Biologically induced mineralization in the tree Milicia excelsa (Moraceae): its causes and consequences to the environment

Authors

  • Olivier Braissant,

    1. Institut de Géologie de l’Université de Neuchâtel, Laboratoire de Géodynamique de la Biosphère, Rue Emile Argand 11, 2007 Neuchâtel, Switzerland
    Search for more papers by this author
  • Guillaume Cailleau,

    1. Institut de Géologie de l’Université de Neuchâtel, Laboratoire de Géodynamique de la Biosphère, Rue Emile Argand 11, 2007 Neuchâtel, Switzerland
    Search for more papers by this author
  • Michel Aragno,

    1. Institut de Botanique de l’Université de Neuchâtel, Laboratoire de Microbiologie, Rue Emile Argand, 9 Case Postale 2 , 2007 Neuchâtel, Switzerland
    Search for more papers by this author
  • Eric P. Verrecchia

    Corresponding author
    1. Institut de Géologie de l’Université de Neuchâtel, Laboratoire de Géodynamique de la Biosphère, Rue Emile Argand 11, 2007 Neuchâtel, Switzerland
      Corresponding author: Dr Eric P. Verrecchia. Tel.: +41 (0)32 718 2579; fax: +41 (0)32 718 2601; e-mail: eric.verrecchia@unine.ch
    Search for more papers by this author

Corresponding author: Dr Eric P. Verrecchia. Tel.: +41 (0)32 718 2579; fax: +41 (0)32 718 2601; e-mail: eric.verrecchia@unine.ch

ABSTRACT

Iroko trees (Milicia excelsa) in Ivory Coast and Cameroon are unusual because of their highly biomineralized tissues, which can virtually transform the trunk into stone. Oxalic acid (C2O4H2) and metal-oxalate play important roles in their ecosystems. In this study, the various forms of oxalate and carbonate mineralization reactions are investigated by using scanning electron microscopy and X-ray diffraction. Calcium oxalate monohydrate is associated with stem, bark and root tissues, whereas calcium oxalate dihydrate is found with wood rot fungi in soils, as well as in decaying wood. Laboratory cultures show that many soil bacteria are able to oxidize calcium oxalate rapidly, resulting in an increase in solution pH. In terms of Mexcelsa, these transformations lead to the precipitation of calcium carbonate, not only within the wood tissue, but also within the litter and soil. We calculate that c. 500 kg of inorganic carbon is accumulated inside an 80-year-old tree, and c. 1000 kg is associated with its surrounding soil. Crucially, the fixation of atmospheric CO2 during tree photosynthesis, and its ultimate transformation into calcite, potentially represents a long-term carbon sink, because inorganic carbon has a longer residence time than organic carbon. Considering that calcium oxalate biosynthesis is widespread in the plant and fungal kingdoms, the biomineralization displayed by M. excelsa may be an extremely common phenomena.

Ancillary