SHOULD ORGANIC COMPOUNDS FOUND IN ASSOCIATION WITH MINERALS BE CALLED BIOMINERALS?

Most higher organisms form exo- or endoskeletons by the direct precipitation of minerals (e.g. silica, calcite, apatite), a process called biomineralization. This involves the selective uptake of elements and their incorporation into macro-molecular mineral structures under direct biological control. However, minerals are also biologically induced. The recognition of biominerals (or their remnant chemical signatures) in ascertaining the presence of past life is important when considering the veracity of Archaean fossils, life preserved in meteorites, or microbial deposits (e.g. Metallogenium). Can we clearly define which minerals are indicators of life? Does the presence of organic compounds support the existence of past life? While studying desert varnish, silica sinter and silica glazes, DNA, amino acids, pigments, and polymorphic organic compounds were identified along with silica and metal oxides. For instance, many researchers have identified living microorganisms on rock coatings. However, there have been only few, if any, viable models for a direct microbial contribution in producing, for example desert varnish. Formation of these “biologically-induced” silica-related coatings through non-biological means poses the question: can any of these deposits be called biominerals? Life-related organics have measurable effects (biosignatures) when found in association with minerals, but when living cells are not directly responsible, existing terminology can be misleading. Biomineralization is a useful descriptive term. However, the indirect formation of minerals by organisms, such as by the alteration of local Eh-pH conditions, the contribution of biological organic compounds to altering biogeochemical reactions and the fossilized biopolymers, could be more appropriately be termed organominerals. This term has been used in the hydrocarbon and agricultural industries but has not generally been applied to natural minerals that are of special interest to microbiologists, paleontologists, and astrobiologists. We propose that the term organomineral provides a more succinct definition of minerals not produced directly by organisms, but formed with organopolymers, bio(organic), and/or organic compounds without the direct evidence of skeleton or intracellular formation.