BPB, followed by Danio rerio and T. obliquus. D. magna naturally occurs in the lentic freshwater system, and it is often a standard test organism for the aquatic toxicity of chemicals (Nagato et al. 2016). In order to explore the environmental risk of BPB, it is crucial to further study the chronic toxicity of BPB to D. magna, under a low dose for a long-term exposure.
Due to the ubiquity in environment and the well-known endocrine disrupting effect of BPA, we collected the data on the aquatic toxicity of BPA from previous literatures. The acute toxicity data were presented in Table S3. As shown, the EC50 values of BPA to algae ranged from 8.65 to 63.5 mg/L (Li et al. 2009, Tišler et al. 2016, Zhang et al. 2014), and those to D. magna ranged from 7.30 to 14.4 mg/L (Alexander et al. 1988, Brennan et al. 2006, Hirano et al. 2004, Ike et al. 2002, Liu et al. 2019, Mansilha et al. 2013, Nagato et al. 2016). In the case of zebrafish, the range of LC50 values was 8.04–12.8 mg/L (Blanc et al. 2019, Chan &Chan 2012, Corrales et al. 2017, Moreman et al. 2017, Mu et al. 2018). By contrast, BPB was more hazardous to aquatic organisms than BPA, even though it has a lower detectable rate in the environment. In addition, BPB has a larger hydrophobicity than BPA, which means that it has higher bioaccumulation in the body of organisms (Chen et al. 2016). All of these further demonstrate that more attention should be paid to BPB in water in the future.
Chronic toxicity of bisphenol B to D. magna
Although the detectable rate and the concentration of BPB in environmental media are quite low, the aquatic organisms live in water for a long time and they are always inevitably exposed to BPB. In this study, the most sensitive organism, i.e., D. magna, was employed to evaluate the chronic toxicity of BPB, through a series of parameters relating to reproductive ability, which were the time of the first brood, the number of neonates in the first brood, the number of broods and the total neonates in the 21-day exposure.
As shown from Fig. 2A, almost all of females produced the first brood on the 7th–11th day. Relative to the control, no visible change in the first reproduction time was observed in all the treatments even at concentration up to 0.10 mg/L. The number of the first brood progeny exhibited a somewhat decreasing trend with increasing BPB concentration (Fig. 2B). However, no statistically significant differences between the treatments and the control were found. This may be because the exposure time was too short, and the slight changes in the two indicators were just a stress response, which was not enough to cause significant differences. The number of broods and the total neonates were measured during the whole 21 days period, and they showed a descending trend with the increase of BPB concentration (Fig. 2C and 2D). The decrease in the number of broods may be due to the delayed spawning time. In addition, a significantly negative correlation between the total neonates and the BPB concentrations was observed (p < 0.01). It further proved that BPB, as an endocrine disrupting compound, impeded the process of oogenesis and limited the birth of offspring. Moreover, under the high BPB concentrations (> 0.04 mg/L), some ephippia were found at the bottom of test solution, which suggesting that D. magna shifted from parthenogenesis to sexual reproduction, in response to the adverse environment caused by BPB exposure.
In addition to the reproductive ability, the growth status of parent D. magna is also an important indicator of chronic toxicity. After 21 days exposure, the average body length of parent D. magna in each group was presented in Fig. 3A. As shown, even though no regular trend was observed, the D. magna in 0.04, 0.08 and 0.10 mg/L BPB groups has smaller body length than that in control group. Thus, BPB inhibited the growth of parent D. magna, which was unfavorable for their performance to produce offspring. As shown from Fig. 3B, comparing with the control group, exposure to 0.08 and 0.10 mg/L BPB could result in a significant reduction in rm value. The reduction of rm meant that BPB inhibited the growth and renewal of population. Hence, once D. magna was exposed to BPB for a long time, it would encounter a devastating damage, not only from the individual level but also from the population level.
Ecological risk assessment of BPB and BPA
According to the protocol of risk quotient method, the calculation of risk quotient required the no-observed effect concentration (NOEC) and the measured concentration in environment (MEC). Among multiple endpoints, the most sensitive endpoint was the number of broods and the corresponding 21 d-NOEC value was estimated at 0.01 mg/L. In this case, 0.01 mg/L, was employed as the NOEC value of BPB. In addition, we also collected the toxicity data of BPA from previous literatures (Table S3). Results indicated that D. magna was also the most sensitive organism to BPA, and the NOEC values of BPA in chronic toxicity were reported as 0.86–5.00 mg/L (Brennan et al. 2006, Jemec et al. 2012, Mansilha et al. 2013, Tišler et al. 2016). Obviously, the NOEC value of BPB was two orders of magnitude lower than that of BPA, while the EC50 value of BPB was just one order of magnitude lower than that of BPA (as shown in Section 3.1 and Table 1). Comparing the two compounds, the difference in NOEC from chronic toxicity was much larger than that in EC50 from acute toxicity. This difference further demonstrated that in the long run, BPB was much more hazardous to aquatic organisms than BPA.
The concentration data of BPB and BPA in surface waters around the world were collected from previous studies (Tables S4–S6). BPA, as a well-known endocrine disrupting compound, was one of the frequently detected compounds in environmental monitoring. There were a large number of studies about BPA, and nearly all of them had 100% detectable rate. The average concentrations of BPA were 5.56–930 ng/L, and China and India were the most detected area (Jin &Zhu 2016, Lalwani et al. 2020, Si et al. 2019, Yamazaki et al. 2015, Zhao et al. 2019). Comparing to the abundant data of BPA the available data of BPB was limited. Up to now, BPB was reported in only five samples from surface waters, which all located at China (Shan et al. 2014, Yan et al. 2017, Zhao et al. 2019). The mean of BPB concentrations ranged from 3.32 to 20 ng/L, and the highest concentration occurred in Taihu Lake with 46 ng/L (Yan et al. 2017). In most samples, BPB was often detected at trace levels or even couldn’t reach a detectable level. The low concentration and detectable rate of BPB should be accounted to its small consumption, because the main BPA alternatives were BPF and BPS, rather than BPB.
The ecological risk of BPA and BPB were calculated using the above NOEC values and concentration data (Fig. 4). Due to the limit of data for BPB, few of risk values of BPB were presented, and all of them occurred in China waters. As shown from Fig. 4A, the BPB in inland lakes, i.e., Taihu Lake, Luoma Lake and Chaohu Lake, behaved as higher ecological risks than that in open coastal water, i.e., Pearl River Estuary. This result implied that anthropogenic activities mainly contributed to the risk of BPB, and good hydrodynamic condition could mitigate the risk of BPB to the ecosystem to some extent. Nevertheless, BPA from the same waters all had lower risks than BPB, even though it had much higher concentrations than BPB. Additionally, the risk of BPA from the waters around the world were presented in Fig. 4A and 4B, and results indicated that except for Jialu River (Henan Province, China), nearly all of them had a low ecological risk with RQ < 0.1, whether in the rivers/lakes from China or from other countries. Overall, the risk of BPA may not be as serious as it has been thought, although it was well-known because of its high health risk to humans.