Acute Toxic Effects And Mechanisms Of PBDEs On Two Species Of Marine Bait-microalgae

Polybrominated diphenyl ethers, PBDEs, are a class of additive flame retardants widely used in plastics, textiles, electronic circuits, building materials and household electric appliances and other fields. Over the past three decades, PBDEs have been identified in lakes, seas and streams around the world. The predominant PDBEs congener found in marine fish, bird and mammal tissues was 2,2',4,4'-tetraBDE (BDE-47), followed by 2,2',4,4',5-pentaBDE (BDE-99) and 2,2',4,4',6-pentaBDE (BDE-100). Significant amounts of 2,2',4,4',5,5'-hexaBDE (BDE-153), 2,2',4,4',5,6'-hexaBDE (BDE-154), 2,2',3,3',4,4',5,5',6,6'-decaBDE(BDE-209) as well as the lower congener 2,4,4'-triBDE (BDE-28) have also been found in animal tissues depending on the species. Biomagnification of PBDEs in the food chain has been demonstrated in top predators, such as marine mammals.Microalgae are among the most important microorganisms in the marine ecosystems because they are primary producers. Its species diversity and primary production had the dramatic impacts on aquatic ecosystem structure and function. Recently, study on acute toxicity effects of PBDEs on marine microalgae has just started. A few studies have reported the median effective concentrations (96h EC₅₀) of BDE-47 on several species of marine microalgae, as well as the response characteristics of superoxide dismutase (SOD) and catalase (CAT). These studies are insufficient to conduct a comprehensive evaluation of the toxicity effects of PBDEs on marine ecosystem. In this study, 5 PBDEs homologues (BDE-28, BDE-47, BDE-99, BDE-153 and BDE-209) with high concentrations in marine organisms and 2 species of marine bait-algae (Platymonas subcordiformis, Dunaliella salina) were selected for acute toxicity test. First, the median effect concentrations (96h EC₅₀) of 5 homologues PBDEs on two algae species were calculated. The sensitivity of these two algae species to PBDEs and the toxicity differences between PBDEs homologues were evaluated according to the 96h EC₅₀ values. On the basis of this, BDE-28 and D. salina were chosen as research materials for the following experiments. Then the variations of photosynthetic pigments (chlorophyll a(Chla), chlorophyll b(Chlb) and carotenoids(Car)), soluble proteins, glutathione (GSH) and its metabolic enzymes (glutathione peroxidase (GPX), glutathione reductase (GR), gluthione S-transferase(GST)) of D. salina were measured under the exposure of high(100μg·L^-1) and low(10μg·L^-1) concentrations of BDE-28. Based on the above research, the toxic mechanisms of PBDEs to marine microalgae were discussed. The main conclusions are listed as follows:(1) The growth of P. subcordiformis and D. salina was inhibited by 5 PBDEs homologues, which showed a dose-response relationship. The degree of inhibition decreased with the increasing number of Br substituent. According to the 96h EC₅₀ values, the toxicity of 5 PBDEs homologues was in the order of BDE-28> BDE-47> BDE-99> BDE-153> BDE-209. The 96h EC₅₀ values of BDE-28 and BDE-209 on P. subcordiformis were 127.72 and 2055.77μg·L^-1, respectively. The 96h EC₅₀ values of BDE-28 and BDE-209 on D. salina were 75.35 and 1867.95μg·L^-1, respectively. According to the "New Chemical Substance Risk Assessment Criteria (HJ/T154-2004)", BDE-28, BDE-47 and BDE-99 were highly toxic substances to marine microalgae, while BDE-153 and BDE-209 were toxic substances.(2) Both the photosynthetic pigment and soluble protein contents of D. salina were influenced by the BDE-28 concentration and exposure time. Chla, Chlb and Car contents increased in low exposure concentration (10μg·L^-1) group. However, in high exposure concentration (100μg·L^-1) group, the contents of 3 kinds of photosynthetic pigments decreased continuously. After a 96h-exposure, Chla and Car contents in high exposure concentration group decreased to 9.98% and 7.51% of those in control group, respectively. The sensitivity of photosynthetic pigments to BDE-28 was in the order of Chla> Car> Chlb. Thus Chla and Car of marine microalgae could be sensitive indicators for monitoring PBDEs pollution. No significant decrease in soluble protein content was observed in low BDE-28 exposure concentration group. However, in high BDE-28 exposure concentration group, soluble protein content of D. salina decreased dramatically to 26% of that in control group after a 96h-exposure. The formation of oxidation stresses in microalgae cells under certain dose of BDE-28 might be the main reason for the reduction of photosynthetic pigments and soluble proteins.(3) Dramatic variations in GSH content were observed in BDE-28 exposed groups. In low exposure concentration group, GSH content decreased significantly at the beginning and increased obviously after, indicating the self-recovery capability of the studied microalgae. Afterwards, the GSH content gradually decreased with the exposure time. In high exposure concentration group, GSH contents were significantly lower than those in control group during the experimental period. The GSH content in high exposure concentration group at 96h was only 19% of that in control group, indicating heavy oxidative stress on the microalgae. Since the change in GSH content, as the stress response in the molecular and cellular level, was sensitive to environmental factors, GSH content might be used as an early indicator for monitoring the PBDEs pollution in marine environment.(4) During the exposure period of BDE-28, changes of 3 kinds of GSH metabolic enzyme (GPX, GR, GST) activities were detected. GPX and GR activities had similar trends, showing a significant dose-response relationship. With the exposure time increasing, both enzyme activities increased first (before 24h) and then decreased. GST activities had different trends, which decreased gradually during the exposure period in all groups, except for an increase in low BDE-28 concentration group during 24-48h exposure period.