Environmental Science and Pollution Research

, Volume 26, Issue 9, pp 9146–9160 | Cite as

State of rare earth elements in the sediment and their bioaccumulation by mangroves: a case study in pristine islands of Indian Sundarban

  • Sanjay K. Mandal
  • Raghab RayEmail author
  • Aridane G. González
  • Vasileios Mavromatis
  • Oleg S. Pokrovsky
  • Tapan K. Jana
Research Article


The mangrove ecosystems are known to efficiently sequester trace metals both in sediments and plant biomass. However, less is known about the chemistry of rare earth elements (REE) in the coastal environments, especially in the world’s largest mangrove province, the Sundarban. Here, the concentration of REE in the sediment and plant organs of eight dominant mangrove species (mainly Avicennia sp.) in the Indian Sundarban was measured to assess REE sources, distribution, and bioaccumulation state. Results revealed that light REE (LREE) were more concentrated than the heavy REE (HREE) (128–144 mg kg−1 and 12–15 mg kg−1, respectively) in the mangrove sediments, with a relatively weak positive europium anomaly (Eu/Eu* = 1.03–1.14) with respect to North American shale composite. The primary source of REE was most likely linked to aluminosilicate weathering of crustal materials, and the resultant increase in LREE in the detritus. Vertical distribution of REE in one of the long cores from Lothian Island was altered by mangrove root activity and dependent on various physicochemical properties in the sediment (e.g., Eh, pH, organic carbon, and phosphate). REE uptake by plants was higher in the below-ground parts than in the above-ground plant tissues (root = 3.3 mg kg−1, leaf + wood = 1.7 mg kg−1); however, their total concentration was much lower than in the sediment (149.5 mg kg−1). Species-specific variability in bioaccumulation factor and translocation factor was observed indicating different REE partitioning and varying degree of mangrove uptake efficiency. Total REE stock in plant (above + live below ground) was estimated to be 168 g ha−1 with LREE contributing ~ 90% of the stock. This study highlighted the efficiency of using REE as a biological proxy in determining the degree of bioaccumulation within the mangrove environment.


Rare earth elements (REE) Bioaccumulation Mangrove Sundarban 



RR is indebted to LabexMER International Postdoctoral Program for providing fellowship (FNP150009-DOCT-RAY). AGG thanks to the Laboratoire d’Excellence LabexMer (ANR-10-LABX-19) and the Postdoctoral program from the Universidad de Las Palmas de Gran Canaria. The authors sincerely thank the Sundarban Biosphere Reserve for giving permission to undertake this study inside the mangrove forest. We thank the editor and reviewer for their comments that helped much improving the manuscript.

Funding information

SKM received minor research project grant from the University Grant Commission, New Delhi (No. F, PSW-076/13-14, ERO).


  1. Analuddin K, Sharma S, Jamili J, Septianaa A, Sahidind I, Riansee U, Nadaoka K (2017) Heavy metal bioaccumulation in mangrove ecosystem at the coral triangle ecoregion, Southeast Sulawesi, Indonesia. Mar Pollut Bull 125:472–480CrossRefGoogle Scholar
  2. APHA 20005 (1995) Standard methods for the examination of water and waste water, Washington, pp 5–15Google Scholar
  3. Åström M (2001) Abundance and fractionation patterns of rare earth elements in streams affected by acid sulphate soils. Chem Geol 175:249–258CrossRefGoogle Scholar
  4. Ayres M, Harris N (1997) REE fractionation and Nd isotope disequilibrium during crustal anatexis: constraints from Himalayan leucogranites. Chem Geol 139:249–269CrossRefGoogle Scholar
  5. Baker AJM, Brooks RR (1989) Terrestrials higher plants which hyper accumulate metallic elements. A review of their distribution, ecology and phytochemistry. Biorecovery 1:81–26Google Scholar
  6. Banfield JF, Eggleton RA (1989) Apatite replacement and rare earth element mobilization, fractionation and fixation during weathering. Clay Clay Miner 37:113–127CrossRefGoogle Scholar
  7. Bau M, Dulski P (1996) Distribution of yttrium and rare-earth elements in the Penge and Kuruman Iron-Formations, Transvaal Supergroup, South Africa. Precambrian Res 79:37–55CrossRefGoogle Scholar
  8. Bau M, Möller P, Dulski P (1997) Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling. Marine Chemistry 56 (1-2):123-131Google Scholar
  9. Bowen HJM (1979) Environmental chemistry of the elements. Academic Press, LondonGoogle Scholar
  10. Brito P, Malvar M, Galinha C, Caçador I, Canário J, Araújo F, Raimundo J (2018a) Yttrium and rare earth elements fractionation in salt marsh halophyte plants. Sci Total Environ 643:1117–1126CrossRefGoogle Scholar
  11. Brito P, Prego R, Mil-Homens M, Caçador I, Caetano M (2018b) Sources and distribution of yttrium and rare earth elements in surface sediments from Tagus estuary, Portugal. Sci Total Environ 621:317–325CrossRefGoogle Scholar
  12. Byrne RH, Kim KH (1990) Rare earth element scavenging in seawater. Geochim Cosmochim Acta 54:2645–2656CrossRefGoogle Scholar
  13. Caccia VG, Millero FJ (2007) Distribution of yttrium and rare earths in Florida Bay sediments. Mar Chem 104:171–185CrossRefGoogle Scholar
  14. Cao XD, Chen Y, Gu ZM, Wang XR (2000) Determination of trace rare earth elements in plant and soil samples by inductively coupled plasma-mass spectrometry. Int J Environ An Ch 76:295–309CrossRefGoogle Scholar
  15. Cao XD, Chen Y, Wang XR, Deng XH (2001) Effects of redox potential and pH value on the release of rare earth elements from soil. Chemosphere 44:655–661CrossRefGoogle Scholar
  16. Carpenter D, Boutin C, Allison JE, Parsons JL, Ellis DM (2015) Uptake and effects of six rare earth elements (rees) on selected native and crop species growing in contaminated soils. PLoS One 10(6):e0129936CrossRefGoogle Scholar
  17. Censi P, Spoto SE, Nardone G, Saiano F, Punturo R, Geronimo D, Mazzola SI, Bonanno S, Patti A, Sprovieri B, Ottonello D (2005) Rare-earth elements and yttrium distributions in mangrove coastal water systems: the western Gulf of Thailand. Chem Ecol 21:255–277CrossRefGoogle Scholar
  18. Clark MW, McConchie D, Lewis DW, Saenger P (1998) Redox stratification and heavy metal partitioning in Avicennia dominated mangrove sediments: a geochemical model. Chem Geol 149:147–171CrossRefGoogle Scholar
  19. Cluis C (2004) Junk-greedy greens: phytoremediation as a new option for soil decontamination. Biotechnol J 2:60–67Google Scholar
  20. Das S, Jana TK, De TK (2014) Vertical profile of phosphatase activity in the Sundarban mangrove forest, north east coast of Bay of Bengal. India Geomicrobiol J 31:716–725CrossRefGoogle Scholar
  21. Davranche M, Grybos M, Gruau G, Pédrot M, Dia A, Marsac R (2011) Rare earth element patterns: a tool for identifying trace metal sources during wetland soil reduction. Chem Geol 284:127–137CrossRefGoogle Scholar
  22. de Oliveira C, Ramos SJ, Siqueira JO, Faquin V, de Castro EM, Amaral DC, Techio VH, Coelho LC, e Silva PHP, Schnug E, LRG G (2015) Bioaccumulation and effects of lanthanum on growth and mitotic index in soybean plants. Ecotoxicol Environ Saf 122:136–144CrossRefGoogle Scholar
  23. Dong WM, Wang XK, Bian XY, Wang AX, Du JZ, Tao ZY (2001) Comparative study on sorption/desorption of radioeuropium on alumina, bentonite and red earth: effects of pH, ionic strength, fulvic acid, and iron oxides in red earth. Appl Radiat Isot 54:603–610CrossRefGoogle Scholar
  24. Duarte CM, Middelburg JJ, Caraco N (2005) Major role of marine vegetation on the oceanic carbon cycle. Biogeosci 2:1–8CrossRefGoogle Scholar
  25. França EJ, De Nadai Fernandes EA, Turra C, Bacchi MA, Elias C, Tagliaferro F et al (2011) Survey of lanthanoids in plants from a tropical region. Int J Environ Heal 5:32–48CrossRefGoogle Scholar
  26. Fu F, Akagi T, Shinotsuka K (1998) Distribution pattern of rare earth elements in fern: implication for intake of fresh silicate particles by plants. Biol Trace Elem Res 64:13–26CrossRefGoogle Scholar
  27. Gaillardet J, Dupre B, Allegre CJ, Negrel P (1997) Chemical and physical denudation in the Amazon river basin. Chem Geol 142:141–173CrossRefGoogle Scholar
  28. Graf JL Jr (1977) Rare earth elements as hydrothermal tracers during the formation of massive sulfide deposits in volcanic rocks. Econ Geol 72:527–548CrossRefGoogle Scholar
  29. Grasshoff K, Ehrhardt M, Kremling K (1983) Methods of seawater analysis, 2nd edn. Verlag Chemic, GermanyGoogle Scholar
  30. Grawunder A, Merten D, Büchel G (2014) Origin of middle rare earth element enrichment in acid mine drainage-impacted areas. Environ Sci Pollut Res 21:6812–6823CrossRefGoogle Scholar
  31. Gromet LP, Dymek RF, Haskin LA, Korotev RL (1984) The North American shale composite; its compilation, major and trace element characteristics. Geochim Cosmochim Acta 48:2469–2482CrossRefGoogle Scholar
  32. Hannigan RE, Sholkovitz ER (2001) The development of middle rare earth element enrichments in freshwater: weathering of phosphate minerals. Chem Geol 175:495–508CrossRefGoogle Scholar
  33. Hannigan R, Dorval E, Jones C (2010) The rare earth element chemistry of estuarine surface sediments in the Chesapeake Bay. Chem Geol 272:20–30CrossRefGoogle Scholar
  34. Haskin LA, Frey FA, Schmitt RA, Smith RH (1966) Meteoritic, solar and terrestrial rare earth distributions. In: Ahrens LH, Press F, Runcorn SK, Urey HC (eds) Physics and Chemistry of the Earth. Pergamon Press, Oxford, pp 169–321Google Scholar
  35. Haskin LA, Wildeman TR, Haskin MA (1968) An accurate procedure for the determination of the rare earths by neutron activation. J Radioanal Nucl Ch 1:337–348CrossRefGoogle Scholar
  36. Heidam NZ (1982) Atmospheric aerosol factor models, mass and missing data. Atmos Environ 16:1923–1931CrossRefGoogle Scholar
  37. Hu Z, Richter H, Sparovek G, Schnug E (2004) Physiological and biochemical effects of rare earth elements on plants and their agricultural significance: a review. J Plant Nutr 27:183–220CrossRefGoogle Scholar
  38. Hu Z, Haneklaus S, Sparovek G, Schnug E (2006) Rare earth elements in soils. Commun Soil Sci Plant Anal 37:1381–1420CrossRefGoogle Scholar
  39. Khan AM, Abu Bakar NK, Abu Bakar AF, Ashraf MA (2017) Chemical speciation and bioavailability of rare earth elements (REEs) in the ecosystem: a review. Environ Sci Pollut Res 24:22764–22789CrossRefGoogle Scholar
  40. Kötschau A, Büchel G, Einax JW, von Tümpling W, Merten D (2014) Sunflower (Helianthus annuus): phytoextraction capacity for heavy metals on a mining-influenced area in Thuringia, Germany. Environ Earth Sci 72:2023–2031Google Scholar
  41. Lecomte KL, Sarmiento AM, Borrego J, Nieto JM (2017) Rare earth elements mobility processes in an AMD-affected estuary: Huelva Estuary (SW Spain). Mar Pollut Bull 121:282–291CrossRefGoogle Scholar
  42. Li FL, Shan XQ, Zhang TH, Zhang SZ (1998) Evaluation of plant availability of rare earth elements is soils by chemical fractionation and multiple regression analysis. Environ Pollut 102:269–277CrossRefGoogle Scholar
  43. Ma YJ, Huo RK, Liu CQ (2002) Speciation and fractionation of rare earth elements in a lateritic profile southern China: identification of the carriers of Ce anomalies. Proceedings of the Goldschmidt conference, Davos, SwitzerlandGoogle Scholar
  44. Mandal SK, Ray R, Chowdhury C, Majumder N, Jana TK (2013) Implication of organic matter on arsenic and antimony sequestration in sediment: evidence from Sundarban mangrove forest, India. Bull Environ Contam Toxicol 90:451–455CrossRefGoogle Scholar
  45. Markert B, Li ZD (1991) Natural background concentrations of rare-earth elements in a forest ecosystem. Sci Total Environ 103:27–35CrossRefGoogle Scholar
  46. Migaszewski ZM, Gałuszka A, Dołęgowska S (2016) Rare earth and trace element signatures for assessing an impact of rock mining and processing on the environment: Wiśniówka case study, south-central Poland. Environ Sci Pollut Res 23:24943–24959CrossRefGoogle Scholar
  47. Millero FJ (1992) Stability constants for the formation of rare earth inorganic complexes as a function of ionic strength. Geochim Cosmochim Acta 56:3123–3132CrossRefGoogle Scholar
  48. Mleczek P, Borowiak K, Budka A, Niedzielski P (2018) Relationship between concentration of rare earth elements in soil and their distribution in plants growing near a frequented road. Environ Sci Pollut Res 25:23695–23711CrossRefGoogle Scholar
  49. Mohanty AK, Bramha SN, Satpathy KK, Padhi RK, Panigrahi SN, Samantara MK, Barath Kumar S, Sarkar SK, Prasad MVR (2018) Geochemical distribution of forms of phosphorus in marine sediment of Bay of Bengal, southeast coast of India. Indian Journal of Geo-Marine Sciences 47:1132–1141Google Scholar
  50. Morrison JF, Cleland WW (1983) Lanthanide ATP complexes determination of their dissociation constants and mechanism of action as inhibitors of yeast hexo kinase. Biochemistry-US 22:5507–5513CrossRefGoogle Scholar
  51. Nakanishi TM, Takahashi J, Yagi H (1997) Rare earth element, Al, and Sc partition between soil and Caatinger wood grown in north-east Brazil by instrumental neutron activation analysis. Biol Trace Elem Res 60:163–174CrossRefGoogle Scholar
  52. Olivares E, Aguiar G, Pean E, Colonnello G, Benitez M, Herrera F (2014) Rare earth elements related to aluminium in Rhynchanthera grandiflora growing in palm swamp communities. Interciencia 39:32–39Google Scholar
  53. Olmez I, Sholkovitz ER, Hermann D, Eganhouse RP (1991) Rare earth elements in sediments of southern California: a new anthropogenic indicator. Environ Sci Technol 25:310–316CrossRefGoogle Scholar
  54. Perez-Lopez R, Macias F, Canovas CR, Sarmiento AR, Perez-Moreno AM (2016) Pollutant flows from a phosphogypsum disposal area to an estuarine environment: an insight from geochemical signatures. Sci Total Environ 553:42–51CrossRefGoogle Scholar
  55. Pickard BG (1970) Comparison of calcium and lanthanum ions in the Avena-coleoptile growth test. Planta 91:314–320CrossRefGoogle Scholar
  56. Pourret O, Davranche M, Gruau G, Dia A (2007) Rare earth elements complexation with humic acid. Chem Geol 243:128–141CrossRefGoogle Scholar
  57. Prasad MBK, Ramanathan A (2008) Distribution of rare earth elements in the Pichavaram mangrove sediments of the southeast coast of India. J Coast Res 24:126–134CrossRefGoogle Scholar
  58. Rajkumar K, Ramanathan AL, Behera PN (2012) Characterization of clay minerals in the Sunda, Mangrove River sediments by SEM/EDS. J Geol Soc India 80:429–434CrossRefGoogle Scholar
  59. Ramesh R, Ramanathan AL, Arthur James R, Subramanian V, Jacobsen SB, Holland HD (1999) Rare earth elements and heavy metal distribution in estuarine sediments of east coast of India. Hydrobiol 397:89–99CrossRefGoogle Scholar
  60. Ray R, Ganguly D, Chowdhury C, Dey M, Das S, Dutta MK, Mandal SK, Majumder N, De TK, Mukhopadhyay SK, Jana TK (2011) Carbon sequestration and annual increase of carbon stock in a mangrove forest. Atmos Environ 45:5016–5024CrossRefGoogle Scholar
  61. Ray R, Majumder N, Das S, Chowdhury C, Jana TK (2014) Biogeochemical cycle of nitrogen in a tropical mangrove ecosystem, east coast of India. Mar Chem 167:33–43CrossRefGoogle Scholar
  62. Ray R, Majumder N, Chowdhury C, Das S, Jana TK (2017) Phosphorus budget of the Sundarban mangrove ecosystem: Box model approach. Estuar Coasts 41:1036–1049CrossRefGoogle Scholar
  63. Ray R, Baum A, Rixen T, Gleixner G, Jana TK (2018) Exportation of dissolved (inorganic and organic) and particulate carbon from mangroves and its implication to the carbon budget in the Indian Sundarbans. Sci Total Environ 621:535–547CrossRefGoogle Scholar
  64. Rice AJ, Maccarthy P (1989) Characterization of stream sediment humin. In: Suffet H, Maccarthy P (eds) Aquatic humic substances. American Chemical Society, Washington, p 54Google Scholar
  65. Rodrıiguez H, Fraga R, Gonzalez T, Bashan Y (2006) Genetics of phosphate solubilization and its potential applications for improving plant growth-promoting bacteria. Plant Soil 287:15–21CrossRefGoogle Scholar
  66. Santos IR, DIT F´v, CEGR S, Silva-Filho EV (2007) Sediment geochemistry in coastal maritime Antarctica (Admiralty Bay, King George Island): evidence from rare earths and other elements. Mar Chem 107:464–474CrossRefGoogle Scholar
  67. Sappal SM, Ramanathan AL, Ranjan RK, Singh G, Kumar A (2014) Rare earth elements as biogeochemical indicators in mangrove ecosystems (Pichavaram, Tamilnadu, India). J Sediment Res 84:781–791CrossRefGoogle Scholar
  68. Sharpe AG (1999) Inorganic Chemistry, 3rd edn. Addison-Wesley-Longman, Inc., EnglandGoogle Scholar
  69. Sholkovitz ER (1988) Rare earth elements in the sediments of the North Atlantic Ocean, Amazon Delta, and East China Sea: reinterpretation of terrigenous input patterns to the oceans. Am J Sci 288:236–281CrossRefGoogle Scholar
  70. Sholkovitz ER (1990) REE’s in marine sediments and geochemical standards. Chem Geol 88:333–347CrossRefGoogle Scholar
  71. Silva-Filho EV, Sanders CJ, Bernat M, Figueiredo AMG, Sella SM, Wasserman J (2011) Origin of rare earth element anomalies in mangrove sediments, Sepetiba Bay, SE Brazil: used as geochemical tracers of sediment sources. Environ Earth Sci 64:1257–1267CrossRefGoogle Scholar
  72. Sonke JE (2006) Lanthanide-humic substances complexation. II. Calibration of humic ionbinding model V. Environ Sci Technol 40:7481–7487CrossRefGoogle Scholar
  73. Thomas P, Carpenter D, Boutin C, Allison JE (2014) Rare earth elements (REEs): effects on germination and growth of selected crop and native plant species. Chemosphere 96:57–66CrossRefGoogle Scholar
  74. Tranchida G, Oliveri E, Angelone M, Bellanca A, Censi P, D’Elia M, Neri R, Placenti F, Sprovieri S, Mazzola S (2011) Distribution of rare earth elements in marine sediments from the Strait of Sicily (western Mediterranean Sea): evidence of phosphogypsum waste contamination. Mar Pollut Bull 62:182–191CrossRefGoogle Scholar
  75. Tyler G (2004) Rare earth elements in soil and plant systems-a review. Plant Soil 267:191–206CrossRefGoogle Scholar
  76. Tyler G, Olsson T (2002) Conditions related to solubility of rare and minor elements in forest soils. J Plant Nutr Soil Sci 165:594–601CrossRefGoogle Scholar
  77. Tyler G, Olsson T (2005) Rare earth elements in forest-floor herbs as related to soil conditions and mineral nutrition. Biol Trace Elem Res 106:177–191CrossRefGoogle Scholar
  78. Vermeire M, Cornu S, Fekiacova Z, Detienne M, Delvaux B et al (2016) Rare earth elements dynamics along pedogenesis in a chronosequence of podzolic soils. Chem Geol 446:163–174CrossRefGoogle Scholar
  79. Vischer PT, Beukema J, van Gemerden H (1991) In situ characterization of sediments: m measurements of oxygen and sulfide profiles with a novel combined needle electrode. Limnol Oceanogr 36:1476–1480CrossRefGoogle Scholar
  80. Volokh AA, Gorbunov AV, Gundorina SF, Revich BA, Frontasyeva MV, Pal CS (1990) Phosphorus fertilizer production as a source of rare-earth elements pollution of the environment. Sci Total Environ 95:141–148CrossRefGoogle Scholar
  81. Wan YX, Liu CQ (2006) The effect of humic acid on the adsorption of REE on kaolin. Colloids Surf A Physicochem Eng Asp 290:112–117CrossRefGoogle Scholar
  82. Wen B, Yuan DA, Shan XQ, Li FL, Zhang SZ (2001) The influence of rare earth element fertilizer application on the distribution and bioaccumulation of rare earth elements in plants under field conditions. Chem Speciat Bioavailab 13:39–48CrossRefGoogle Scholar
  83. Wiche O, Kummer N-A, Heilmeier H (2016) Interspecific roots interactions between white lupin and barley enhance the uptake of rare earth elements (REEs) and nutrients in shoots of barley. Plant Soil 402:235–245CrossRefGoogle Scholar
  84. Wiche O, Tischler D, Fauser C, Lodemann J, Heilmeier H (2017) Effects of citric acid and the siderophore desferrioxamine B (DFO-B) on the mobility of germanium and rare earth elements in soil and uptake in Phalaris arundinacea. Int J Phytoremediat 19:746–754CrossRefGoogle Scholar
  85. Windom HL, Schropp SJ, Calder FD, Ryan JD, Smith RG, Burney LC, Lewis FG, Rawlinson CH (1989) Natural trace metal concentrations in estuarine and coastal marine sediments of the southeastern United States. Environ Sci Technol 23:314–320CrossRefGoogle Scholar
  86. Wright J, Schrader H, Holster WT (1987) Paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite. Geochim Cosmochim Acta 51:631–644CrossRefGoogle Scholar
  87. Wu ZH, Luo J, Guo HY, Wang XR, Yang CS(2015) Adsorption isotherms of lanthanum to soil constituents and effects of pH, EDTA and fulvic acid on adsorption of lanthanum onto goethite and humic acid. Chemical Speciation & Bioavailability 13 (3):75-81Google Scholar
  88. Xiaoqing L, Hao H, Chao L, Min Z, Fashui H (2009) Physico-chemical property of rare earths—effects on the energy regulation of photosystem II in Arabidopsis thaliana. Biol Trace Elem Res 130:141–151CrossRefGoogle Scholar
  89. Yoshida S, Muramatsu Y (1997) Determination of major and trace elements in mushroom, plant and soil samples collected from Japanese forests. Int J Environ An Ch 67:49–58CrossRefGoogle Scholar
  90. Zhang R, Yan C, Liu J (2013) Effect of mangroves on the horizontal and vertical distributions of rare earth elements in sediments of the Zhangjiang Estuary in Fujian Province, Southeastern China. J Coast Res 29:1341–1350CrossRefGoogle Scholar

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© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Marine ScienceCalcutta UniversityKolkataIndia
  2. 2.Department of ChemistrySundarban Hazi Desarat CollegePathankhaliIndia
  3. 3.LEMAR (Laboratoire des Sciences de l’Environnement Marin), UMR 6539, (CNRS-UBO-IRD- IFREMER)PlouzanéFrance
  4. 4.Department of Chemical Oceanography, Atmosphere and Ocean Research InstituteThe University of TokyoKashiwaJapan
  5. 5.Instituto de Oceanografía y Cambio Global, IOCAGUniversidad de Las Palmas de Gran Canaria, ULPGCLas Palmas de Gran CanariaSpain
  6. 6.GET (Géosciences Environnement Toulouse) UMR 5563 CNRSToulouseFrance
  7. 7.BIO-GEO-CLIM LaboratoryTomsk State UniversityTomskRussia
  8. 8.N. Laverov Federal Center for Integrated Arctic Research, IEPSRussian Academy of SciencesArkhangelskRussia

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