- 學位論文
水域生態系中魚介類對金屬傳輸與生物攝取之結構與機制
Structure and Mechanism of Fish/Shellfish Involved in Transport and Biouptake of Metals in Aquatic Ecosystems
並列摘要
The goal of this dissertation is to develop predictive models to better understand the structure and mechanism of fish/shellfish involved in transport and biouptake of metals in aquatic ecosystems. The overall paradigm is trying to establish a relationship among total, external/ internal effect concentrations and target concentration profiles of metals in aquatic organisms. Emphases of the dissertation are on metal biouptake/ bioavailability dynamics and time-dependent toxicity. When these approaches successfully applied, the inherent hazard potential of heavy metals can be better estimated. We developed a mortality model, by coupling an acute toxicity model and a pharmacodynamic model, to predict survival of abalone (Haliotis diversicolor supertexta) exposed to waterborne zinc (Zn). We developed a mechanistic model based on the pharmacological area-under-curve (AUC) concept associated with a biokinetic rate model to predict relative bioavailable Zn to abalone. A laboratory 14-day exposure experiment was conducted to obtain biokinetic parameters for soft tissue and shell of abalone and their food source, red alga Gracilaria tenuistipitata var. liui. A 7-day acute toxicity test was conducted to obtain LC50(t) and time course of lethal body burden of Zn in abalone. Results of the mortality model demonstrate that 96-h LC50 and incipient LC50 for H. diversicolor supertexta exposed to Zn are 1.1 and 1.05 mg l-1, respectively. Our predictions show that equilibrium lethal body burden at site of action is about 198 µg g-1, whereas the mortalities never reach 50% when H. diversicolor supertexta exposed to Zn is ≤ 1 mg l-1. In addition, the AUC-based model demonstrates that during uptake phase of the exposure experiment, estimated relative bioavailable Zn to soft tissue and shell of abalone were 71.04 ± 9.71% and 68.44 ± 8.29%, respectively. Sensitivity analysis indicates that relative bioavailable Zn to abalone are greatly affected by growth rate and depuration rate constants of abalone. We performed the stage-classified demographic method to investigate the effects of increased waterborne Zn concentrations on the population dynamics of abalone Haliotis diversicolor supertexta. We reanalyzed the results of a 7-d acute and a 28-d chronic toxicity bioassays to examine the survival and growth performances when exposing abalone to different levels of zinc stresses. An energy-based biological approach was adopted to model the effects of zinc on fecundity. These data provided stage-specific schedules of vital rates that were used to parameterize a projection matrix model for abalone. Simulations were carried out to produce temporal population abundance changes under seven exposure concentrations ranged from 0.03 to 1 mg l-1 Zn. Model manipulations indicate that a reduction of individual growth rate is observed at an exposed Zn concentration greater than 0.12 mg l-1, whereas the significant influence of survivorship is occurred until the Zn concentration reached 0.25 mg l-1. The asymptotic population growth rate decreases from λ = 1.00 for the control group to λ = 0.9968 for abalone population exposed to 1 mg l-1 Zn, indicating a potential risk of population intrinsic growth rates for abalone exposed to higher levels of waterborne Zn. We coupled the Michaelis-Menten (M-M) type flux and the Fick’s type of dynamic mass transfer flux to arrive at the Best equation to quantitatively model the transport and biouptake mechanism of the gills of freshwater tilapia (Oreochromis mossambicus) exposed to waterborne arsenic (As). A 15-day uptake/depuration bioassay was conducted to examine the accumulation kinetics of As in tilapia gills. A diffusion-based permeability was calculated using the physiological and allometric-related parameters. The bioaffinity parameter and the limiting uptake flux in M-M equation were acquired by fitting the experimental values from published literature. A linear relationship between As bioconversion rate and As concentration in ambient water was obtained. The fitted bioaffinity parameter and limiting uptake flux were 3.07 mg l-1 and 2.17 mg l-1 d-1, respectively, suggesting a low As binding affinity of tilapia gills, yet a relative high binding capacity was obtained. The As permeability through tilapia gills membrane decreases from 1.42 µm d-1 to a steady-state value of 0.82 µm d-1 after two months, indicating the non-equilibrium aspects of biouptake processes is involved. Our studies suggest that we have to take into account both kinetic and dynamic processes in predicting the transport and biouptake of metals to aquatic animals. The proposed models have potential applications in assessing the environmental risks associated with aquatic ecosystems of metal exposure.