582
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
to the seagrass. From a management perspective we
also need to know which species are able to accumulate/concentrate which metals.
Different populations of the same species within
a small geographic distribution can have widely different tolerances to metal impacts. Three in situ populations of Zostera capricorni within 80 km in the
Sydney region exposed to copper showed different
declines in photosynthetic efficiency (Macinnis-Ng
and Ralph, 2004). It is not clear whether this was
due to genotypic or phenotypic variation between
the populations. Organisms can become resistant to
pollution either through acquiring tolerance as individuals via physiological adaptation or through genetically based resistance, which renders a population resistant (Klerks and Weis, 1987). Background
levels of metals in the sediment, water column, and
seagrass did not correlate with the different metal
impacts and the mechanisms of tolerance remain unclear. Further investigations of interactions between
metals and other stress factors (e.g. eutrophication
and turbidity) are needed, as well as understanding the process of compartmentalization of metals
within the tissue, since tolerant populations may
have the ability to isolate certain metals, reducing the
toxic impacts (Pergent-Martini and Pergent, 2000).
It is important to consider why different physiological endpoints show different degrees of sensitivity. Heavy metals have a number of impact sites
on the photosynthetic apparatus; these include thylakoid membrane stability, photophosphorylation,
inhibition of chlorophyll synthesis, blocking electron transport or inhibition of the Calvin Cycle enzymes (Clijsters et al., 1999). The effect of different
impact sites can be seen in the growth rate of Z. capricorni being reduced by 10 mg L
−1 of copper, cadmium, lead, and zinc after a 10 day exposure period,
while changes in photosynthetic rate (measured as
O 2 production) were less pronounced (Conroy et al.,
1991). Thus, endpoint selection can influence the
perceived threat of a particular contaminant but a
good understanding of mechanisms of toxicity can
minimize this problem.
Detoxification is also an important aspect of tolerating contaminants. Generation of active oxygen
species is an important part of metal toxicity. To
avoid accumulation of these toxic intermediates,
plant tissues have a series of detoxifying agents
that involve enzymatic and non-enzymatic mechanisms (Clijsters et al., 1999). An absorbed pollutant
compound has one of three fates within an organism (Pergent-Martini and Pergent, 2000). It can be
trapped in a neutral fraction and stored in a compartment of the organism, it can find its way to a fixation
site or it can be metabolized by biotransformation
enzymes. Exposure to metals generally stimulates
enzyme activity (Clijsters et al., 1999), since natural elimination of toxic substances is encouraged
by biotransformation enzymes increasing their water solubility (Pergent-Martini and Pergent, 2000).
This process involves oxidation of the pollutant, followed by binding with glucuronic acid, glutathione,
sulfates or water molecules. The bioavailability of
pollutants is decreased when they are metabolized
by the P450 or peroxidase systems and incorporated into the lignin of the plant cell wall (PergentMartini and Pergent, 2000). Peroxidase activity has
been found to increase with the onset of metal toxicity as a detoxifying mechanism (MacFarlane and
Burchett, 2001).
5. Recovery from Heavy Metal Exposure
Heavy metals generally disrupt the function of photosynthesis; however, permanent damage may not be
caused, since the impact can be reversed by the removal of the metals such as cadmium, copper and
lead (Prasad and Strzalka, 1999). Therefore, recovery is possible, and acclimation to chronic metal contamination has been documented (Ward, 1987). For
example, Prange and Dennison (2000) were able to
demonstrate recovery of Z. capricorni after exposure
to copper, once the Cu-laden water was removed, the
photosynthesis recovered. These types of toxicitybased investigations provide an ecological perspective of the impact of metals. The potential for recovery from heavy metal exposure is as important
as understanding of the degree of damage caused by
exposure.
Metal-binding polypeptides (phytochelatins) are
another mechanism employed to confer metal tolerance. Phytochelatins reduce the concentration of
toxic free metal ions by binding in the cytoplasm
(Verkleij and Schat, 1990; Turner, 1994). The influence of phytochelatins on tolerance is thought to be
minimal since there is little evidence of quantitative
differences between the rate and capacity of phytochelatin production between sensitive and tolerant
plants (Verkleij and Schat, 1990). However, these
detoxification processes that promote recovery have
not been fully investigated in seagrasses.
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
to the seagrass. From a management perspective we
also need to know which species are able to accumulate/concentrate which metals.
Different populations of the same species within
a small geographic distribution can have widely different tolerances to metal impacts. Three in situ populations of Zostera capricorni within 80 km in the
Sydney region exposed to copper showed different
declines in photosynthetic efficiency (Macinnis-Ng
and Ralph, 2004). It is not clear whether this was
due to genotypic or phenotypic variation between
the populations. Organisms can become resistant to
pollution either through acquiring tolerance as individuals via physiological adaptation or through genetically based resistance, which renders a population resistant (Klerks and Weis, 1987). Background
levels of metals in the sediment, water column, and
seagrass did not correlate with the different metal
impacts and the mechanisms of tolerance remain unclear. Further investigations of interactions between
metals and other stress factors (e.g. eutrophication
and turbidity) are needed, as well as understanding the process of compartmentalization of metals
within the tissue, since tolerant populations may
have the ability to isolate certain metals, reducing the
toxic impacts (Pergent-Martini and Pergent, 2000).
It is important to consider why different physiological endpoints show different degrees of sensitivity. Heavy metals have a number of impact sites
on the photosynthetic apparatus; these include thylakoid membrane stability, photophosphorylation,
inhibition of chlorophyll synthesis, blocking electron transport or inhibition of the Calvin Cycle enzymes (Clijsters et al., 1999). The effect of different
impact sites can be seen in the growth rate of Z. capricorni being reduced by 10 mg L
−1 of copper, cadmium, lead, and zinc after a 10 day exposure period,
while changes in photosynthetic rate (measured as
O 2 production) were less pronounced (Conroy et al.,
1991). Thus, endpoint selection can influence the
perceived threat of a particular contaminant but a
good understanding of mechanisms of toxicity can
minimize this problem.
Detoxification is also an important aspect of tolerating contaminants. Generation of active oxygen
species is an important part of metal toxicity. To
avoid accumulation of these toxic intermediates,
plant tissues have a series of detoxifying agents
that involve enzymatic and non-enzymatic mechanisms (Clijsters et al., 1999). An absorbed pollutant
compound has one of three fates within an organism (Pergent-Martini and Pergent, 2000). It can be
trapped in a neutral fraction and stored in a compartment of the organism, it can find its way to a fixation
site or it can be metabolized by biotransformation
enzymes. Exposure to metals generally stimulates
enzyme activity (Clijsters et al., 1999), since natural elimination of toxic substances is encouraged
by biotransformation enzymes increasing their water solubility (Pergent-Martini and Pergent, 2000).
This process involves oxidation of the pollutant, followed by binding with glucuronic acid, glutathione,
sulfates or water molecules. The bioavailability of
pollutants is decreased when they are metabolized
by the P450 or peroxidase systems and incorporated into the lignin of the plant cell wall (PergentMartini and Pergent, 2000). Peroxidase activity has
been found to increase with the onset of metal toxicity as a detoxifying mechanism (MacFarlane and
Burchett, 2001).
5. Recovery from Heavy Metal Exposure
Heavy metals generally disrupt the function of photosynthesis; however, permanent damage may not be
caused, since the impact can be reversed by the removal of the metals such as cadmium, copper and
lead (Prasad and Strzalka, 1999). Therefore, recovery is possible, and acclimation to chronic metal contamination has been documented (Ward, 1987). For
example, Prange and Dennison (2000) were able to
demonstrate recovery of Z. capricorni after exposure
to copper, once the Cu-laden water was removed, the
photosynthesis recovered. These types of toxicitybased investigations provide an ecological perspective of the impact of metals. The potential for recovery from heavy metal exposure is as important
as understanding of the degree of damage caused by
exposure.
Metal-binding polypeptides (phytochelatins) are
another mechanism employed to confer metal tolerance. Phytochelatins reduce the concentration of
toxic free metal ions by binding in the cytoplasm
(Verkleij and Schat, 1990; Turner, 1994). The influence of phytochelatins on tolerance is thought to be
minimal since there is little evidence of quantitative
differences between the rate and capacity of phytochelatin production between sensitive and tolerant
plants (Verkleij and Schat, 1990). However, these
detoxification processes that promote recovery have
not been fully investigated in seagrasses.