Global searches for microalgae and aquatic plants that can eliminate radioactive cesium, iodine and strontium from the radio-polluted aquatic environment: a bioremediation strategy

Strontium-90 pollution can be bioremediated with the green microalga Tetraselmis chui

Strontium-90 (90Sr) is an artificial radioisotope produced by nuclear fission, with a relatively long half-life of 29 years. This radionuclide is released into the environment in the event of a nuclear incident, posing a serious risk to human and ecosystem health. There is a need to develop new efficient methods for the remediation of 90Sr, as current techniques for its removal have significant technical limitations and involve high energy and economic costs. Recently, several species of green microalgae within the class Chlorodendrophyceae have been found to form intracellular mineral inclusions of amorphous calcium carbonate (ACC), which can be highly enriched in natural (non-radiogenic) Sr. As bioremediation techniques are an attractive option to address radioactive pollution, we investigated the capacity of the unicellular alga Tetraselmis chui (class Chlorodendrophyceae) to sequester 90Sr. The 90Sr uptake capacity of T. chui cells was assessed in laboratory cultures by monitoring the time course of radioactivity in the culture medium using liquid scintillation counting (LSC). T. chui was shown to effectively sequester 90Sr, reducing the initial radioactivity of the culture medium by up to 50%. Thus, this study demonstrates the potential of the microalga T. chui to be used as a bioremediation agent against 90Sr pollution.

Conceptualization of the comprehensive phyto-radiotoxicity incurred by radiocesium and strategies to expunge the metal using biotechnological and phytoremediative approaches

Agricultural pollution with 137Cs is an ecological threat due to its sustained half-life and radioactivity. Release of radiocesium isotopes after major nuclear power plant accidents like the Fukushima Dai-ichi and the Chernobyl nuclear power plant disasters have severely affected the surrounding growth of agricultural crops and vegetables cultivated across extensive areas. Even years after the nuclear accidents, biosafety in these agricultural fields is still questionable. Due to similarity in charge and ionic radius between radiocesium and K+, the radionuclides are promiscuously uptaken via K+ channels expressed in plants. Bioaccumulation of radiocesium reportedly promotes physiological and anatomical anomalies in crops due to radiation and also affects the rhizospheric architecture. Due to radiation hazard, the ecological balance and quality are compromised and ingestion of such contaminated food results in irreversible health hazards. Recently, strategies like exogenous supplementation of K+ or genetic engineering of K+ channels were able to reduce radiocesium bioaccumulation in plants taking the advantage of competition between radiocesium and K+ translocation. Furthermore, bioremediation strategies like phycoremediation, mycoremediation, phytoremediation and rhizofiltration have also showed promising results for removing radiocesium from polluted sites. It has been proposed that these eco-friendly ways can be adopted to de-pollute the contaminated sites prior to subsequent cultivation of crops and vegetables. Hence it is essential to: 1) understand the basic radiotoxic effects of radiocesium on agricultural crops and surrounding vegetation and, 2) design sustainable ameliorative strategies to promote radiocesium tolerance for ensuring food and social security of the affected population.

Phytotoxicity of radionuclides: A review of sources, impacts and remediation strategies

Various anthropogenic activities and natural sources contribute to the presence of radioactive materials in the environment, posing a serious threat to phytotoxicity. Contamination of soil and water by radioactive isotopes degrades the environmental quality and biodiversity. They persist in soils for a considerable amount of time and disturb the fauna and flora of any affected area. Hence, their removal from the contaminated medium is inevitable to prevent their entry into the food chain and the organisms at higher levels of the food chain. Physicochemical methods for radioactive element remediation are effective; however, they are not eco-friendly, can be expensive and impractical for large-scale remediation. Contrastingly, different bioremediation approaches, such as phytoremediation using appropriate plant species for removing the radionuclides from the polluted sites, and microbe-based remediation, represent promising alternatives for cleanup. In this review, sources of radionuclides in soil as well as their hazardous impacts on plants are discussed. Moreover, various conventional physicochemical approaches used for remediation discussed in detail. Similarly, the effectiveness and superiority of various bioremediation approaches, such as phytoremediation and microbe-based remediation, over traditional approaches have been explained in detail. In the end, future perspectives related to enhancing the efficiency of the phytoremediation process have been elaborated.

Characterization of the chloroplast genome of the marine microalga Tetraselmis marina (Cienkowski) R.E.Norris, Hori & Chihara 1980

Tetraselmis marina (Cienkowski) R.E.Norris, Hori & Chihara 1980, a costal green microalga, is considered as a promising animal feed in aquaculture due to the high content of fatty acids and carotenoid. Furthermore, T. marina plays important roles in bioremediation. In this study, we assembled the complete chloroplast genome of T. marina. Results showed that the full length of the complete chloroplast genome was 96,151 bp, containing a large single-copy region of 62,574 bp, a small single-copy region of 1261 bp, and a pair of inverted repeat regions of 16,158 bp. The GC content of the genome was 36.6%. A total of 125 genes were annotated, including 81 protein coding genes, 38 tRNA genes, and six rRNA genes. Phylogenetic analysis based on 22 chloroplast genomes suggested that T. marina was closely related to Tetraselmis sp. CCMP 881.

MXene/AgNW composite material for selective and efficient removal of radioactive cesium and iodine from water

Toxic fission products, such as cesium (137Cs) and iodine (129I) are of great concern because of their long half-lives and high solubility in water. The simultaneous removal of Cs and I using a single adsorbent is an area of increasing interest. In this study, MXene/silver nanowire (AgNW) composite was synthesized through physical mixing and employed for simultaneous removal of iodide (I-) and cesium (Cs+) ions from contaminated water. The MXene/AgNW composite demonstrated excellent adsorption capacities of 84.70 and 26.22 mg/g for I- and Cs+, respectively. The experimental data supported the hypothesis of multilayer adsorption of Cs+ owing to the inter-lamellar structures and the presence of heterogeneous adsorption sites in MXene. The interaction between I- and the AgNW involved chemisorption followed by monolayer adsorption. MXene/AgNW composite material exhibited promising results in the presence of competitive ions under extreme pH conditions. Thus, synthesized composite materials holds promising potential as an adsorbent for the remediation of radioactive liquid waste.

Microbial involvement in iodine cycle: mechanisms and potential applications

Stable iodine isotopes are essential for humans as they are necessary for producing thyroid gland hormones. However, there are hazardous radioactive iodine isotopes that are emitted into the environment through radioactive waste generated by nuclear power plants, nuclear weapon tests, and medical practice. Due to the biophilic character of iodine radionuclides and their enormous biomagnification potential, their elimination from contaminated environments is essential to prevent the spread of radioactive pollution in ecosystems. Since microorganisms play a vital role in controlling iodine cycling and fate in the environment, they also can be efficiently utilized in solving the issue of contamination spread. Thus, this paper summarizes all known on microbial processes that are involved in iodine transformation to highlight their prospects in remediation of the sites contaminated with radioactive iodine isotopes.

The interplay between Cs and K in Pseudanabaena catenata; from microbial bloom control strategies to bioremediation options for radioactive waters

Pseudanabaena dominates cyanobacterial blooms in the First-Generation Magnox Storage Pond (FGMSP) at a UK nuclear site. The fission product Cs is a radiologically significant radionuclide in the pond, and understanding the interactions between Cs and Pseudanabaena spp. is therefore important for determining facility management strategies, as well as improving understanding of microbiological responses to this non-essential chemical analogue of K. This study evaluated the fate of Cs following interactions with Pseudanabaena catenata, a laboratory strain most closely related to that dominating FGMSP blooms. Experiments showed that Cs (1 mM) exposure did not affect the growth of P. catenata, while a high concentration of K (5 mM) caused a significant reduction in cell yield. Scanning transmission X-ray microscopy elemental mapping identified Cs accumulation to discrete cytoplasmic locations within P. catenata cells, indicating a potential bioremediation option for Cs. Proteins related to stress responses and nutrient limitation (K, P) were stimulated by Cs treatment. Furthermore, selected K⁺ transport proteins were mis-regulated by Cs dosing, which indicates the importance of the K⁺ transport system for Cs accumulation. These findings enhance understanding of Cs fate and biological responses within Pseudanabaena blooms, and indicate that K exposure might provide a microbial bloom control strategy.

Kinetic properties of 137Cs uptake by the cesium-accumulating eustigmatophycean microalga

Cesium-137 (¹³⁷Cs) is one of the radioactive substances that was released into the environment as a result of the Fukushima nuclear disaster. Radiocesium exposure is of great concern due to its potential environmental implications. However, research on ¹³⁷Cs removal using algae is still limited. This is the first report to describe the kinetic properties of ¹³⁷Cs uptake by Vacuoliviride crystalliferum in the presence and absence of potassium. In this work, we studied the kinetic properties of ¹³⁷Cs uptake using a freshwater microalga, V. crystalliferum (NIES 2860). We also analyzed the effects of temperature, light, and potassium (K) on the ¹³⁷Cs uptake. Results showed that V. crystalliferum can remove up to 90% of 157 nM ¹³⁷Cs within an hour. At 20 °C, the removal increased by up to 96%, compared to less than 10% at 5 °C. However, the removal was inhibited by nearly 90% in the dark compared to the removal in the light, implying that V. crystalliferum cells require energy to accumulate ¹³⁷Cs. In the inhibition assay, K concentrations ranged from 0 to 500 µM and the inhibitory constant (K_i) for K was determined to be 16.7 µM. While in the uptake assay without potassium (- K), the Michaelis constant (K_m) for Cs was 45 nM and increased to 283 nM by the addition of 20 µM potassium (+ K), indicating that V. crystalliferum had a high affinity for ¹³⁷Cs. In addition, the maximum uptake velocity (V_max) also increased from 6.75 to 21.10 nmol (mg Chl h)^-1, implying the existence of Cs active transport system. In conclusion, V. crystalliferum is capable of removing radioactive ¹³⁷Cs from the environment and the removal was favorable at both normal temperature and in the light.

Massive Accumulation of Strontium and Barium in Diplonemid Protists

Barium and strontium are often used as proxies of marine productivity in palaeoceanographic reconstructions of global climate. However, long-searched biological drivers for such correlations remain unknown. Here, we report that taxa within one of the most abundant groups of marine planktonic protists, diplonemids (Euglenozoa), are potent accumulators of intracellular barite (BaSO₄), celestite (SrSO₄), and strontiobarite (Ba,Sr)SO₄. In culture, Namystinia karyoxenos accumulates Ba²⁺ and Sr²⁺ 42,000 and 10,000 times higher than the surrounding medium, forming barite and celestite representing 90% of the dry weight, the greatest concentration in biomass known to date. As heterotrophs, diplonemids are not restricted to the photic zone, and they are widespread in the oceans in astonishing abundance and diversity, as their distribution correlates with environmental particulate barite and celestite, prevailing in the mesopelagic zone. We found diplonemid predators, the filter-feeding zooplankton that produces fecal pellets containing the undigested celestite from diplonemids, facilitating its deposition on the seafloor. To the best of our knowledge, evidence for diplonemid biomineralization presents the strongest explanation for the occurrence of particulate barite and celestite in the marine environment. Both structures of the crystals and their variable chemical compositions found in diplonemids fit the properties of environmentally sampled particulate barite and celestite. Finally, we propose that diplonemids, which emerged during the Neoproterozoic era, qualify as impactful players in Ba²⁺/Sr²⁺ cycling in the ocean that has possibly contributed to sedimentary rock formation over long geological periods. IMPORTANCE We have identified that diplonemids, an abundant group of marine planktonic protists, accumulate conspicuous amounts of Sr²⁺ and Ba²⁺ in the form of intracellular barite and celestite crystals, in concentrations that greatly exceed those of the most efficient Ba/Sr-accumulating organisms known to date. We propose that diplonemids are potential players in Ba²⁺/Sr²⁺ cycling in the ocean and have possibly contributed to sedimentary rock formation over long geological periods. These organisms emerged during the Neoproterozoic era (590 to 900 million years ago), prior to known coccolithophore carbonate biomineralization (~200 million years ago). Based on reported data, the distribution of diplonemids in the oceans is correlated with the occurrence of particulate barite and celestite. Finally, diplonemids may provide new insights into the long-questioned biogenic origin of particulate barite and celestite and bring more understanding of the observed spatial-temporal correlation of the minerals with marine productivity used in reconstructions of past global climate.

Morphological and Molecular Genetic Analyses of the Genus Vischeria (Eustigmatophyceae, Ochrophyta) in the Algal Collection of Soil Science Institute

Four soil eustigmatophyte algal strains isolated from gray forest soils in Moscow and Tula regions of Russia and deposited in the Algal Collection of Soil Science Institute (ACSSI) were examined by morphological and molecular genetic methods. The strains were assigned to the genus Vischeria on evidence of 18S rRNA gene and ITS2 phylogeny. The strains were morphologically similar to V. magna. However, only one of them, ACSSI 026, clustered with the authentic strain SAG 2554, while the other strains formed a separate independent group. The taxonomy of the genus is problematic because its phylogenetic tree based on the 18S rRNA gene and ITS2 is unresolved, the variable regions V4-V5 and V8-V9 of the 18S rRNA gene are noninformative, and the compensatory base change (CBC) concept fails to work (the concept states that closely related species are distinct if even a single CBC occurs in conserved secondary structure regions of ITS2). The concept of species is presumably possible to develop for Eustigmatophyceae and the genus Vischeria in particular when a greater number of eustigmatophyte algal strains are isolated from various biotopes; plastid genes are used or the total plastid genome is deeply sequenced; and ultrastructural, physiological, and biochemical characteristics are studied in more detail.

Synthesis and applications of bismuth-impregnated biochars originated from spent coffee grounds for efficient adsorption of radioactive iodine: A mechanism study

The adsorption of radioactive iodine, which is capable of presenting high mobility in aquatic ecosystems and generating undesirable health effects in humans (e.g., thyroid gland dysfunction), was comprehensively examined using pristine spent coffee ground biochar (SCGB) and bismuth-impregnated spent coffee ground biochar (Bi@SCGB) to provide valuable insights into the variations in the adsorption capacity and mechanisms after pretreatment with Bi(NO₃)₃. The greater adsorption of radioactive iodine toward Bi@SCGB (adsorption capacity (Q_e) = 253.71 μg/g) compared to that for SCGB (Q_e = 23.32 μg/g) and its reduced adsorption capability at higher pH values provide evidence that the adsorption of radioactive iodine with SCGB and Bi@SCGB is strongly influenced by the presence of bismuth materials and the electrostatic repulsion between their negatively charged surfaces and negatively charged radioactive iodine (IO₃^-). The calculated R² values for the adsorption kinetics and isotherms support that chemisorption plays a crucial role in the adsorption of radioactive iodine by SCGB and Bi@SCGB in aqueous phases. The adsorption of radioactive iodine onto SCGB was linearly correlated with the contact time (h^1/2), and the diffusion of intra-particle predominantly determined the adsorption rate of radioactive iodine onto Bi@SCGB (C_{stage II} (129.20) > C_{stage I} (42.33)). Thermodynamic studies revealed that the adsorption of radioactive iodine toward SCGB (ΔG° = -8.47 to -7.83 kJ/mol; ΔH° = -13.93 kJ/mol) occurred exothermically and that for Bi@SCGB (ΔG° = -15.90 to -13.89 kJ/mol; ΔH° = 5.88 kJ/mol) proceeded endothermically and spontaneously. The X-ray photoelectron spectroscopy (XPS) analysis of SCGB and Bi@SCGB before and after the adsorption of radioactive iodine suggest the conclusion that the change in the primary adsorption mechanism from electrostatic attraction to surface precipitation upon the impregnation of bismuth materials on the surfaces of spent coffee ground biochars is beneficial for the adsorption of radioactive iodine in aqueous phases.

Molecular reaction and dynamic mechanism of iodate reduction to molecular iodine by nitrogen(III) in aqueous solution

This work studies the molecular reaction and dynamic mechanism of iodate reduction by nitrogen(III) in aqueous solution using the ab initio molecular dynamics (AIMD) method based on density functional theory (DFT). Two possible reaction pathways (without and with H⁺) are proposed. The thermodynamic parameters of the proposed reaction pathways are calculated. The theoretical calculation aspects of iodate reduction, including the atomic dipole moment corrected Hirshfeld population (ADCH) atomic charge values, the intrinsic reaction coordinate (IRC) curves, the calculated interaction regional indicator (IRI) isosurfaces with the corresponding sign(λ₂)ρ scatter plots, electrostatic potential (ESP) analysis and molecular reaction dynamics are discussed in-depth. The results show that the reaction pathway with H⁺ is confirmed based on the Gibbs free energy analysis. The transition state proved that the iodate reduction with nitrous acid undergoes four steps according to oxygen-atom deprivation. The IRC curves describe the energy change of the chemical bonds of the reactant conformations in the four steps, with an energy reduction of 71.95, 69.35, 130.15, and 125.87 kJ mol^-1, respectively. The ESP interpenetration diagram and IRI isosurfaces provide detailed information on the nucleophilicity and electrophilicity of the reactant conformations. By decreasing the O atom number in HIO_x (x = 1, 2, 3), the maximum positive charge decreases, and the positive charge coverage area increases, thus resulting in energy reduction and consequently a more stable conformation. Molecular reaction dynamics analytical results indicated that a relatively stable status of the reactants of the four steps was achieved after around 200 fs, and that the HIO₃-HNO₂ reaction released the highest energy.

Efficacious bioremediation of heavy metals and radionuclides from wastewater employing aquatic macro- and microphytes

Cytotoxic, mutagenic, and carcinogenic contaminants, such as heavy metals and radionuclides, have become an alarming environmental concern globally, especially for developed and developing nations. Moreover, inefficient prevalent wastewater treatment technologies combined with increased industrial activity and modernization has led to increase in the concentration of toxic metals and radioactive components in the natural water bodies. However, for the improvement of ecosystem of rivers, lakes, and other water sources different physicochemical methods such as membrane filtration, reverse osmosis, activated carbon adsorption, electrocoagulation, and other electrochemical treatment are employed, which are uneconomical and insufficient for the complete abatement of these emerging pollutants. Therefore, the application of bioremediation employing aquatic macrophytes and microphytes have gained considerable importance owing to the benefits of cost-effectiveness, eco-friendly, and higher energy efficiency. Thus, the present review aims to enlighten the readers on the potential application of algae, cyanobacteria, plant, and other aquatic micro- and macrophytes for the elimination of carcinogenic metals and radioactive isotopes from wastewater. Additionally, the use of transgenic plants, genetically modified species, algal-bacterial symbiosis for the enhancement of removal efficiency of mutagenic contaminants are also highlighted. Furthermore, species selection based on robustness, mechanism of different pathways for heavy metal and radionuclide detoxification are elucidated in this review article.

Two newly identified Haematococcus strains efficiently accumulated radioactive cesium over higher astaxanthin production

In this study, we investigated the morphological, genomic and bioaccumulation characteristics of two isolated Haematococcus strains (namely Goyang and Sogang), which were newly discovered in South Korea. Morphological analysis revealed that the isolated strains were unicellular and bi-flagellated green microalgae that formed thickened walls at the palmelloid or red-cyst phase. Phylogenetic analysis of 18S rRNA and rbcL gDNA sequences demonstrated that both strains were taxonomically related to the genus Haematococcus. The two strains showed growth pattern that was similar to a typical Haematococcus strain, and accumulated astaxanthin within 48 h of exposure to intensive light. Both red-cyst cells effectively removed radioactive cesium to more than 50% within 48 h from low-level cesium-contaminated water of 5 Bq/ml concentration. The cesium-accumulation mechanism is largely associated with the replacement of cellular potassium in thick cell walls during biouptake, and the cesium-removal rate highly depends on the corresponding astaxanthin accumulation involving the potassium-transporting protein (P-type ATPase).

The effects of ionizing radiation on the structure and antioxidative and metal-binding capacity of the cell wall of microalga Chlorella sorokiniana

The impact of ionizing radiation on microorganisms such as microalgae is a topic of increasing importance for understanding the dynamics of aquatic ecosystems in response to environmental radiation, and for the development of efficient approaches for bioremediation of mining and nuclear power plants wastewaters. Currently, nothing is known about the effects of ionizing radiation on the microalgal cell wall, which represents the first line of defence against chemical and physical environmental stresses. Using various microscopy, spectroscopy and biochemical techniques we show that the unicellular alga Chlorella sorokiniana elicits a fast response to ionizing radiation. Within one day after irradiation with doses of 1-5 Gy, the fibrilar layer of the cell wall became thicker, the fraction of uronic acids was higher, and the capacity to remove the main reactive product of water radiolysis increased. In addition, the isolated cell wall fraction showed significant binding capacity for Cu²⁺, Mn²⁺, and Cr³⁺. The irradiation further increased the binding capacity for Cu²⁺, which appears to be mainly bound to glucosamine moieties within a chitosan-like polymer in the outer rigid layer of the wall. These results imply that the cell wall represents a dynamic structure that is involved in the protective response of microalgae to ionizing radiation. It appears that microalgae may exhibit a significant control of metal mobility in aquatic ecosystems via biosorption by the cell wall matrix.

Enhanced removal of cesium by potassium-starved microalga, Desmodesmus armatus SCK, under photoheterotrophic condition with magnetic separation

This study investigated the feasibility of using photoheterotrophic microalga, Desmodesmus armatus SCK, for removal of cesium (Cs⁺) followed by recovery process using magnetic nanoparticles. The comparison of three microalgae results indicated that D. armatus SCK removed the most Cs⁺ at both 25 °C and 10 °C. The results also revealed that the use of microalga grown in potassium (K⁺)-starved condition improves the accumulation of Cs⁺. Heterotrophic mode with addition of volatile fatty acids (VFAs), especially acetic acids (HAc), also enhanced removal of Cs⁺ by K⁺-starved D. armatus SCK; maximum removal efficiency of Cs⁺ was almost 2-fold higher than that of cells grown without organic carbon source. The Cs⁺ taken up by this microalga was efficiently harvested using magnetic nanoparticles, polydiallyldimethylammonium (PDDA)-FeO₃. Finally, this strain eliminated more than 99% of radioactive ¹³⁷Cs from solutions of 10, 100, and 1000 Bq mL^-1. Therefore, use of K⁺-starved microalga, D. armatus SCK, with VFAs could be promising means to remove the Cs from the liquid wastes.

Uptake of 133Cs and 134Cs by Ceratophyllum demersum L. under field and greenhouse conditions

The phytoremediation abilities of Hornwort (Ceratophyllum demersum L.) were tested under greenhouse and field conditions. Plants were exposed for 8, 16, and 24 days (greenhouse with stable isotope ¹³³Cs), 8 days (field with ¹³³Cs), and 8 days (climabox with radioactive isotope ¹³⁴Cs). The plants were exposed to different concentration of stable Cs provided as CsCl (0.008, 0.033, 0.133, 0.267, 0.533, 0.800, 1.067, and 1.333 mM) and different activities of ¹³⁴Cs (4.46, 4.46, 4.74, 4.64, 2.23 and 2.26 kBq). The results of the experiment revealed a significant effect (p < 0.001) of exposure time on Cs uptake. The results showed highest average ¹³³Cs removal rates of 11%, 17% and 19% for 8, 16, and 24 days, respectively, in the greenhouse, 10% for the ¹³⁴Cs experiment, and 27% for the field experiment with ¹³³Cs. The results indicated that increasing the length of exposure lowered the uptake ability, hence indicating that the plant has limited capacity for Cs removal. The accumulated amount of Cs by plants is significantly dependent (p < 0.001) on the concentration of treatment and complies to a sigmoid curve. Comparison of experiments revealed the greenhouse experiment with ¹³³Cs and the experiment with ¹³⁴Cs did not differ significantly in their removal rate. However, the field experiment was significantly different from the previous two (p < 0.001), providing a higher removal rate. C. demersum was also able to resist phytotoxic effects of Cs in the greenhouse experiment for 16 days without significant effects (p > 0.05) on health. Even after 24 days of exposure, the plant resisted up to 0.267 mM treatment concentration with no significant tissue lesion (p > 0.05). These results indicate that C. demersum has potential for remediating aquatic habitats, especially in the case of acute events, where a short duration of phytoremediation may take place.

Microbial bloom formation in a high pH spent nuclear fuel pond

Microorganisms are able to colonise a wide range of extreme environments, including nuclear facilities. In this study, the First Generation Magnox Storage Pond (FGMSP) a high pH, legacy spent nuclear fuel pond (SNFP) situated at Sellafield, Cumbria, UK, was studied. Despite the inhospitable conditions in the FGMSP, microorganisms can cause "blooms" within the facility which to date have not been studied. These microbial blooms significantly reduce visibility in the engineered facility, disrupting fuel retrieval operations and slowing decommissioning. The microbial community colonising the pond during two microbial bloom periods was determined by using physiological measurements and high throughput next generation sequencing techniques. In situ probes within the ponds targeting photosynthetic pigments indicated a cyanobacterial bloom event. Analysis of the 16S rRNA gene data suggested that a single cyanobacterial genus was dominant during the bloom events, which was most closely related to Pseudanabaena sp. Comparisons between the microbial community of FGMSP and an adjacent SNFP that is periodically purged into the FGMSP, showed different community profiles. Data confirm the onset of the microbial blooms occurred when the pond purge rate was reduced, and blooms could be controlled by re-establishing the purging regime. The presence of Pseudanabaena sp. that can colonise the pond and dominate during bloom periods is notable since they have received little attention for their role in cyanobacterial bloom formation. This work also informs bioremediation efforts to treat waters contaminated with radionuclides, which could benefit from the use of cyanobacteria able to tolerate extreme environments and accumulate priority radionuclides.

Relationship of the mobile forms of calcium and strontium in soils with their accumulation in meadow plants in the area of Kashin-Beck endemia

This study was aimed at assessment of strontium and calcium mobility in soils and their accumulation with plants in the areas endemic for Kashin-Beck disease in Eastern Transbaikalia. The strontium and calcium mobility levels were determined using the method of sequential chemical extraction for 7 samples of meadow soils collected from the endemic region and 7 soil samples taken from conditionally control sites. To measure the Ca and Sr levels in the soil and plant samples, XRF analysis and AAS were used. The increased strontium level in the meadow soils of the endemic areas is accompanied by the element's higher mobility. The highest strontium yield was observed in the course of soil extraction using 1 M ammonium acetate, while the soils taken from the control sites gave lower amounts of the trace element. Furthermore, there is a positive correlation between the amount of the strontium extracted and its content in plants (r = + 0.86 - 0.98). At the sequential chemical extraction of calcium from the soils using the above method, the calcium yield was maximal in the ammonium acetate fraction (background sites) and in ammonium acetate and 6 M HCl fractions (endemic areas). The correlation between the amount of the calcium extracted in 1 M ammonium acetate and the macroelement levels found in plants was + 0.968. In addition, a peculiarly high accumulation of strontium in various willow species as compared to other meadow plants was revealed for the first time ever. Thus, the work introduces new data into the trace element biogeochemistry and environmental monitoring.

Sequestration of Radionuclides Radium-226 and Strontium-90 by Cyanobacteria Forming Intracellular Calcium Carbonates

²²⁶Ra is a naturally occurring radionuclide with a half-life of 1600 years. In contrast, ⁹⁰Sr is a radionuclide of sole anthropogenic origin, produced by nuclear fission reactions and has a half-life of 29 years; each of these radionuclides poses potential threats to human and ecosystem health. Here, the cyanobacterium Gloeomargarita lithophora, capable of forming intracellular amorphous calcium carbonate inclusions, was investigated for its ability to uptake ²²⁶Ra and ⁹⁰Sr. In BG-11 medium, G. lithophora accumulated 3.9 μg g^-1 of ²²⁶Ra within 144 h and 47.9 ng g^-1 of ⁹⁰Sr within 1 h, corresponding to ∼99% removal of trace radionuclides. The presence of high-concentration Ca²⁺ in the background media solution did not inhibit ⁹⁰Sr and ²²⁶Ra uptake by G. lithophora. In contrast, dead biomass of G. lithophora accumulated 0.8 μg g^-1 of ²²⁶Ra and 8.87 ng g^-1 of ⁹⁰Sr. Moreover, Synechocystis, a nonbiomineralizing cyanobacteria, removed only 14 and 25% of ²²⁶Ra and ⁹⁰Sr, respectively. This suggested that sequestration of ⁹⁰Sr and ²²⁶Ra was not intrinsic to all cyanobacteria but was likely a specific biological trait of G. lithophora related to the formation of intracellular amorphous Ca-carbonates. The unique ability of G. lithophora to uptake ⁹⁰Sr and ²²⁶Ra at high rates makes it an attractive candidate for further studies involving bioremediation of these radionuclides.

Chemical Analysis of Selected Seaweeds and Seagrass from the Adriatic Coast of Montenegro

Three seaweeds (Halimeda tuna, Codium bursa and Cystoseira barbata) and one seagrass (Cymodocea nodosa) were collected from the Coast of Montenegro, Gulf of Boka Kotorska and their chemical analysis was performed. In seagrass C. nodosa, three phenolic compounds were identified (diosmetin 7-sulfate, caftaric and coutaric acid). The content of β-glucan, fatty acids, sterols and micro- and macro-elements were investigated among all samples. The highest content of β-glucan was detected in C. nodosa seagrass (13.04±0.42 g/100 g). The highest polyunsaturated fatty acids (PUFAs) level was reported in C. barbata, the brown alga (7.157 mg/g), which also had the significant sterol content (fucosterol, 21.76±0.1 μg/g). Green algae, C. bursa and H. tuna, showed the highest level of sterols (β-sitosterol, 95.21±0.16 μg/g and 73.90±0.08 μg/g, respectively). H. tuna had the highest content of calcium (Ca) in amount of 55125 μg/g. In C. bursa, C. barbata and C. nodosa, the Na/K ratio was low (0.43, 0.46 and 0.69, respectively).

Application of Arthrospira (Spirulina) platensis against Chemical Pollution of Water

The basis of phytoremediation technology for cleaning chemically polluted water was developed in the framework of the presented work. This technology is based on the ability of blue-green alga Arthrospira platensis to eliminate different environmental toxicants from water. This technological approach was conducted for the following pollutants: 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT), 2,4,6-trinitrotoluene (TNT), and cesium ions. The effectiveness of the technology was tested in model experiments, which were carried out in glass containers (volume 40 L). In particular, the different concentrations of alga biomass with the aforementioned pollutants were incubated with permanent illumination conditions and air barbotage, at a temperature of 25 °C. The results of the model experiments showed that after two weeks from the start of remediation Arthrospira effectively cleaned artificially polluted waters. Particularly in the case of TNT 56 mg/L concentration, the effect of water remediation was 97%. In the case of DDT 10 mg/L concentration, the degree of cleaning was 90%. Similar results were obtained in the case of 100 mg/L concentration of cesium ions. Thus, the model experiments confirmed that the alga Arthrospira effectively removed tested pollutants from water. That is the basis of phytoremediation technology.

Removal of radioactive cesium from an aqueous solution via bioaccumulation by microalgae and magnetic separation

We evaluated the potential sequestration of cesium (Cs⁺) by microalgae under heterotrophic growth conditions in an attempt to ultimately develop a system for treatment of radioactive wastewater. Thus, we examined the effects of initial Cs⁺ concentration (100–500 μM), pH (5–9), K⁺ and Na⁺ concentrations (0–20 mg/L), and different organic carbon sources (acetate, glycerol, glucose) on Cs⁺ removal. Our initial comparison of nine microalgae indicated that Desmodesmus armatus SCK had removed the most Cs⁺ under various environmental conditions. Addition of organic substrates significantly enhanced Cs⁺ uptake by D. armatus, even in the presence of a competitive cation (K⁺). We also applied magnetic nanoparticles coated with a cationic polymer (polyethylenimine) to separate ¹³⁷Cs-containing microalgal biomass under a magnetic field. Our technique of combining bioaccumulation and magnetic separation successfully removed more than 90% of the radioactive ¹³⁷Cs from an aqueous medium. These results clearly demonstrate that the method described here is a promising bioremediation technique for treatment of radioactive liquid waste.

Effects of Cobalt on Spore Germination, Gametophyte Growth and Gametogenesis of Undaria pinnatifida (Phaeophyceae)

With rapid development of the construction of nuclear power plants along the coast, the concern of negative effects of potentially unexpected release of nuclides on marine organisms has increased. Cobalt (Co) is one of the crucial nuclides in nuclear polluted seawater. The effect of its presence in seawater on life cycle of macroalgae has seldom been studied. In this investigation, a series of Co concentrations including 1, 10, 100 μg L-1, and 1, 10 mg L-1 (the background concentration of Co in culture seawater was determined to be at the level of 0.75 ± 0.11 μg L-1) were used to test the effects of their presence on spore germination, gametophyte growth and gametogenesis of the important brown macroalga Undaria pinnatifida. It was found that the spore germination rate of 10 mg L-1 group was significantly lower than that of the control group after 1- and 2 days exposure. The gametophyte sizes of 1 and 10 mg L-1 groups were much smaller than that of the control group after 6- and 12-days exposure. Oogonia and juvenile sporophytes were observed to appear in 1, 10 μg L-1 and the control groups after 12 and 15 days, respectively, but not in the higher concentration groups. In the recovery test, sporophytes appeared in the 100 μg L-1 group on the 5th day, but not in 1 and 10 mg L-1 groups. These results demonstrate that presence of Co at high concentrations in seawater disturbs the life cycle by suppressing both the gametophyte growth and gametogenesis in U. pinnatifida.

Biosorption of radioactive cesium from contaminated water by microalgae Haematococcus pluvialis and Chlorella vulgaris

The biosorption properties of water-soluble radioactive cesium (¹³⁷Cs) by microalga Haematococcus pluvialis were evaluated with different cell conditions, and its cesium-uptake rate was compared with that by other microalgae, Chlorella vulgaris and Anabaena sp. Photo-induced H. pluvialis red cyst rapidly removed radioactive cesium from the solution by bioaccumulation. We showed that the effectiveness of ¹³⁷Cs uptake is dependent on the specific cell condition of even the same microalgal species. While the H. pluvialis red cyst removed almost 95% of the soluble ¹³⁷Cs in 48 h, both H. pluvialis intermediate cells and C. vulgaris showed 90% uptake efficiency of ¹³⁷Cs with slow uptake rate. The energy dispersive spectrometer data demonstrated that the cesium uptake acceleration by inducing astaxanthin in H. pluvialis red cyst involves the cesium accumulation through the potassium transport channel. The long-term monitoring experiments of the cesium uptake showed that only 40% of ¹³⁷Cs remained in collapsed H. pluvialis cell fragments after 12 months.

Biomineralization of strontium and barium contributes to detoxification in the freshwater alga Micrasterias

The unicellular model alga Micrasterias denticulata inhabits acid peat bogs that are highly endangered by pollutants due to their high humidity. As it was known from earlier studies that algae like Micrasterias are capable of storing barium naturally in form of BaSO₄ crystals, it was interesting to experimentally investigate distribution and sequestration of barium and the chemically similar alkaline earth metal strontium. Additionally, we intended to analyze whether biomineralization by crystal formation contributes to diminution of the generally toxic effects of these minerals to physiology and structure of this alga which is closely related to higher plants. The results show that depending on the treatment differently shaped crystals are formed in BaCl₂ and Cl₂Sr exposed Micrasterias cells. Modern microscopic techniques such as analytical TEM by electron energy loss spectroscopy and Raman microscopy provide evidence for the chemical composition of these crystals. It is shown that barium treatment results in the formation of insoluble BaSO₄ crystals that develop within distinct compartments. During strontium exposure long rod-like crystals are formed and are surrounded by membranes. Based on the Raman signature of these crystals their composition is attributed to strontium citrate. These crystals are instable and are dissolved during cell death. During strontium as well as barium treatment cell division rates and photosynthetic oxygen production decreased in dependence of the concentration, whereas cell vitality was reduced only slightly. Together with the fact that TEM analyses revealed only minor ultrastructural alterations as consequence of relatively high concentrated BaCl₂ and Cl₂Sr exposure, this indicates that biomineralization of Sr and Ba protects the cells from severe damage or cell death at least within a particular concentration range and time period. In the case of Sr treatment where ROS levels were found to be elevated, hallmarks for autophagy of single organelles were observed by TEM, indicating beginning degradation processes.

Potential of higher plants, algae, and cyanobacteria for remediation of radioactively contaminated waters

The potential of photosynthetic organisms to remediate radioactively contaminated water was evaluated for scenarios related to nuclear installations and included the following radionuclides: ¹³⁷Cs, ¹³⁴Cs, ¹³⁶Cs, ⁹⁰Sr, ¹³¹I, ²³⁹Pu, ²⁴¹Am, ¹³²Te/¹³²I, ⁵⁸Co, ⁶⁰Co, ⁵¹Cr, ^110mAg, ⁵⁴Mn, ¹²⁴Sb, ⁵⁹Fe, ⁶⁵Zn, ⁹⁵Zr, and ⁹⁵Nb. An extensive literature review was undertaken leading to the creation of a database including more than 20,000 entries from over 100 references in which terrestrial and aquatic plants, macro- and microalgae, cyanobacteria and biosorbents derived from these organisms were used to clean water from these specific radionuclides or their stable isotopes. In a first phase, the remediation potential of the organisms and biosorbents was evaluated for the individual elements based on parameters such as plant uptake, removal percentage, and bioconcentration factor, and for two radionuclide mixtures based on the ability of the organisms/biosorbents to work under mixture conditions. As the experimental and environmental conditions will influence the performance of the organisms and biosorbents, a literature-based evaluation of the most influencing or restricting parameters was made and water pH, competing ions, and the chemical modification of biosorbents showed to be of major importance. Finally, the most promising organisms and biosorbents were identified using a specifically developed selection procedure taking into account their performance and robustness. Ranking was done based on clear criteria with a distinct weight and scoring scheme. As such, 20 organisms/biosorbents were identified that showed high potential to clean waters contaminated with (mixtures of) radionuclides related to nuclear installations and which can be used for further experimental investigations.

Stable and radioactive cesium: A review about distribution in the environment, uptake and translocation in plants, plant reactions and plants' potential for bioremediation

Radiocesium in water, soil, and air represents a severe threat to human health and the environment. It either acts directly on living organisms from external sources, or it becomes incorporated through the food chain, or both. Plants are at the base of the food chain; it is therefore essential to understand the mechanisms of plants for cesium retention and uptake. In this review we summarize investigations about sources of stable and radioactive cesium in the environment and harmful effects caused by internal and external exposure of plants to radiocesium. Uptake of cesium into cells occurs through molecular mechanisms such as potassium and calcium transporters in the plasma membrane. In soil, bioavailability of cesium depends on the chemical composition of the soil and physical factors such as pH, temperature and tilling as well as on environmental factors such as soil microorganisms. Uptake of cesium occurs also from air through interception and absorption on leaves and from water through the whole submerged surface. We reviewed information about reducing cesium in the vegetation by loss processes, and we extracted transfer factors from the available literature and give an overview over the uptake capacities of 72 plants for cesium from the substratum to the biomass. Plants with high uptake potential could be used to remediate soil and water from radiocesium by accumulation and rhizofiltration. Inside plants, cesium distributes fast between the different plant organs and cells, but cesium in soil is extremely stable and remains for decades in the rhizosphere. Monitoring of contaminated soil therefore has to continue for many decades, and edible plants grown on such soil must continuously be monitored.

Application of an ecosystem model to evaluate the importance of different processes and food web structure for transfer of 13 elements in a shallow lake

In environmental risk assessments of nuclear waste, there is need to estimate the potential risks of a large number of radionuclides over a long time period during which the environment is likely to change. Usually concentration ratios (CRs) are used to calculate the activity concentrations in organisms. However, CRs are not available for all radionuclides and they are not easily scalable to the varying environment. Here, an ecosystem transport model of elements, which estimates concentrations in organisms using carbon flows and food transfer instead of CR is presented. It is a stochastic compartment model developed for Lake Eckarfjärden at Forsmark in Sweden. The model was based on available data on carbon circulation, physical and biological processes from the site and identifies 11 functional groups of organisms. The ecosystem model was used to estimate the environmental transfer of 13 elements (Al, Ca, Cd, Cl, Cs, I, Ni, Nb, Pb, Se, Sr, Th, U) to various aquatic organisms, using element-specific distribution coefficients for suspended particles (K_{d PM}) and upper sediment (K_{d sed}), and subsequent transfer in the foodweb. The modelled CRs for different organism groups were compared with measured CRs from the lake and literature data, and showed good agreement for many elements and organisms, particularly for lower trophic levels. The model is, therefore, proposed as an alternative to measured CR, though it is suggested to further explore active uptake, assimilation and elimination processes to get better correspondence for some of the elements. The benthic organisms (i.e. bacteria, microphytobenthos and macroalgae) were identified as more important than pelagic organisms for transfer of elements to top predators. The element transfer model revealed that most of the radionuclides were channelled through the microbial loop, despite the fact that macroalgae dominated the carbon fluxes in this lake. Thus, element-specific adsorption of elements to the surface of aquatic species, that may be food sources for organisms at higher trophic levels, needs to be considered in combination with generic processes described by carbon fluxes.

Radionuclides: Accumulation and Transport in Plants

Application of radioactive elements or radionuclides for anthropogenic use is a widespread phenomenon nowadays. Radionuclides undergo radioactive decays releasing ionizing radiation like gamma ray(s) and/or alpha or beta particles that can displace electrons in the living matter (like in DNA) and disturb its function. Radionuclides are highly hazardous pollutants of considerable impact on the environment, food chain and human health. Cleaning up of the contaminated environment through plants is a promising technology where the rhizosphere may play an important role. Plants belonging to the families of Brassicaceae, Papilionaceae, Caryophyllaceae, Poaceae, and Asteraceae are most important in this respect and offer the largest potential for heavy metal phytoremediation. Plants like Lactuca sativa L., Silybum marianum Gaertn., Centaurea cyanus L., Carthamus tinctorius L., Helianthus annuus and H. tuberosus are also important plants for heavy metal phytoremediation. However, transfer factors (TF) of radionuclide from soil/water to plant ([Radionuclide]plant/[Radionuclide]soil) vary widely in different plants. Rhizosphere, rhizobacteria and varied metal transporters like NRAMP, ZIP families CDF, ATPases (HMAs) family like P1B-ATPases, are involved in the radio-phytoremediation processes. This review will discuss recent advancements and potential application of plants for radionuclide removal from the environment.

Selective Uptake of Alkaline Earth Metals by Cyanobacteria Forming Intracellular Carbonates

The uptakes of calcium (Ca), strontium (Sr), and barium (Ba) by two cyanobacterial strains, Cyanothece sp. PCC7425 and Gloeomargarita lithophora, both forming intracellular carbonates, were investigated in laboratory cultures. In the culture medium BG-11 amended with 250 μM Ca and 50 or 250 μM Sr and Ba, G. lithophora accumulated first Ba, then Sr, and finally Ca. Sr and Ba were completely accumulated by G. lithophora cells at rates between 0.02 and 0.10 fmol h^-1 cell^-1 and down to extracellular concentrations below the detection limits of inductively coupled plasma atomic emission spectroscopy. Accumulation of Sr and Ba did not affect the growth rate of the strain. This sequential accumulation occurred mostly intracellularly within polyphosphate and carbonate granules and resulted in the formation of core-shell structures in carbonates. In contrast, Cyanothece sp. PCC7425 showed neither a preferential accumulation of heavier alkaline earth metals nor core-shell structures in the carbonates. This indicated that fractionation between alkaline earth metals was not inherent to intracellularly calcifying cyanobacteria but was likely a genetically based trait of G. lithophora. Overall, the capability of G. lithophora to sequester preferentially Sr and Ba at high rates may be of considerable interest for designing new remediation strategies and better understanding the geochemical cycles of these elements.

Growth and Oxidative Stress of Brittlewort (Nitella pseudoflabellata) in Response to Cesium Exposure

The present study evaluated the impact of cesium ((133)Cs) at four concentrations (0, 0.001, 0.01, and 0.1 mg L(-1)) on growth, concentrations of chlorophyll and carotenoid pigments, and oxidative stress responses in the charophyte, Nitella pseudoflabellata, over 30 days. Oxidative stress was quantified by measuring anti-oxidant enzyme activities and H2O2 content. When compared with the control, significantly elevated activity levels of the anti-oxidative enzymes ascorbic peroxidase, catalase and guaiacol peroxidase were observed at 0.1 mg L(-1) (all p < 0.05), even though the H2O2 level was not significantly elevated. Carotenoid and chlorophyll a and b pigment levels were significantly reduced (all p < 0.05) at Cs exposures of 0.01 and 0.1 mg L(-1). Photosynthetic efficiency (i.e., Fv/Fm) was significantly reduced (p < 0.05) at Cs concentrations ≥0.001 mg L(-1). Significant reduction (p < 0.05) of plant growth (i.e., shoot length) was also observed after 1 week of exposure at Cs concentrations ≥0.001 mg L(-1). Our results suggested that Cs exposure reduced plant growth and affected plant functioning via activating the defense mechanism against oxidative stress in Nitella.

Remediation of radiocesium-contaminated liquid waste, soil, and ash: a mini review since the Fukushima Daiichi Nuclear Power Plant accident

The radiation contamination after the Fukushima Daiichi Nuclear Power Plant accident attracts considerable concern all over the world. Many countries, areas, and oceans are greatly affected by the emergency situation other than Japan. An effective remediation strategy is in a highly urgent demand. Though plenty of works have been carried out, progressive achievements have not yet been well summarized. Here, we review the recent advances on the remediation of radiocesium-contaminated liquid waste, soil, and ash. The overview of the radiation contamination is firstly given. Afterwards, the current remediation strategies are critically reviewed in terms of the environmental medium. Special attentions are paid on the adsorption/ion exchange and electrically switched ion exchange methods. Finally, the present review outlines the possible works to do for the large-scale application of the novel remediation strategies.

Taxonomic study of a new eustigmatophycean alga, Vacuoliviride crystalliferum gen. et sp. nov

This study investigated the taxonomic affiliation of the algal strain nak-9, which has been reported to absorb radioactive cesium with high efficiency, using light and electron microscopy, and molecular phylogenetic analysis based on 18S ribosomal RNA gene (rDNA) sequences. This alga is spherical and coccoid, with a smooth cell wall, large vacuole, crystalline structure, reddish globule, and refractile granules (lamellate vesicles). The cells possess one to several greenish parietal chloroplasts with a bulging pyrenoid surrounded by lamellate vesicles. The chloroplasts include orderly thylakoid lamellae but no girdle lamella. Molecular phylogenetic analysis suggests that strain nak-9 is a member of the eustigmatophycean clade, which includes Goniochloris, Pseudostaurastrum, and Trachydiscus. On the basis of these results, we propose that strain nak-9 (NIES-2860) comprises a new species and new genus of the Eustigmatophyceae, Vacuoliviride crystalliferum gen. et sp. nov.

Removal efficiency of radioactive cesium and iodine ions by a flow-type apparatus designed for electrochemically reduced water production

The Fukushima Daiichi Nuclear Power Plant accident on March 11, 2011 attracted people’s attention, with anxiety over possible radiation hazards. Immediate and long-term concerns are around protection from external and internal exposure by the liberated radionuclides. In particular, residents living in the affected regions are most concerned about ingesting contaminated foodstuffs, including drinking water. Efficient removal of radionuclides from rainwater and drinking water has been reported using several pot-type filtration devices. A currently used flow-type test apparatus is expected to simultaneously provide radionuclide elimination prior to ingestion and protection from internal exposure by accidental ingestion of radionuclides through the use of a micro-carbon carboxymethyl cartridge unit and an electrochemically reduced water production unit, respectively. However, the removability of radionuclides from contaminated tap water has not been tested to date. Thus, the current research was undertaken to assess the capability of the apparatus to remove radionuclides from artificially contaminated tap water. The results presented here demonstrate that the apparatus can reduce radioactivity levels to below the detection limit in applied tap water containing either 300 Bq/kg of ¹³⁷Cs or 150 Bq/kg of ¹²⁵I. The apparatus had a removal efficiency of over 90% for all concentration ranges of radio–cesium and –iodine tested. The results showing efficient radionuclide removability, together with previous studies on molecular hydrogen and platinum nanoparticles as reactive oxygen species scavengers, strongly suggest that the test apparatus has the potential to offer maximum safety against radionuclide-contaminated foodstuffs, including drinking water.

Photosynthesis at the forefront of a sustainable life

The development of a sustainable bio-based economy has drawn much attention in recent years, and research to find smart solutions to the many inherent challenges has intensified. In nature, perhaps the best example of an authentic sustainable system is oxygenic photosynthesis. The biochemistry of this intricate process is empowered by solar radiation influx and performed by hierarchically organized complexes composed by photoreceptors, inorganic catalysts, and enzymes which define specific niches for optimizing light-to-energy conversion. The success of this process relies on its capability to exploit the almost inexhaustible reservoirs of sunlight, water, and carbon dioxide to transform photonic energy into chemical energy such as stored in adenosine triphosphate. Oxygenic photosynthesis is responsible for most of the oxygen, fossil fuels, and biomass on our planet. So, even after a few billion years of evolution, this process unceasingly supports life on earth, and probably soon also in outer-space, and inspires the development of enabling technologies for a sustainable global economy and ecosystem. The following review covers some of the major milestones reached in photosynthesis research, each reflecting lasting routes of innovation in agriculture, environmental protection, and clean energy production.