No abstract

Non-skeletal carbonate particles in microbial mats were studied using thin sections and scanning electron microscopy. The microbial mats form biolaminated units (so-called potential stromatolites) in salterns. This study emphasizes the coexistence of different particle forms and makes a genetic connection between the heterogeneity of the organic substrate built by bacteria and diatoms and their extracellular polymeric substances (EPS). Whereas allochthonous particles are scarce, Lanzarote microbial mats provide various autochthonous surfaces for the attachment of cells and EPS, including sheaths and capsules of cyanobacteria, frustules of diatoms, metabolic products such as gas bubbles, liquid globules and faecal pellets, as well as the carbonate precipitates themselves. Morphologically different carbonate precipitates are: (i) calcified organic clumps (peloids), (ii) particles composed of concentric aragonite and biofilm laminae (ooids and oncoids), (iii) isolated particles floating in gel-supported mats and coated by rims of fibrous cement (cortoids), (iv) particles bound by cryptocrystalline matrices or cement, resulting in aggregate grains and (v) lobate cement which fills out spaces and pores and fixes the particles. Peloids are suggested to represent faecal pellets although microbial systems also generate cell clumps by non-faecal processes. Ooid and oncoid constructions clearly record alternating processes of biofilm accumulation and aragonite encrustation. Further characteristic features of carbonate particles generated within a microbial mat are: (i) an irregular distribution ranging from isolated particles floating within the gel-like matrix to closely packed particles, (ii) the amalgamation of different particle types (e.g. peloids and ooids) in aggregate grains, (iii) the heterogeneous nature of nuclei comprising bacterial clumps, intraclasts, individual cells, cell colonies and bubbles, (iv) the enrichment of remains, casts and imprints of cells within precipitates and (v) deformation (e.g. truncated cortices) of particles.