Barium is relatively abundant in the Earth's crust, but is found at low levels in aquatic environments due to the insolubility of BaSO₄ (barite) and BaCO₃ (witherite). No biolgical role for barium has been identified in prokaryotes, but protozoan ciliates of the genus Loxodes have organelles composed of barite crystals (Mueller's bodies) that function as mechanoreceptors (Rieder et al, 1975; reviewed by Hemmersbach et al, 1999).

Dissolution of barite by microbial action has been demonstrated (Bolze et al, 1974), which could increase soluble barium levels in localized environments (ToxFAQs: Barium). BaCl₂ is commonly added to uranium mine wastes to immobilize Ra⁺ in an insoluble Ba/RaSO₄ co-precipitate (Fedorak et al, 1986). The sulfate-reducing bacterium Desulfovibrio vulgaris can use barite as a sulfate source (McCready and Krouse, 1980) and can liberate both Ba²⁺ and Ra²⁺ from Ba/RaSO₄ precipitates (McCready et al, 1980).

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McCready RGL, Bland CJ, Gonzales DE. Preliminary studies on the chemical, physical, and biological stability of Ba/RaSO₄ precipitates. Hydrometallurgy. 1980;5:109-116.

McCready RGL, Krouse HR. Sulfur isotope fractionation by Desulfovibrio vulgaris during metabolism of BaSO₄. Geomicrobiol J. 1980;2:55-62.

Rieder HG, Schmitt G, Send W. Accumulation of barium in Mueller's bodies of the Loxodidae (Ciliata, Holotricha). Zeitschrift fuer Naturforschung. 1975;30C:422.