**5. Bioaccumulation of metals in algae**

The effects of single lanthanides and monazite on growth rate, lipid profile, and pigments in two biotechnologically interesting algae (*Parachlorella kessleri* and *Trachydiscus minutus*) were evaluated. The impact of lanthanides depended on the combination of species, element, and light intensity. For example, the presence of Ce, La, and Sc caused the growth rate of *T. minutus* to rapidly rise at low light intensity. The saturated fatty acid content increased at the expense of polyunsaturated fatty acids in both species. The effect on pigments was variable [60].

The use of lanthanides in agriculture and in aquatic cultures is gradually increasing although their impact on the environment has not been sufficiently verified. Lanthanides are not yet commercially available to increase the production of algal biomass despite the fact that their effects on economically interesting pigments and lipids are known. In the alga *Haematococcus pluvialis*, cellular growth and production of astaxanthin increased after the addition of Ce3+ at a concentration of 1 mg/L. However, this effect was dose-dependent and growth at higher

The toxicity of lanthanides has been reported as low, but is dependent on their chemical form and processing, as reported by Hodge-Sterner's classification system [62]. In soil and water, however, a surplus of lanthanides has a negative to toxic effect on human beings and animals [63]. Human exposure to lanthanides and effects on health are discussed by Pagano et al. [64]. The best studied effects on health are for Ce, La, and Gd, and the rest remain unclear [64]. The toxicity of lanthanides to various organisms is described in several reports [31, 42, 65], but maximum admissible concentrations, thresholds, and toxicity levels are poorly defined [66]. For each organism or species, the toxicity of different lanthanides differs, but the exact effects

The ability of lanthanides to be involved in the metabolism of several basic elements has been considered as a possible cause of their toxicity [36]. Due to this phenomenon, differences in normal functions of several enzymes have been found, as demonstrated by work describing ATPase and pectate lyase [69, 70], ion channel blocking [71], or mineral transport [42, 72].

Although toxic effects of lanthanides have been reported for various microorganisms (**Table 3**), there is little evidence to generalize their effect on algae. Only a few orders of Charophyta [73], Chlorophyta [46, 48, 74], Dinophyta [75], Euglenophyta [49], Bacillariophyceae [76, 77] and Haptophyta [50], and Cyanobacteria [78, 79] have been studied. Most other algal studies, however, contained little or no data on the bioavailability of lanthanides. The relationship between lanthanide concentrations and stimulatory or inhibitory effects on the same algal species are therefore inconsistent. Moreover, many algal groups or species have not yet been tested for toxicity and no tests for macroalgae have been developed. The database on bioas-

The transfer of lanthanides is expected through the food chain, as algae are primary producers [66, 81]. The toxicity of lanthanide on algae therefore needs to be addressed because any harmful effects may result in the transfer of negative effects to organisms at higher trophic levels [67, 82, 83].

concentrations of Ce3+ was inhibited [61].

**4. Toxicity of lanthanides**

92 Lanthanides

remain unknown [67, 68] (**Table 3**).

says for algal toxicity is summarized in Guida et al. [80].


In recent decades, metal uptake by algal biomass has been studied with great interest. Uptake can be by passive binding, so-called "biosorption," or an active process of "bioaccumulation,"

Algal divisions Chlorophyta (C), Ochrophyta (O), and Rhodophyta (R), and Cyanobacteria (B), and the protist classes Dinophyceae (D) and Euglenophyceae (E) are specified. If microalgae were utilized, they are annotated with an (m). If an algal species has a new name, it is referred to with the actual name and an asterisk (\*); names are according to Algaebase, see Guiry et al. [53].

**Table 4.** Studies on algal accumulation, biosorption and/or desorption of lanthanides.

where uptake or removal of elements is metabolically controlled [86, 87]. Some metals belong to the group of essential micronutrients, being important for growth and development of plant cells, and are involved in active metabolism [88]. Bioaccumulation of chemical compounds depends on rates of uptake and metabolism, and on the ability of the organism to degrade or store compounds. In essence, the process of accumulation of elements in algal cells is very complicated and depends on the properties of the species (type, size, form, and state of development), the element (charge, chemical form, and concentration), and the medium (pH, type, and concentration of metal salts or presence of complexing agents) [89]. As can be seen in **Table 4**, accumulation, biosorption, and desorption of lanthanides occurs in micro- and macroalgae, including brown, green, and red algae, algal flagellates, and also cyanobacteria. The potential for biosorption of cerium ions by cyanobacteria *Arthrospira* (*Spirulina*) was also tested [100]. Live and dead algae were shown to efficiently accumulate these metals because

of their ability to create chelated metabolites, e.g., with proteins, sugars, nucleic acids, amino acids, nucleotides, etc. [32]. Moreover, lanthanides in algae also have the ability to bind to pigments, and polysaccharides such as cellulose, alginic acid, carrageenan, fucoidan, etc., which are present in algal cells in great quantities and varieties [91, 95, 101–104]. The bioaccumulation of lanthanum by different organisms, including algae, and its ecotoxicity in the aquatic environment is reviewed in [105]. A recent database of studies evaluating lanthanide

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Precise data about mechanisms of entry for lanthanides into algae and their accumulation are sparse. Even in higher plants, which are much more researched, cell processes responsible for lanthanide intake have only recently been described [38]. Several studies have shown that lanthanides concentrate in chloroplasts [93, 94, 106–108]. It was demonstrated that selective deposition of individual lanthanides in chloroplasts or the cytoplasm occurs in the green alga *Desmodesmus quadricauda* [109]. Nd and Ce were located in the chloroplast while La and Gd were found in the cytoplasm (**Figure 2**). Lanthanides increased the total amount of chlorophyll by up to 21% and changed the chlorophyll *a/b* ratio. They also changed the relative incorporation of heavy Mg isotopes into chlorophyll

However, many questions regarding the transfer and accumulation of lanthanides remain unanswered. For example, mechanisms of transport through the complex cell wall of algae or cyanobacteria, and whether they are stored in some specific structures or just loosely in the cytoplasm are unclear. Research into resistant strains or natural hyper-accumulators might

In biological systems, lanthanides are applied for different purposes such as growth promoters, fertilizers, water bloom killers, or as detection tools (bioindicators, tracers, and markers). Lanthanides have been proposed as growth stimulators for various animals such as pigs and other livestock [110]. Algae were also used as a feed additive to improve the condition of domestic animals [111]. Lanthanide-rich algae are a potential alternative to food supplements or functional foods. However, only one study on young abalones was performed to demonstrate that lanthanide-enriched algal biomass was an effective growth promoter [82]. Therefore, it would be important to increase the number of studies, to obtain relevant data on the effects of lanthanide transmission and to assess the risk of human exposure through food

Many microorganisms, including blue-green algae (e.g., *Microcystis* or *Alexandrium* spp.), cause water blooms with negative impacts on health, ecology, and economics. Water blooms produce harmful toxins (e.g., microcystins and saxitoxins) with detrimental effects on humans and animals [84]. Lanthanides affect algal physiology and their impact on the level of microcystins was demonstrated in *Microcystis aeruginosa* [112, 113]. There was a close relationship between lanthanides, phosphorus content and the growth characteristics of cyanobacteria [113].

bioaccumulation in algae is reviewed by Guida et al. [80].

**6. Biological applications of lanthanides**

molecules [109].

bring some answers.

derived from animals [35].

**Figure 2.** Intracellular localization of different lanthanides in *Desmodesmus quadricauda.* The absorbed lanthanides (horizontal rows) were visualized in cells stained with the fluorescent dye Fluo-4 (left column). Chloroplasts are visualized by autofluorescence of chlorophyll (middle column). In merged photos (right column), the localization of lanthanides seen either inside chloroplasts (Nd, Ce) or in the cytoplasm (La, Gd) (according to Řezanka et al. [109]).

of their ability to create chelated metabolites, e.g., with proteins, sugars, nucleic acids, amino acids, nucleotides, etc. [32]. Moreover, lanthanides in algae also have the ability to bind to pigments, and polysaccharides such as cellulose, alginic acid, carrageenan, fucoidan, etc., which are present in algal cells in great quantities and varieties [91, 95, 101–104]. The bioaccumulation of lanthanum by different organisms, including algae, and its ecotoxicity in the aquatic environment is reviewed in [105]. A recent database of studies evaluating lanthanide bioaccumulation in algae is reviewed by Guida et al. [80].

Precise data about mechanisms of entry for lanthanides into algae and their accumulation are sparse. Even in higher plants, which are much more researched, cell processes responsible for lanthanide intake have only recently been described [38]. Several studies have shown that lanthanides concentrate in chloroplasts [93, 94, 106–108]. It was demonstrated that selective deposition of individual lanthanides in chloroplasts or the cytoplasm occurs in the green alga *Desmodesmus quadricauda* [109]. Nd and Ce were located in the chloroplast while La and Gd were found in the cytoplasm (**Figure 2**). Lanthanides increased the total amount of chlorophyll by up to 21% and changed the chlorophyll *a/b* ratio. They also changed the relative incorporation of heavy Mg isotopes into chlorophyll molecules [109].

However, many questions regarding the transfer and accumulation of lanthanides remain unanswered. For example, mechanisms of transport through the complex cell wall of algae or cyanobacteria, and whether they are stored in some specific structures or just loosely in the cytoplasm are unclear. Research into resistant strains or natural hyper-accumulators might bring some answers.
