Toxicity of Dietary Cadmium to Aquatic Organisms Part 1

Regulatory assessments of cadmium toxicity are originally mostly based on dissolved cadmium concentrations with the assumption that toxicity is caused by dissolved (or waterborne) cadmium only, and does not include the possible impact of cadmium associated with the particulate phase (e.g., phytoplankton, suspended particulate matter, sediment; food particles = dietary exposure). In recent years, more and more studies have shown that dietary cadmium uptake and accumulation is possible and very important (Wang et al., 2007; Wang and Fisher, 1999). Although situations exist where dissolved uptake alone describes most of bioaccumulation, the correlation between model forecasts and observations would be weak overall if only dissolved uptake was considered. Cadmium bioaccumulation from dissolved and dietary sources was 50% of total cadmium bioaccumulation separately in the bivalves Mytilus edulis and Mytilus balthica, and bioaccumulation of cadmium in nature agreed well with the forecasts when waterborne and dietary routes of uptake were considered. Assimilation efficiency of cadmium from food is much greater than that from water in Daphnia magna (Geffard et al., 2008). All of which suggested that dietary metals could play a crucial role in their health as strong evidence, although the toxicity of dietary metals on primary consumers was need to be understood further.

Figure 11. Diagrammatic representati ons of various aquatic organisms indicating cadmium sources at each trophic level exposed to dissolved and dietary cadmium.

Figure 11 suggests that the predator was fed herbivore prey, which also was fed with cadmium-rich food, the algae phytoplankton, so fish and zooplankton were exposed to cadmium in both food and water. Hence, dietary exposure may occur, because the food algae and zooplankton can adsorb and take up dissolved cadmium from the exposure solution before being ingested by the zooplankton and fish.

The development and implementation of effective remedial measures depend on our ability to predict the fate and effects of metals in these systems (Hare and Tessier, 1996). So, cadmium uptake through dietary intake is an important source and the toxicity of dietary cadmium is significantly important for aquatic organisms (Sofyan et al., 2007).

Phytoplankton Exposed to Cadmium

Cadmium has been ranked as one of the major potential metal hazards, which can produce biological effects in aquatic environments and can increase cell volume, lipid relative volume, and vacuole relative volume in algae. These biological effects may result in structural changes in planktonic communities. In particular, they reduce the richness of micro-algal species, and micro-algal production and alter the micro-algal community structure. The toxic effect of cadmium on microalgae is relevant since these organisms constitute the base of the marine food chain.

Phytoplankton, such as algae, exhibit a net negative charge resulting in an affinity for positively charged species, such as toxic cadmium cations, which will readily adsorb to algal cell surfaces (Taylor et al., 1998) and accumulate significant concentrations of cadmium throughout the food chain (Kremling et al., 1978), posing serious hazards to microalgae. Moreover trace metals such as cadmium typically have a potential affinity for sulfur and nitrogen, and proteins are made up of amino acids, many of which contain sulfur and/or nitrogen, so there is no shortage of potential binding sites for trace metals within algal cells. Such affinities make cadmium potentially toxic, binding to proteins or other molecules and preventing them from functioning in their normal metabolic role (Rainbow, 2002).

Toxic Values of Microalgae Exposure to Cadmium

Table 3 gives different median effective concentrations (EC50, the cadmium concentration that reduces the population growth to 50% of the control) and the rank of various algal species exposed to cadmium, the values of which are highly dependent on and/or are influenced by factors such as strains of algal species, continuous or batch test, composition of the cultures medium, exposure time, and other experimental conditions.

Toxicity values are available for six species of saltwater diatoms, one species of dinoflagellate and green alga, and two species of macroalgae (Table 3). Concentrations causing fifty percent reductions in the growth rates of diatoms range from 60 g/L for Ditylum brightwelli to 22,390 g/L for Phaeodactylum tricornutum, the most resistant to cadmium. The brown macroalga (kelp) exhibited mid-range sensitivity to cadmium, with an EC50 of 860 ^g Cd/L. The most sensitive saltwater plant tested was the red alga, Champia parvula, with significant reductions in the growth of both the tetrasporophyte plant and female plant occurring at 22.8 g Cd/L.

Toxicity values are available for six species of saltwater diatoms, one species of dinoflagellate and green alga, and two species of macroalgae (Table 3). Concentrations causing fifty percent reductions in the growth rates of diatoms range from 60 g/L for Ditylum brightwelli to 22,390 g/L for Phaeodactylum tricornutum, the most resistant to cadmium. The brown macroalga (kelp) exhibited mid-range sensitivity to cadmium, with an EC50 of 860 g Cd/L. The most sensitive saltwater plant tested was the red alga, Champia parvula, with significant reductions in the growth of both the tetrasporophyte plant and female plant occurring at 22.8 g Cd/L.

Table 3. Median effective concentrations and rank of various algal species exposed to cadmium.

Strains of Algal Species		EC50 fog Cd/L)	Effect	Rank of Sensitive
Kelp, Laminana saccharina		860	8-day growth rate	7
Green alga, Chlorella pyrenoidosa		81.16	96-hour growth rate	3
Dinoflagellate, Prorocentrum micans		60	30-day growth rate	2
Diatom	Phaeodactylum tricornutum	22390	growth	9
	Tetraselmis suecia	7900	6-day growth	8
	Asterionella japonica	224.8	72-hour growth rate	6
	Skeletonema costatum	175	96-hour growth rate	5
	Thalassiosira pseudonana	160	96-hour growth rate	4
	Ditylum brightwellii	60	5-day growth	2
Red alga, Champia parvula		22.8	Reduced female growth or stopped sexual reproduction Reduced tetrasporophyte growth	1
		24.9		Note: only d from sensitive stag were used
		77	NOEC sexual reproduction
		189	Reduced tetrasporangia production

The second sensitive alga was the dinoflagellate Prorocentrum micans, the growth rate of which was inhibited by 1.2 ^g Cd/L with resulting cell numbers in the cultures being less than one tenth of the control values. No effect was found at 0.4 ^g Cd/L. However other researchers found that the growth rate of this species was only slightly affected at 5 ^g Cd/L and then only after 22 days exposure, 50% reduction occurred at 60 ^g Cd/L with 30 days exposure. Concentrations greater than 10 ^g Cd/L were found to increase the vacuolation and number of lysosomes in this species. Researchers also recorded a reduced growth rate of Isochrysis galbana when exposed to 1 ^g Cd/L for 10 days and found that 10 ^g Cd/L temporarily reduced the growth rate of Scrippsiella faeroense, but there was no such effect at 2.0 ^g Cd/L (UNEP, 1985 and literature cited by this report).

The third sensitive alga was the green alga Chlorella pyrenoidosa, the 96-h EC50 and its 95% CIs (confidence intervals) of which were determined to be 81.16 (71.87-95.12) ^g Cd/L for biomass followed ASTM (American Society for Testing and Materials) guidelines without EDTA (ethylenediaminete traacetic acid) addition. These values were similar to the values reported by some other researchers (Lin et al., 2007), although their experimental conditions such as temperature and testing periods were slightly different from these tests.

The reason for the different toxic values above is probably that the exclusion mechanisms and detoxification processes of different strains exposed to cadmium are different (Wang et al., 2009a). Indeed, microalgae can develop a tolerance toward metallic pollutants by the development or the activation of exclusion processes and/or internal sequestration of the pollutants. The exclusion may occur by sequestration and excretion of pollutants in the cell wall or may be a result of the cellular release of organic material that can chelate pollutants in solution. Internal sequestration is generally the result of the production of specific soluble ligands such as metal-binding polypeptides synthesized by algal species from several classes or may occur by the compartmentalization of pollutants within lysosomal/vacuolar systems or precipitation in the form of granules.

Algal Cadmium Burden

Phytoplankton can accumulate significant concentrations of cadmium, although decomposing cells rapidly release cadmium into the water (Kremling et al., 1978). The concentrations of cadmium are usually lower in surface waters than in deep ocean waters, probably because of cadmium uptake into certain species of phytoplankton in surface waters. It is assumed that the metal is loosely bound to the cell’s surface, and this process appears to be important in the biogeochemical cycling of cadmium in the marine system.

Single-celled green algae are capable of concentrating cadmium in the cells by both adsorption and active uptake, and the total and intracellular cadmium accumulation increased with aqueous cadmium cations [Cd2+]aq, but the growth rate decreased with the increase of [Cd2+]aq, presumably due to cadmium toxicity, which may additionally result in an increase in the cellular cadmium concentration. The amount accumulated is also directly proportional to the concentration of cadmium present initially and is dependent upon the pH of the medium. Adsorbed cadmium is proportional to [Cd2+]aq over almost three orders of magnitude at constant pH, suggesting a roughly 1:1 stoichiometric interaction between [Cd2+]aq and the surface adsorption sites.

Green alga such as saltwater Chlorella pyrenoidosa accumulated cadmium in a dose-dependent manner (Figure 12), with algal cadmium burdens increasing from less than the detection limit in control treatments to 73.86*10-16 g Cd/cell in algae exposed to 70.29 ^g Cd/L, which was related to the adsorption and uptake of cadmium by the growing algae (Wang et al., 2009b).

Metal uptake by living algal cells is said to arise from two sequential processes: an initial rapid passive uptake due to surface binding of ions to the cell walls and a subsequent relatively slow active membrane transport of the metal ions through the wall into the cytoplasm of the cell (Khoshmanesh et al., 1996). Figure 12 shows that in the ranges of 12.9 to 47.1 ^g Cd/L, there was a rapid increase in the mass of cadmium adsorbed to the cell as the dissolved cadmium concentration rises, whereas at dissolved cadmium concentrations from 47.1 to 70.3 ^g Cd/L, the cadmium uptake in the algae becomes essentially constant and reaches an apparent "plateau". Maybe cellular binding saturation has been reached (Wang and Wang, 2008) or the algal cells have become saturated by adsorbed cadmium. Saturation levels corresponding to 79% surface coverage have been shown for C. pyrenoidosa through the adsorption isotherms (Khoshmanesh et al., 1996). However, cell density was inversely correlated with cadmium exposure, decreasing from 670*104 to 38*104 cells/ml with enhanced cadmium concentrations (Figure 12), so growth rate decreased as cadmium concentration increased in the medium and the inhibition was proportional to cadmium concentration (Wang et al., 2009b).

The ability of Chlorella pyrenoidosa to accumulate large concentrations of cadmium before showing adverse effects may be related to the presence of cadmium-sequestering agents within the cell. Moreover, algae are not the only suspended solid-phase component found in natural waters. The suspended solids in the water are a mixed collection of different materials, including resuspended sediment material, colloids, fecal material, and organic decomposition materials, which can easily adsorb metals and could constitute a source of contamination and toxicity for filter feeders, and could even pose a hazard to the food chain of all aquatic ecosystems.

Figure 12. Effects of cell density and algal cadmium burdens of Chlorellapyrenoidosa for 96-h exposure to a control and five cadmium concentrations. Vertical error bars represent standard deviation of mean (n=3) of algal cadmium burdens and cell density. Means with a different letter are significantly different from one another (two-sided Student’s t test, p<0.05). BD = below the limit of detection.

Cadmium Removed by Unicellular Microalgae

Algae have the ability to concentrate cadmium from aqueous solutions, which has the potential for bioremediation of cadmium-polluted seawater at a lower cost than wastewater treatment processes such as ion exchange, electrochemical treatment or evaporation. Both living and dead algal cells are also known to accumulate metals as "biosorbants", but cadmium was more quickly adsorbed by dried cells than that by living cells.

Cells of the Chlorella species are reported to be spherical or ellipsoidal with diameters in the range 2 to 10 ^m. Other cells are also spherical elongate or egg-shaped and/or the same size. The mean specific areas were calculated on the assumption that the algal cells are spherical in shape. If we assume that the hydrated ions are essentially spherical, their apparent cross-sectional area can be estimated. The hydrated radius of the cadmium ion has been calculated to be 4.37*10-8 cm. Therefore the cross-sectional area of a hydrated ion will be about 6*10-5 cm2, and hence the surface coverage of the ions on the cell at saturation may be estimated from:

Where A is the cross-sectional area of a hydrated ionis the metal adsorbed on the cell surface,is Avogardro’s number and S is the specific surface area (Khoshmanesh et al., 1996).

The uptake cadmium by algae can be calculated through adsorption isotherms, for example, the maximum cadmium adsorptionof marine green alga Chlorella sp. NKG16014 was estimated to be 37.0 mg Cd (g dry cells)-1 using the Langmuir sorption isotherms (Tadashi et al., 1999).