3.6 Heavy Metal Pathways in Corals and Understanding the Role of Zooxanthallae
63
The speciation of the heavy metals in the marine environment determines their
biological availability or potential toxicity of the metals. Factors such as elevated
pCO 2 levels and/or low pH in seawater increases the bioavailability of trace elements
due to increased solubility and/or changes in the metal speciation. Increased pCO 2
concentrations also affect the uptake/excretion mechanisms of trace elements in
marine organisms.
The symbiosis in corals is highly complex with the zooxanthallae contributing to
the energy budget of the host and enhancing calcification, thus influencing the skeletal
deposition rates of heavy metals. Studies have revealed that high loads of heavy
metal concentrations enable the release of symbiotic algae and secretion of mucus in
cnidarians. Hence, algae expulsion is used as a mechanism of metal detoxification,
mainly because algae have been found to accumulate metals to a larger extent and
be more tolerant than their symbiotic hosts. Studies such as Harland et al. (1990);
Harland and Nganro (1990) and Reichelt-Brushett and McOrist (2003) have revealed
that zooxanthallae play an important role in the uptake of heavy metals. Mucus
production (Marshall 2002) and gamete production (Reichelt-Brushett and McOrist
2003) also influence uptake of heavy metals.
Loss of zooxanthallae is a suggested mechanism by which accumulated metal
concentrations could be regulated by corals (Reichelt-Brushett and McOrist 2003;
Hardefeldt and Reichelt-Brushett 2015). The expulsion of symbiotic zooxanthellae
by cnidarians has been reported in response to a number of natural and anthropogenic
perturbations including metal exposure (Meehan and Ostrander 1997; Peters et al.
1997; Bastidas and Garcia 2004). The expulsion of metal rich zooxanthallae influences the whole body metal concentrations in corals. However, there is no clear
correlation between metal exposure and metal accumulation in corals nor there is for
the rate of transfer of metals. Essentially, consideration of uptake and depuration of
metals in corals is vital in understanding metal accumulation in corals.
Hardefeldt and Reichelt-Brushett (2015) state that zooxanthellae display similar
patterns of metal bioaccumulation to other marine algae, and hence suggest that
metal accumulation in zooxanthellae may be a function of uptake and detoxification
mechanisms similar to those occurring in other marine algae. The regulation of metals
in the living components of corals influences the concentrations of metals transferred
to and stored within coral skeleton.
Hardefeldt and Reichelt-Brushett (2015) studied anemone and hence suggest that
zooxanthellae exert a strong influence on the whole anemone metal concentrations.
Hardefeldt and Reichelt-Brushett (2015) mentioned that variations in zooxanthellae
density due to stress and other factors represents an important factor in metal regulation in the living components of cnidarians and must be considered when interpreting
skeletal metal concentrations in biomonitoring studies.
Coral bleaching is yet another mechanism through which zooxanthallae is lost and
hence metal loads in cnidarians could also be regulated. Coral bleaching transpires
when coral-algal symbioses are stressed by high temperatures, high irradiance (both
in the visual and ultraviolet regions of the solar spectrum) or other environmental
changes, such as manmade pollutants (Bielmyer et al. 2010). Studies such as Harland
and Brown (1989); Peters et al. (1997); Reichelt-Brushett and McOrist (2003) and
63
The speciation of the heavy metals in the marine environment determines their
biological availability or potential toxicity of the metals. Factors such as elevated
pCO 2 levels and/or low pH in seawater increases the bioavailability of trace elements
due to increased solubility and/or changes in the metal speciation. Increased pCO 2
concentrations also affect the uptake/excretion mechanisms of trace elements in
marine organisms.
The symbiosis in corals is highly complex with the zooxanthallae contributing to
the energy budget of the host and enhancing calcification, thus influencing the skeletal
deposition rates of heavy metals. Studies have revealed that high loads of heavy
metal concentrations enable the release of symbiotic algae and secretion of mucus in
cnidarians. Hence, algae expulsion is used as a mechanism of metal detoxification,
mainly because algae have been found to accumulate metals to a larger extent and
be more tolerant than their symbiotic hosts. Studies such as Harland et al. (1990);
Harland and Nganro (1990) and Reichelt-Brushett and McOrist (2003) have revealed
that zooxanthallae play an important role in the uptake of heavy metals. Mucus
production (Marshall 2002) and gamete production (Reichelt-Brushett and McOrist
2003) also influence uptake of heavy metals.
Loss of zooxanthallae is a suggested mechanism by which accumulated metal
concentrations could be regulated by corals (Reichelt-Brushett and McOrist 2003;
Hardefeldt and Reichelt-Brushett 2015). The expulsion of symbiotic zooxanthellae
by cnidarians has been reported in response to a number of natural and anthropogenic
perturbations including metal exposure (Meehan and Ostrander 1997; Peters et al.
1997; Bastidas and Garcia 2004). The expulsion of metal rich zooxanthallae influences the whole body metal concentrations in corals. However, there is no clear
correlation between metal exposure and metal accumulation in corals nor there is for
the rate of transfer of metals. Essentially, consideration of uptake and depuration of
metals in corals is vital in understanding metal accumulation in corals.
Hardefeldt and Reichelt-Brushett (2015) state that zooxanthellae display similar
patterns of metal bioaccumulation to other marine algae, and hence suggest that
metal accumulation in zooxanthellae may be a function of uptake and detoxification
mechanisms similar to those occurring in other marine algae. The regulation of metals
in the living components of corals influences the concentrations of metals transferred
to and stored within coral skeleton.
Hardefeldt and Reichelt-Brushett (2015) studied anemone and hence suggest that
zooxanthellae exert a strong influence on the whole anemone metal concentrations.
Hardefeldt and Reichelt-Brushett (2015) mentioned that variations in zooxanthellae
density due to stress and other factors represents an important factor in metal regulation in the living components of cnidarians and must be considered when interpreting
skeletal metal concentrations in biomonitoring studies.
Coral bleaching is yet another mechanism through which zooxanthallae is lost and
hence metal loads in cnidarians could also be regulated. Coral bleaching transpires
when coral-algal symbioses are stressed by high temperatures, high irradiance (both
in the visual and ultraviolet regions of the solar spectrum) or other environmental
changes, such as manmade pollutants (Bielmyer et al. 2010). Studies such as Harland
and Brown (1989); Peters et al. (1997); Reichelt-Brushett and McOrist (2003) and