Mechanism of Cadmium to Microalgae
Cadmium uptake is described in terms of two distinct steps that are believed to occur when cadmium cations are taken from solution by living algal cells: an initial rapid uptake of [Cd2+] due to attachment to the cell wall, followed by a relatively slow uptake due to membrane transport of the ion through the cell wall into the cytoplasm (Khoshmanesh et al., 1996). The first step in cadmium uptake by biota is the adsorption of aqueous ions or complexes on external layers of the algal cell wall, so these "primary" adsorption processes are crucial for modeling the impact on cadmium transport in natural settings and the ecotoxicological consequences.
Similar to other metals, cadmium can interact with both high-affinity (phosphoryl, sulfhydryl) and low-affinity (carboxyl) sites on cell surfaces, depending on its concentration in solution. Complexation with carboxylate surface groups is the primary process responsible for cadmium binding on algal cell walls, so carboxyl and phosphoryl sites play dominant roles in cadmium bio-sorption by marine algae. Furthermore, some detoxification mechanisms via glutathione and phytochelatin (PCn) production triggered by cadmium incorporation in the cells appear to play a role both in metal storage and in detoxification since low-level PCn concentrations are produced by many species of phytoplankton even at inorganic cadmium contents far below those that impede the growth (Pokrovsky et al., 2008).
The diatom cell wall structure can be viewed as a layer of amorphous silica (frustule) attached to a protein template from the interior of the cell and covered by a polysaccharide layer bearing negatively charged >R-COO moieties according to macroscopic and spectroscopic measurements. Therefore, the metal speciation at the outermost diatom cell walls resulting from short-term adsorption is likely to be controlled by>R-COO-Cd+ surface complexes, and solely carboxylate moieties were found to be sufficient to provide an adequate fit to the sorption data (Ge’labert et al., 2007). So the order of cadmium toxicity to the diatoms’ cell was S. costatum>A. japonica>T. suecia>P. tricornutum (EC50, Table 3), in agreement with the decrease of carboxylate group concentration in the surface layers.
Subcellular partitioning of metals may provide a mechanistic approach to investigate metal toxicity and tolerance. The cellular cadmium can be separated into soluble and insoluble fractions, or into the five biologically relevant fractions (namely MRG, cellular debris, organelle, HDP, and HSP), which also were considered to two subcellular compartments comprised of these fractions (MSF and BDM). The presumed metal sensitive fraction was defined as organelles + HDP, and the biological detoxified metal was defined as HSP + MRG. Soluble cadmium was defined as HSP + HDP, and insoluble cadmium was MRG + cellular debris + organelles (Wang and Wang, 2008). For diatoms, HSP was the largest subcellular pool for cadmium, followed by organelles, cellular debris, and MRG, with the least concentration in HDP. Thus, metal detoxification was presumably the major pool for cellular cadmium in diatoms. Metal detoxification can take place by binding to inducible metal-binding proteins or through the precipitation of metals into insoluble forms. Such internal storage and detoxification of metals by MTs and MRG may increase metal tolerance. Further, the ‘spillover’ of cadmium from these detoxified fractions to other subcellular fractions, i.e., redistribution of cadmium to the sensitive sites, may cause deleterious effects on the photosynthetic PSH system and the growth of the diatoms (Wang and Rainbow, 2006).
Toxicological end points exhibited by diatoms would be more closely related to partitioning of cadmium to organelles and enzymes (the metal-sensitive fractions) than to surface-absorbed or total cellular cadmium (Wallace et al., 2003). Moreover, the correlation between the growth rate inhibition and MSF or organelles was the most significant, both of which strongly demonstrated that cadmium toxicity in diatoms was best predicted by its distribution in MSF or organelles. The reason maybe mainly that the organelles are the principal component in MSF (>87% in most treatments), but it was difficult to determine if cadmium in HDP is a good predictor of cadmium toxicity given its small quantity, because the HDP only made up a small fraction of MSF.
In the diatom, HSP was the most important subcellular fraction for cadmium accumulation and most cadmium was distributed in the insoluble fraction (a combination of metal-rich granules, cellular debris, and organelles). Toxicity differences were the smallest among the different nutrient conditioned cells when the cadmium concentration in the soluble fraction (a combination of HDP and HSP) was used, suggesting that intracellular soluble cadmium may be the best predictor of cadmium toxicity under different nutrient conditions (Miao and Wang, 2006). For example, there was similarly a strong correlation between the cellular accumulation (surface-adsorbed, total cellular and intracellular cadmium) and the growth rate inhibition, and the differences of EC50 based on these parameters were smaller than those of [Cd2+]. Furthermore, among the different types of cadmium concentrations, the soluble cadmium was the best predictor of cadmium inhibition of f and 0M for the diatom T. weissflogii under different nutrient conditions (Wang and Wang, 2008; Miao and Wang, 2006).
Dietary Toxicity
Cadmium uptake in the aquatic food chain can be considered as occurring through three main vectors:
(i) it can be taken up directly from water column or from interstitial water in sediment for plants and microorganisms;
(ii) sediment-ingesting organisms have a geochemical source of cadmium as well as water source for heterotrophs;
(iii) food items provided by other biota represent a potential source of cadmium as does the water column and diet is an important route for cadmium accumulation in aquatic organisms. Food choice influences body loading, which poses a serious hazard to other consumers.
Previous studies of cadmium toxicity were conducted based on dissolved metal concentrations, with the assumption that toxicity was caused by waterborne metal only. In recent years forever, more and more researchers have considered that cadmium uptake through dietary intake is also an important source for aquatic organisms. The reproductions of marine copepods (Acartia tonsa and Acartia hudsonica) decreased when fed with the diatom Thallasiosira pseudonana contaminated with cadmium (Hook and Fisher, 2002). Toxic effects of reproduction were observed when freshwater cladoceran Ceriodaphnia dubia expose to dietary cadmium (Sofyan et al., 2007). Conversely, reproduction and growth increased when the freshwater cladoceran Daphnia magna was fed with the cadmium-contaminated algae Chlamydomonas reinhardtii and Pseudokirchneriella subcapit (Garcia et al., 2004).
Figure 13 shows that dietary cadmium negatively affects net reproductive rate per brood of M. monogolica, which is a keystone species in estuarine food webs of China. The net reproductive rate considered two reproductive traits of the brood size and the fraction of parent females producing broods, the dramatic reductions of which from broods 2 to 4 occurred with increasing dietary cadmium and prolonged exposure. Only one female produced neonates or all female were dead in the fourth brood, which demonstrated that the net reproductive rate was severely affected in algal cadmium burdens greater than or equal to 21.06*10-16 g Cd/cell (Wang et al., 2009b).
However, when examining the effects of dietary cadmium, no mortality was observed in short term exposures, which were significantly different from those of waterborne cadmium.
Figure 13. Net reproductive rate per brood of Moina monogolica in 21-d dietary cadmium experiments after feeding with Chlorella pyrenoidosa exposed for 96 h to a control and five cadmium concentrations. Vertical error bars represent the standard deviations among five replicates. Means with the same letter are not significantly different (Student-Newman-Keuls multiple-range tests, p<0.05). The "&" means that only one female produced neonates, so comparisons could not be made because of no standard deviations. The "#" means that all exposed organisms died in the fourth brood before day 21.
The presence of algae in the culture medium could change the speciation and bioavailability of cadmium because of the possible release of algal exudates, which make it difficult to separate effects of dietary and aqueous exposure, particularly for zooplankton, which feed on small particles that rapidly exchange contaminants with water. Moreover, cadmium transfer in aquatic planktonic food chains is controlled by several important physiological and geochemical parameters, including cadmium assimilation and elimination. Fortunately through specific experimental procedures, low cadmium desorption was observed with values below the detection limit, demonstrating that algae were the main cadmium source during dietary exposure.
Mechanisms of Dietary Cadmium
For aquatic organisms, it is generally assumed that metal accumulated from water exposure is more likely to be deposited in the respiratory organs (such as gills), whereas metal from particulate-bound form or via the diet following ingestion and digestion is assimilated and mainly deposited in internal tissues.
Figure 14. Cadmium accumulation patterns of aquatic organisms from dietary sources showing net accumulation with some excretion of cadmium accumulated in a detoxified form, other details as for Figure 10.
Tolerance or resistance to metal toxicity is based on controlling metal cellular speciation. Sequestration by cellular ligands such as MTs, lysosomes, and mineralized organically based concretions appears to be one of the most commonly adopted strategies by invertebrates (Figure 14).
Because of this differential distribution, dietary metal can exert toxicity via mechanisms different from those of dissolved metal, and, finally, metal distribution and detoxification mechanisms induced by dietary contamination could be different from the contamination of waterborne exposure (Fraysse et al., 2006).
Identification of subcellular metal distribution in aquatic animals and quantification of the subcellular fates of metals in relation to metal toxicity or metal transfer to a higher trophic level can be achieved through different biochemical fractionation techniques (Geffard et al., 2008).
After partitioning, different detoxification processes including detoxification proteins (MTs) and target molecules such as enzymes could be separated into different sub-cellular fractions as insoluble and soluble fractions, the latter often involving binding to physiologically important proteins such as enzymes, or MTs, or the partitioning of cadmium to subcellular compartment containing trophically available metals (TAM) (i.e., HSP, HDP, organelles, ‘insoluble’ components [e.g., exoskeleton and metal-rich granules] and cellular debris) of specific target tissues or cells (Seeebaugh et al., 2006), especially accumulation in which is most probably related to toxic effects on reproduction (McCarty and Mackay, 1993).
Cadmium distributions into soluble (cytosolic) and insoluble fractions were observed in aquatic organisms of exposure to waterborne and dietary routes, showing their similar potential toxicity. The soluble fraction is made up of MT-like proteins, and also metals binding target molecules such as enzymes, which also seem to have an important role in the detoxification and toxicology of metal. The metal distribution can be related to metal toxicity or trophic transfer. Dietary cadmium was mainly accumulated in the soluble fraction (from 75 to 85% of total accumulated cadmium), which induces deleterious effects on zooplankton reproduction (Figure 14).
