Coastal and Estuarine Risk Assessment - Chapter 5 ppt

©2002 CRC Press LLC

Bioavailability,

Biotransformation,

and Fate of Organic

Contaminants

in Estuarine Animals

Richard F. Lee

CONTENTS

5.1 Introduction

5.2 Bioavailability

5.3 Uptake

5.3.1 Uptake from Water

5.3.2 Uptake from Sediment

5.3.3 Uptake from Food

5.4 Fate of Xenobiotics after Uptake by Estuarine Animals

5.4.1 Biotransformation (Metabolism)

5.4.1.1 Phase-One Reactions

5.4.1.2 Phase-Two Reactions

5.4.2 Fates and Metabolic Pathways for Xenobiotics and Metabolites

within Tissues and Cells

5.4.3 Binding of Xenobiotics to Cellular Macromolecules

5.5 Elimination

5.6 Summary

References

5.1 INTRODUCTION

An important component of ecological risk assessment studies in oceans and

estuaries includes the characterization of the exposure of estuarine animals to

contaminants. Data on the bioavailability, uptake, accumulation, and elimination

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of contaminants by animals are necessary to characterize contaminant exposure.1

Contaminants found in estuarine and marine waters and sediments include aro￾matic hydrocarbons, organometallics, organohalogens, and pesticides, often

referred to as organic xenobiotics. The high concentrations of various xenobiotics

in aquatic animals from contaminated sites are indicative of the efficient uptake

and accumulation of these xenobiotics.2–14 As a result of the presence of these

contaminants in tissues, many toxicological effects may be manifested including

the following: growth, reproduction, and development.

The extent of uptake of xenobiotics by an estuarine animal depends on their

bioavailability from various matrices, including water, sediment, or food. After

entering from one of these matrices via the gill or digestive tract, the xenobiotic can

be accumulated in the liver (fish) or hepatopancreas/digestive gland (annelid, crus￾tacean, mollusk). Hemolymph or blood functions as an important avenue for trans￾porting xenobiotics and xenobiotic metabolites (Figure 5.1). After entrance into an

animal, the processes of accumulation, biotransformation, and elimination determine

the fate of the xenobiotic. The relative importance of these different processes

depends on a number of factors including the physicochemical properties of the

xenobiotic, the ability of the animal’s enzyme system to metabolize the compound,

and the lipid content of the animal.

This chapter discusses bioavailability of contaminants in estuaries, followed by

sections on the uptake, accumulation, metabolism, and elimination of xenobiotics.

The focus is on fish and three groups of marine estuarine invertebrates, i.e., crusta￾ceans, mollusks, and annelids. There are a number of reviews that have discussed

the uptake, metabolism, and elimination of toxicants by aquatic animals.5–22

5.2 BIOAVAILABILITY

In this chapter, the bioavailable fraction is that fraction of a xenobiotic available for

uptake by estuarine and marine animals. Matrices in the estuarine environment

include water, sediment, and food. Bioaccumulation is a general term describing the

processes by which bioavailable xenobiotics are taken up by estuarine animals from

FIGURE 5.1 Uptake and bioaccumulation of organic contaminants by crabs.

contaminant in water

contaminant in food

Gill

Stomach

Hemolymph Hepatopancreas

muscle

nervous tissues

gonadal tissues

green gland (urine/excretion)

Crab

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the water, sediment, or food. To determine bioavailability, it is necessary to determine

the relative partitioning between these matrices and the animal’s gill or stomach (see

Figure 5.1). The partitioning can be illustrated by the following expressions:

Water/gills of animal

Sediment/pore water or digestive juices/gills or stomach of animal

Food/stomach of animal

Xenobiotics in estuarine and marine waters are associated with both the dissolved

and particulate phases. A xenobiotic in the dissolved phase can be freely dissolved,

but in natural waters xenobiotics tend to bind to dissolved organic matter, primarily

the humic fraction.23–27 Landrum et al.24 using the amphipod, Pontoporeia hoyi, found

that the uptake rate constants for a series of xenobiotics increased as the dissolved

organic carbon decreased. Thus, binding of xenobiotics to dissolved organic matter

can reduce the amount that is bioavailable.

Particulates in estuarine water are often in high concentrations, ranging from

10 to 400 mg/l.28,29 These particulates are mixtures of organic matter, living matter,

and small clay particles. Scanning electron micrographs reveal rough surfaces on

these detrital particles, with bacteria fastened by mucoid-like pads and fibrillar

appendages30–32 (Figure 5.2). Xenobiotics can bind to hydrophobic sites on the

particulate surfaces. When radiolabeled benzo(a)pyrene was added to estuarine

water, it was found by autoradiography that most of the benzo(a)pyrene was bound

to detrital particles33 (Figure 5.3). Particulates with associated xenobiotics are

considered to be an important pathway by which contaminants enter estuarine

food webs.

Bioavailability of xenobiotics in sediments is generally not related to the sedi￾ment concentration, but rather to organic carbon content and physicochemical prop￾erties of the sediment. Xenobiotics in sediments are partitioned among particles,

pore water, and organisms. Estuarine sediments are composed of particles of various

sizes with xenobiotics associated with particles in the 30 to 60 m size range, which

is in the silt-clay fraction.34–36 In addition to the mineral phase, estuarine sediments

can be high in organic carbon and xenobiotics bind to hydrophobic sites within the

organic phase of the sediments. Three factors that are important in controlling the

bioavailability of contaminants associated with sediment include the aqueous solu￾bility of the xenobiotic, rate and extent of desorption from the solid phase into the

pore water, and the ability of digestive juices of infaunal animals to solubilize the

xenobiotic.37–39 Some infaunal animals pass sediment particles through their digestive

tract. Surfactants in their digestive juices solubilize a certain fraction of xenobiotics

off the sediment particles.38 Sediment organics can be labile or refractory. Xenobi￾otics bound to labile organics are more bioavailable because, during digestion, these

xenobiotics are released within the animal.40–42 There is some desorption of xeno￾biotics from sediment particles into pore water and xenobiotics in pore water are

highly bioavailable.36 Because of tight binding to humin-kerogen polymers in sed￾iment, there are very low desorption rates of uncharged lipophilic xenobiotics, e.g.,

5- and 6-ringed polycyclic aromatic hydrocarbons (PAHs) and polychlorinated

hydrocarbons in organic-rich sediment.38,43–46