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Micropeloidal textures – Definition, formation and potential as biosignatures
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DOI：10.1016/j.earscirev.2026.105640.png)
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Abstract
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Microbial biosignatures result from the interactions between microbial populations, metabolic processes, organic and mineral products, and the surrounding physicochemical environment. During the last decades, the biogenicity of mineral signatures has gained considerable attention in geobiology and astrobiology. When considering the origin of life on Earth or the search for life beyond it, it is critical to distinguish between organominerals (minerals formed in association with organic matter, regardless of its origin) and biominerals (minerals produced under direct genetic control of an organism). In this paper, we propose the micropeloidal texture (MT) as a candidate biosignature of enzymatically driven organomineralization in microbial mats and biofilms. The formation of MT can be summarized in five sequential steps: (1) enzymatic breakdown of extracellular organic matter (EOM) locally releases calcium and reduces the cation-binding capacity of the matrix, allowing amorphous calcium carbonate (ACC) and residual organics to combine into nanospherulites; (2) the nanospherulites coalesce into micrite-sized crystals that aggregate as organic-rich patches, i.e., the micropeloids; (3) continued crystallization, supported by metabolic alkalinity production and/or extrinsic supersaturation, displaces organic matter outward and microsparite precipitates around and cements the micritic core; (4) the release of intracellular enzymes (i.e., lyases) triggers a "self-destruction sequence" of the biofilm, and a microsparite front migrates past the micropeloid; (5) when the biofilm is fully degraded, the loss of organic kinetic inhibition and the increased accommodation space allow sparite to precipitate, completing the texture. In contrast with "classical" peloids, micropeloids are not transported sedimentary particles: they form in situ, and the surrounding microsparite and sparite are precipitated during early lithification rather than during late diagenesis. The transition from micrite to sparite reflects a progressive decrease in the influence of organic matter on precipitation, expressed as a gradual increase in grain size and euhedral character. We also discuss the relationship between this biologically driven model and possible abiotic pathways, a distinction that is essential when applying MT as a biosignature in early-Earth and astrobiological contexts.
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