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How Chemicals Move from the Water into Fish and Other Aquatic Organisms


Quiz Questions

1.Bioacumulation is Uptake and retention of a substance by an organism from the surrounding media (e.g., water column, pore water, sedimentisoil, and air), from food, and in some cases ingested particulates; bioaccumulated residues are referred to as the body burden (strictly speaking, excludes gut contents) and represent the contribution of all sources.
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2. Assimilation is the fractional amount of a chemical taken up or absorbed by an organism’s skin, gills, or gut relative to the amount of the chemical present in the contiguous exposure medium; for example, gastrointestinal assimilation efficiency for PC6s in ingested fish food (excludes sediments) ranges from about 0.5 to 0.85 or 50 to 85%.
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3.The lipids or fats of fish and wildlife, and humans as well, act as a natural sink for certain types of chemicals. Chemicals that have a propensity to accumulate in aquatic organisms to levels higher than those found in water are called bioconcentratable or bioaccumulatable.
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4.Four major interrelated factors affect the bioaccumulation of chemicals by aquatic organisms. These include the physical-chemical properties of the contaminant, environmental conditions, and characteristics of the exposed organism and its food chain.
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5.Compounds with polar functional groups generally bioaccumulate to a lesser degree than nonpolar lipophilic chemicals.
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6.The presence of lipophilic chemicals in an environment or an effluent does not necessarily mean that they will be significantly bioaccumulated.
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7.Degradation of a bioconcentratable organic compound occurs when its structure is changed by the loss and/or gain and substitution of an atom or a group of atoms, as well as the accompanied changes in the affected chemical bonds.
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8.Relatively large volumes of water (often liters per day) are ventilated across the gills of fish and bivalves for respiration and feeding (bivalves). In order for an organic compound to be extracted from ventilated water, it must passively diffuse through several barriers between the ambient water and lipid storage sites in the organism. The barriers include an exterior stagnant water layer, a mucus layer, and the absorbing bilayer membrane or gills. After passing through these barriers, residues are then transported via blood to fatty tissues, where they are concentrated. One note of interest is that the lipid content of the organism has little or no effect on uptake rates but it does affect the capacity of an organism to bioconcentrate a chemical.
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9.The gill bilayer membrane is composed of a lipid matrix with embedded proteins that have receptors and enzymes. Lipophilic organic compounds diffuse through the fatty regions of gill cell membranes (Barron, 1990) rather than moving through intercellular channels (spaces between cells) in the bilayer membrane where specific inorganic ions are actively transported to maintain homeostasis.
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10.The average distance from the external water to an organism’s blood (across the aforementioned barriers) is only about 6 micrometers (Hayton and Barron, 1990). Because the matrix of the cell membrane has extremely limited free space, a molecular size limit (breadth or cross sectional diameter) of about 1O Angstroms has been suggested for bioconcentratable compounds (see Figure 2-3). Since most environmental contaminants are small enough to diffuse through cell membranes, these barriers can impede, but cannot exclude the uptake of most lipophilic contaminants. For example, McKim et a/. (1 985) have shown that the assimilation efficiency of a variety of lipophilic compounds across fish gills ranges from about 20 to 90% of the contaminant residues present in ventilated water.
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11.Currently, three approaches are recommended by EPA to assess the presence of bioconcentratable or bioaccumulative substances (not covered by water quality criteria) in surface waters and effluents. These are the tissue residue measurement, effluent measurement, and sediment assessment options.
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