⁹⁰Sr is a radionuclide produced during nuclear fission and has a half life of ~29 years. Concern over radioactive strontium as an environmental pollutant (ToxFAQs: Strontium) has driven research investigating the potential of microorganisms to immobilize soluble strontium through biosorption, bioaccumulation, and biomineralization. Zooplankton in the protozoan subclass Acantharia secrete skeletons composed of SrSO₄ (celestite) and have an important role in the marine strontium cycle (Bernstein et al, 1987). No essential biological function for strontium has been identified in prokaryotes or fungi, but Sr²⁺ can replace Ca²⁺ in some biochemical and physiological processes.

Direct reduction of strontium by microorganisms has not been observed, but precipitation of insoluble strontium complexes mediated by bacteria and fungi has been described. When grown on citrate complexed with SrCl₂ as a sole carbon source, Pseudomonas fluorescens deposited strontium in extracellular SrCO₃ crystals (Anderson and Appanna, 1994). The white-rot fungus Resinicium bicolor was shown to mobilize strontium from solid SrCO₃, translocate solubilized strontium through hyphae, and then precipitate strontium in extracellular calcium oxalate crystals (Connolly et al, 1999). Soluble strontium can also be immobilized in siderite (FeCO₃) complexes during growth of dissimilatory Fe(III)-reducing bacteria in bicarbonate-buffered medium with added hydrous ferric oxide (Roden et al, 2002).

Biosorption of Sr²⁺ by yeast and filamentous fungal biomass has been demonstrated (de Rome and Gadd, 1991). Active uptake of Sr²⁺ by Saccharomyces cerevisiae occurs during glucose metabolism, and leads to sequestration of Sr²⁺ in the vacuole (Avery and Tobin, 1992). Strontium accumulation by S. cerevisiae can be increased by supplementing the growth medium with fatty acids, which reduces Sr²⁺ efflux (Avery et al, 1999). Strontium accumulation by bacteria has also been described (Zharova, 1961; Belimov et al, 1998)

In the absence of Ca²⁺, Sr²⁺ restored normal S-layer production in Spirillum putridiconchylium (Beveridge and Murray, 1976) and stimulated exopolysaccharide production in Myxococcus xanthus (Kim et al, 1999). Sr²⁺ has also been shown to effectively replace Ca²⁺ in the assembly of complex flagella in Rhizobium melioti (Robinson et al, 1992), endospore formation in Bacillus cereus and B. megaterium (Foerster and Foster, 1966), and the activation of methanol dehydrogenase in Methylobacterium extorquens (Goodwin et al, 1996).

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Anderson S, Appanna VD. Microbial formation of crystalline strontium carbonate. FEMS Microbiol Rev. 1994;116:43-48.

Belimov AA, Kunakova AM, Vasilyeva ND, Kovatcheva TS, Dritchko VF, Kuzovatov SN, Trushkina IR, Alekseyev YV. Accumulation of radionuclides by associative bacteria and the uptake of ¹³⁴Cs by the inoculated barley plants. Developments in Plant and Soil Sciences. 1998;79:275-280.

Zharova TV. Accumulation of radioactive isotopes of strontium, ruthenium, cesium, and cerium by some bacteria. Mikrobiologiia. 1961;30:871-76.

Bernstein RE, Betzer PR, Feely RA, Byrne RH, Lamb MF, Michaels AF. Acantharian fluxes and strontium to chlorinity ratios in the north Pacific Ocean. Science. 1987;237:1490-94.

Connolly JH, Shortle WC, Jellison J. Translocation and incorporation of strontium carbonate derived strontium into calcium oxalate crystals by the wood decay fungus Resinicium bicolor. Can J Bot. 1999;77:179-87.

Roden EF, Leonardo MR, and Ferris FG. Immobilization of strontium during iron biomineralization coupled to dissimilatory hydrous ferric oxide reduction. Geochim Cosmochim Act. 2002;66:2823-39.

de Rome L, Gadd GM. Use of pelleted and immobilized yeast and fungal biomass for heavy metal and radionuclide recovery. J Indust Microbiol. 1991;7:97-104.