Phytoplankton are a key part of marine and freshwater ecosystems as primary producers of the food web. They are exceptional in their role in heavy metals bioremediation. Algae and cyanobacteria when exposed to metals in mixture, show altered responses as compared to the single metal exposure. Algal parameters such as growth, chlorophyll content, photosynthesis, metal uptake and metabolism, or lipid profile are commonly determined to find out the level of stress in algal cells resulting from heavy metals. Phytoplankton have several pathways of metal entry, detoxification and tolerance. It is essential to estimate reciprocal toxicity of metals as in real-time, metals are released in the environment in bulk (reciprocal effects). Phytoplankton can be a powerful tool in such risk assessments.
Understanding the synergistic effects for metal co-exposure in ecotoxicology is required to evaluate the real-time toxicity of metals, as, in natural ecosystems, pollutants are discharged in the form of a bulk. In nature, organisms in any environment experience exposure to mixtures of metals, while the ecotoxicity of metals alone is different compared to a mixture of different metals [23][24].
Different environmental conditions affect the reciprocal toxicity of heavy metals, together with the different properties of toxic elements, as well as their interactions with organic and inorganic substances outside and inside of living cells [23]. The bioaccumulation of heavy metals in plankton cells depends on multiple factors, including the absorptive ability of individual strains, season, pH, temperature, metal bioavailability, and many others [25][26].
There are more articles, including reviews, that are focused on performing a risk assessment and finding the best-fitting modelling calculations of contaminants in aquatic environments, as well as the toxicity of contaminants to aquatic species, including algae, together to determine the relevant important processes and conditions/parameters related to chemical toxicity to organisms [27][28][29][30][31]. Methods as the biotic ligand model (BLM), concentration addition (CA), independent action (IA), sensitivity distribution of EC50 species sensitivity distribution (SSD curves), and others are used to study reciprocal metal toxicity and providing promising results. If the initial assessment shows acute toxicity of a given pollutant (metal, chemical, or any other), a detailed study can be planned to determine the exact nature of, toxicity, and possible damage to this pollutant.
Scientists and environmentalists have been attempting to examine the mixture toxicity of metals in aquatic systems. The objective of such studies is always to determine the possible risk implied by any metal mixture drained into a water body. Phytoplankton are usually used in such assessment tests, as they are a common and reliable indicator organism for the evaluation of aquatic ecosystem pollution. Table 1 summarises the ecotoxicological reciprocal metal assessments made using phytoplankton test species.
Metal Mixture | Species | pH | Concentrations (mg L−1 or M) |
Reciprocal Effect | Reference |
---|---|---|---|---|---|
Al + Zn | Raphidocelis subcapitata | NA | >0.026 mg L−1 Zn and 0.739 mg L−1 Al; 22.24–37.06 µM Al, 0.08–0.46 µM Zn |
Antagonistic | [32][33] |
Cu + Cr + Ni | Chlorella pyrenoidosa 251 | 6.8 | 0.1–1.0 mg L−1 of Cu, Cr and Ni | Synergistic | [34] |
As + Se | Desmodesmus quadricauda | 7.2 | 29.05 mg L−1 As and 3.65 mg L−1 Se | Synergistic | [16] |
Cd + Co | Chlamydomonas reinhardtii | 7 | 2 × 10−8 M Cd and Co | Non-interactive | [35] |
Cd + Fe + Mn + Cu | Chlamydomonas reinhardtii | 7 | 2 × 10−8 M Cd2+, 1 × 10−17 M Fe3+, 1 × 10−6 M Mn2+, 1 × 10−13 M Cu2+ | Non-interactive | [35] |
Cd + Co | Chlorella vulgaris | 6.5 | 0.89 µM Cd and 9.50 µM Co | Antagonistic | [36] |
Cd + Cr | Nile river algal community | NA | 0.05–1.00 mg L−1 Cd and 0.25–3.00 mg L−1 Cr | Synergistic | [37] |
Cd + Cu | Chaetoceros gracilis; Isochrysis sp. | NA | 0, 0.56, 1.00, 1.80, 3.20, and 5.60 mg L−1 Cd and 0, 0.010, 0.018, 0.032, 0.056, 0.100 mg L−1 Cu | Synergistic | [38] |
Cd + Cu | Chlamydomonas reinhardtii | 7.5 | 40, 60, and 80 nM Cd and Cu | Antagonistic | [39] |
Cd + Cu | Chlamydomonas reinhardtii | 8 | 1 × 10−6–1 × 10−5 M Cd, and 1 × 10−6–1 × 10−5 M Cu | Synergistic | [40] |
Cd + Cu | Chlamydomonas reinhardtii | 6 | 3.52 × 10−6 Cu2+ M and 3.52 × 10−6 M Cd2+ | Antagonistic | [41] |
Cd + Cu | Chlorella pyrenoidosa | 8.6 | 13–25 µM Cu and 6 µM Cd | Synergistic | [42] |
Cd + Cu | Chlorella vulgaris | Antagonistic | [43] | ||
Cd + Cu | Chlorella vulgaris | NA | 1.5 μM Cu and 2.0 μM Cd | Synergistic | [44] |
Cd + Cu | Chlorella vulgaris | 6.5 | 2.80 µM Cu and 0.89 µM Cd | Synergistic | [36] |
Cd + Cu | Chlorella sp. | Synergistic | [45] | ||
Cd + Cu | Chlorolobion braunii | NA | 5 µM Cu and 1 µM Cd | Synergistic | [46] |
Cd + Cu | Dunaliella minuta | 7.4 | 7.57 µM Cu and 0.34 µM Cd | Antagonistic | [47] |
Cd + Cu | Navicula pelliculosa | 7 | 0.42–0.54 µM Cu and 0.50–0.59 µM Cd (EC50 values) | Antagonistic | [27] |
Cd + Cu | Nile river algal community | NA | 0.05–1.00 mg L−1 Cd and Cu | Synergistic | [37] |
Cd + Cu | Pseudokirchneriella subcapitata | 8.1 (BLM) | 0.006–0.046 μM Cu and 0–0.500 μM Cd | Synergistic | [28] |
Cd + Zn | Chlorella vulgaris | 6.8 | 2 × 10−5 M Zn and 0–8 × 10−5 M Cd | Antagonistic | [24] |
Cd + Fe | Thalassiosira weissflogii | NA | 1 × 10−10 M Cd2+ and 1 × 10−7.8 to 1 × 10−5.8 M Fe EDTA | Antagonistic | [48] |
Cd + Hg | Anabaena inaequalis | NA | Synergistic | [49] | |
Cd + Ni | Anabaena inaequalis | NA | Antagonistic and synergistic depending upon metal conc. | [49] | |
Cd + Pb | Scenedesmus obliquus | NA | EE-20 for Cd-Pb synergistic, EE-50 additive | Synergistic | [50] |
Cd + Zn | Chlamydomonas reinhardtii | 7 | 1 × 10−9 M Zn2+, 7 × 10−9 M Cd2+ | Antagonistic | [51] |
Cd + Zn | Chlamydomonas reinhardtii | 7 | 7 nM Cd2+ and 6 × 10−9 M | Antagonistic | [35] |
Cd + Zn | Chlorella sp. | Antagonistic | [45] | ||
Cd + Zn | Skeletonema costatum | 7.8 to 9 | 200–400 μg L−1 Zn 100 μg L−1 Cd | Additive to slight synergistic | [52] |
Cd + Zn | Phaeodactylum tricornutum | 7.8 to 9 | 3000 μg L−1 Cd 4000 μg L−1 Zn |
Additive to slight antagonistic | [52] |
Cd + Zn | Scenedesmus obliquus | NA | EE-20 and EE-50 for Cd-Zn additive | Synergistic | [50] |
Co + Cu | Chlorella vulgaris | 6.5 | 9.5 µM Co and 2.8 µM Cu | Synergistic | [36] |
Cu + Fe | Chlamydomonas reinhardtii | 6–8 | 1 × 10−19 M Fe3+ and 1 × 10−13 to 1 × 10−10.5 | Antagonistic | [53] |
Cu + Ni | Pseudokirchneriella subcapitata | 6.2–8.2 | 0.001–2.680 mg L−1 Ni and 0.001–0.659 mg L−1 Cu | Non-interactive | [54] |
Cd + Ca | Micrasterias denticulata | NA | 2 mM CaSO4 and 150 μM CdSO4 | Antagonistic | [55] |
Cu + Pb | Chlamydomonas reinhardtii | 7 | ≤1 mg L−1 of Cu and Pb | Antagonistic | [56] |
Cu + Zn | Chlorella sp. | Antagonistic | [45] | ||
Cu + Zn | Navicula pelliculosa | 7 | 3.48 µM Zn and 0.51 µM Cu (EC50 values) | Additive | [27] |
Cu + Zn | Phaeodactylum tricornutum | NA | 0.25 mg L−1 Cu and 4.00 mg L−1 Zn | Synergistic | [57] |
Cu + Zn | Phaeocystis antarctica; Cryothecomonas armigera | 7.9 | Antagonistic | [58] | |
Cu + Zn | Scenedesmus sp. | 7 | 2.5–40.0 μM CuCl2.2H2O and 5–100 μM ZnCl2 | Synergistic | [19] |
Cu + Zn | Pseudokirchneriella subcapitata | 8.1 (BLM) | 0.20–2.00 μM Zn and 0.006–0.046 μM Cu | Antagonistic | [28] |
Cd + Zn | Pseudokirchneriella subcapitata | 8.1 (BLM) | 0.20–2.0 μM Zn 0.036–2.100 μM Cd | Antagonistic | [28] |
Cr + Cu | Chlorella vulgaris | NA | 0.05, 0.50, 5.00 μM | Additive | [59] |
Hg + Ni | Anabaena inaequalis | Additive | [49] | ||
Mg + Pb | Chlamydomonas reinhardtii | 7 | ≤1 mg L−1 of M and Pb | Antagonistic | [56] |
Ni + Zn | Navicula pelliculosa | 7 | 0.15–0.19 µM Ni and 3.48–3.71 µM Zn (EC50 values) | Synergistic | [27] |
P + Zn | Raphidocelis subcapitata | NA | 0.09 × 10−6 M to 9.08 × 10−6 M Zn and 2.3 × 10−4 M, 2.3 × 10−6 M and 1.0 × 10−6 M P | Additive | [60] |
Pb + Zn | Scenedesmus obliquus | NA | EE-20 and EE-50 for Pb-Zn synergistic | Additive | [50] |
As(V) + Cd + Cu + Ni + Pb | Diacronema lutheri | NA | 450 µg L−1 As(V), 109 µg L−1 Cd, 34 µg L−1 Cu, 126 µg L−1 Ni, 414 µg L−1 Pb | As(V) had the main toxicity in the mixture | [61] |
Cd + Co | Raphidocelis subcapitata | NA | 0.13–0.25 mg L−1 Co, 0.025–0.100 mg L−1 Cd | Synergistic (high Co and low Cd) Antagonistic (low Co and high Cd) |
[62] |
Cd + Co + Cu | Chlorella vulgaris | 6.5 | 2.80 µM Cu, 0.89 µM Cd and 9.50 µM Co | Antagonistic | [48] |
Cd + Cr + Cu | Nile river algal community | NA | 0.05 mg L−1 Cd and 0.10 mg L−1 Cu, Cr | Antagonistic | [63] |
Cd + Ni + Zn | Nile river algal community | NA | 0.05 mg L−1 Cd and 0.10 mg L−1 Cu, Zn | Antagonistic | [63] |
Co + Cu + Zn | Chlorophyceare; Bacilariophyceae; Cyanophyceae | NA | 1 × 10−6 to 1 × 10−10 mg L−1 Cu, Co and Zn | Synergistic | [64] |
Cu + Ni + Zn | Pseudokirchneriella subcapitata | 7.2 | 0.0200 mg L−1 Zn, 0.0010 mg L−1 Ni, 0.0025 mg L−1 Cu | Non-interactive | [65] |
Cu + Ni + Zn | Pseudokirchneriella subcapitata | 6.2–8.2 | 0.001–2.680 mg L−1 Ni, 0.001–0.659 mg L−1 Cu, and 0.001–0.450 mg L−1 Zn | Non-interactive | [54] |
Cu + Pb + Zn | Scenedesmus quadricauda | 8 | 0.1–0.2 mg L−1 Cu, 0.3–0.5 mg L−1 Zn, 0.3–0.6 mg L−1 Pb | Synergistic (growth) | [66] |
Cu + Pb + Zn | Scenedesmus quadricauda | 8 | 0.1–0.2 mg L−1 Cu, 0.3–0.5 mg L−1 Zn, 0.3–0.6 mg L−1 Pb | Antagonistic (photosynthesis) | [66] |
Cu + Ti + Zn (nanoparticles) | Pseudokirchneriella subcapitata | 7.5–8 | 380 mg L−1 TiO, 0.068 mg L−1 ZnO, 6.400 mg L−1 CuO | Non-interactive | [67] |
Cd + Co + Fe + Zn + P | Chlamydomonas reinhardtii | 7 | 1–100 μM P, 5–40 μM CdCl2 | Antagonistic | [68] |
Cd + Cu + Ni + Zn | Nile river algal community | NA | 0.05 mg L−1 Cd and 0.10 mg L−1 Cu, Cr, Zn | Synergistic | [63] |
Cd + Cu + Ni + Zn | Pseudokirchneriella subcapitata | 0.0200 mg L−1 Zn + 0.0010 mg L−1 Ni + 0.0025 mg L−1 Cu | Non-interactive | [65] | |
Cd + Cu + Ni + Pb + Zn | Phaeocystis antarctica; Cryothecomonas armigera | 7.9 | Synergistic while Zn behaves antagonistic | [69] | |
Cd + Cu + Pb + Zn | Pseudokirchneriella subcapitata | NA | 30, 60, 120, 250 and 500 mg L−1 for Cd and Zn; and 500, 1000, 2000, 3000, 4000 mg L−1 for Cu and Pb | Exude formation lowers metal toxicity | [70] |
Co + Cu + Fe + Mn + Mo + Ni + Zn |
Marine phytoplankton communities | 8.1 | Various oceanic conc. comparison | Complex interactions with biogeochemical influence of ocean | [71] |
Fe + Cr + Cd | Micrasterias denticulata | Near 7 with added soil with buffering property | 600 nM Cd, 10 μM Cr, and 100 μM Fe | Antagonistic | [72] |
Zn + Cd + Cr | Micrasterias denticulata | Near 7 with added soil with buffering property | 600 nM Cd, 10 μM Cr, and 300 nM Zn | Antagonistic | [72] |
Marine and freshwater algae show different toxicity response to heavy metals due to their different living conditions, salinity levels, biology and tolerance etc. The possible mechanism of metal toxicity and the defence system of algal cells is not as studied as in higher plants. It assumes that the entry and detoxification system between plants and algae can be very closely related, as previously described, for the toxicity of Cd, together with the synthesis of phytochelatins and their ability to absorb heavy metal ions in the cytoplasm of algae cells [73][74][75]. Biosorption is achieved due to the presence of various biomolecules on the microalgal cell wall, such as proteins, carbohydrates, and lipids, is aided by negatively charged ions such as phosphate (-PO4)3+, carboxyl (-COOH), hydroxyl (-OH), and amino (-NH2) groups [76]. In addition to compartmentalisation or efflux, there is another pivotal step in detoxification, namely, the binding of the metal ions with certain peptides and the consequent chelation process [77]. The cation diffusion facilitator (CDFs) proteins form a family of ubiquitous transporters involved in metal homeostasis and tolerance. These proteins catalyse the efflux of transition metal cations, such as Zn2+, Cd2+, Co2+, Ni2+, or Mn2+, from the cytoplasm to the outside of the cell or into subcellular compartments [78]. The gal Irt-like proteins (ZIPs), mainly in the vacuolar and plasma membrane, that regulate Zn and Fe uptake.
Other authors [79] identified genes encoding multicopper oxidase (CrFOX1) and Fe permease (CrFTR1) in C. reinhardtii that occur in the Fe assimilation pathway. The Fe status in Chlamydomonas is regulated by genes of the ZIP family, such as IRT1 and IRT2 [80]. These algae species also encode members of the NRAMP family, which are also involved in Fe regulation [81]. The tolerance of Chlorella sp. to Cr and Pb is probably caused by the reduction in metal influx across the plasma membrane; metal chelation in the cytosol by ligands (e.g., phytochelatins, metallothionein, organic acids, and amino acids); the transport of metal ligand complexes through the tonoplast and accumulation in the vacuole; sequestration in the vacuole by tonoplast transporters; and/or ROS defence mechanisms [82]. Ferrari et al. [83] studied H+/(SO4)2− (SULTRs) and Na+/(SO4)2− (SULPs) plasma membrane sulfate transporters and a chloroplast-envelope localised ABC-type holocomplex in Scenedesmus acutus with different Cr(VI) sensitivities, and they observed that the SULTRs’ up-regulation, observed after S-starvation, may directly contribute to enhancing Cr tolerance by limiting Cr(VI) uptake and increasing S availability for the synthesis of S-containing defence molecules.
Current research has indicated that Pb, one of the most toxic metals in nature, causes severe depletion of endogenous cytokinins, auxins, and gibberellin and an increase in abscisic acid content in the green alga Acutodesmus obliquus. Exogenous auxins and cytokinins alleviate Pb toxicity through the regulation of endogenous phytohormone levels [84]. Cd and Co induced growth and photosynthetic inhibition in Raphidocelis subcapitata [62]. Antagonism occurs with Cd and Co because they probably compete for the same transport sites on the membrane, since they are bivalent metals [62]. The tolerance of microalgae to metal toxicity with a high supply of P is probably because microorganisms supplied with this nutrient resist metal toxicity better compared to algal cells under severely limited P conditions [60].
Current toxicological assessments mainly focus on single metal toxicity. To fully understand the potential threat of metals, understanding mixture toxicity in an aquatic ecosystem is vital. The use of phytoplankton can be very useful in ecotoxicological studies, as they make up the first level of any aquatic food web. The recent methodologies used to address this issue have some shortcomings. The EC50 value, the community response, microtox assays, BLM, CA, IA, and other statistical methods provide valuable predictions, but have their shortcomings. In the review, it was noted that most metal toxicity research is conducted with Al, Cd, Co, Cr, Cu, Fe, Ni, Pb, and Zn metals and their reciprocal effects on increased or decreased toxicity. Cu mostly showed antagonistic effects with Cd, Pb, and Zn. The Zn metal ion reduced the toxicity of Al, Cd, Cu, and Ni in many studies. The Cd metal showed variable interactions in different species with Cu and other metals. This suggests that, when studying these metals on phytoplankton, it is important to consider the possible reciprocal interactions of these metals in a mixture. While Zn in the mixture of Cd + Cr + Ni + Pb reacts in the marine algal population as an antagonist, all the mentioned metals can be assessed as synergistic reciprocal effects. Transporters and channels that can regulate metal homeostasis in organelles and cells such as CDF, ZIP, HMA, COPT, MTP, MRP, and NRAMP, regulate and transport essential and non-essential metals through the membranes of algal cells and organelles. A brief introduction to metal cellular pathways and detoxification in the review can help researchers to understand this metal toxicity in phytoplankton for various metal ions, helping future researchers in the field to understand these possible reciprocal toxic metal interactions that affect metal toxicological assessments, and in planning their experiments.
This entry is adapted from the peer-reviewed paper 10.3390/phycology4010007