Chlorella pyrenoidosa 在連續式藻類毒性試驗中之比較研究

Abstract

過去本研究群以藻類Selenastrum carprionutum作一系列毒性試驗研究，並且發展出一套連續式藻類試驗方法，使連續式試驗耗時的缺點得以改善。經由此連續式試驗方法與現行標準方法(批次式)的比較，對於相同之毒性物質，我們可以得到較小之EC50及NOEC值，亦即較佳之敏感度(sensitivity)，且在再現性(reproduceability)方面也較現行標準方法為高。 本研究將藉由另一種常用之指標生物藻類 Chlorella pyrenoidosa作為試驗生物，並決定出 Chlorella pyrenoidosa最合理及適切之測試條件（稀釋率、基質濃度、EDTA濃度等），最後則以連續式毒性試驗評估各種重金屬對 Chlorella pyrenoidosa之毒性，以提供此一藻種更合理之毒性數據資料，並與標準方法之結果相互比較。依據試驗結果，在N/P=30、15 μg/l Na2EDTA•2H2O 含量及稀釋率 D=0.3/d 的條件下為最適宜的測試環境，藻類生長良好且試驗敏感度佳。經過修正的連續式試驗方法利用五種重金屬 (Cd、Zn、Co、Ni、Pb)進行其效能評估，與批次試驗結果比較，連續式試驗法有較佳的試驗敏感度和再現性。 在批次式及連續式藻類毒性試驗中，利用細胞密度和平均細胞體積來作為判斷NOEC值的指標兩者敏感度相當，而細胞總體積則較差。再者以細胞總體積計算出的EC50值也皆比用細胞密度所求出的EC50值大，這是因為藻類細胞受到重金屬影響而體積變大時，將縮小處理組與控制組間的生物質量差異。一般的藻類毒性試驗常將實際的生物質量與細胞密度兩參數相互混淆使用，而未考慮細胞中毒後細胞體積變化的因素，可能會造成在毒性效應評估上嚴重的差異。

Our previous studies, based on the alga Selenastrum capricornutum, indicate that the present algal toxicity test protocols (US EPA, ISO, ASTM, and OECD) are less sensitive and, incorrect in the relative toxicity relationships. The primary reasons for the above phenomena are due to the over-enriched growth medium for these test methods. In addition, we have developed a continuous algal toxicity test whereas algae are tested under a nutrient-limited conditions This new method has been proven to be more sensitive and precise. From the history of toxicity assessment, Chlorella pyrenoidosa has been frequently used as an indicator organism to evaluate the impact of various toxicants and waste discharges to phytoplankton communities. The purpose of this research is therefore to study the responses of this alga to various metal toxicants. Comparison well be made with respect to test sensitivity, and accuracy between the continuous test and the traditional batch test. Based on the test results, these parameters at N/P=30, 15 μg/l Na2EDTA•2H2O and dilution rate D=0.5/d are optimum which achieve a better growth condition and sensitivity. The performance of the modified testing technique is evaluated, employing five different metal toxicants(Cd、Zn、Co、Ni、Pb). Compared with batch test results reported by other researchers and our own experiment, the continuous test achieved superior sensitivities and reproducibility.The sensitivities of NOEC values based on cell density and mean cell volume were equal, while that on total cell volume was less than the formers. The EC50 values by total cell volume which relative to actual biomass were larger than those by cell density. These were due to the differences in biomass of treatments and controls were smaller when the mean cell volume increased under the exposure of toxicity. In standard toxicity tests the cell density is a frequently used surrogate measure of biomass without considering the mean cell volume changed. It would lead to serious discrepancies in toxicity assessment.