STRUCTURAL DIVERSITY OF BIOGENIC CARBONATE PARTICLES IN MICROBIAL MATS

Non-skeletal carbonate particles in microbial mats were studied using thin sections and scanning electron microscopy. The microbial mats for m 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 su bstrate built by bacteria and diatoms and their extracellular polymeri c substances (EPS). Whereas allochthonous particles are scarce, Lanzar ote microbial mats provide various autochthonous surfaces for the atta chment of cells and EPS, including sheaths and capsules of cyanobacter ia, frustules of diatoms, metabolic products such as gas bubbles, liqu id 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 concent ric 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 sugges ted to represent faecal pellets although microbial systems also genera te cell clumps by non-faecal processes. Ooid and oncoid constructions clearly record alternating processes of biofilm accumulation and arago nite encrustation. Further characteristic features of carbonate partic les generated within a microbial mat are: (i) an irregular distributio n ranging from isolated particles floating within the gel-like matrix to closely packed particles, (ii) the amalgamation of different partic le types (e.g. peloids and ooids) in aggregate grains, (iii) the heter ogeneous nature of nuclei comprising bacterial clumps, intraclasts, in dividual cells, cell colonies and bubbles, (iv) the enrichment of rema ins, casts and imprints of cells within precipitates and (v) deformati on (e.g. truncated cortices) of particles.