Significance of the biomineralizing microniche in a lyngbya (cyanobacterium) travertine

Observations of the spatial arrangement of Lyngbya (Phormidium) incrustatum tri‐chomes in a travertine crust allowed prediction of the frequency of four types of microniche favorable for photosynthetically induced calcification. The most frequent type of microniche occurred at “cross‐over”; points where two trichomes made contact at a single point. Such points occurred very frequently (～6000/mm² travertine). Microniches more favorable to photocalcification consist of groups of three or more trichomes lying side by side, enclosing narrow tubes of interstitial water. These configurations were far less frequent and their effects would be negligible in Lyngbya travertines. In the calcified cyanobacteria Rivularia and Schizothrix such configurations may be much more common. The existence of such microniches demonstrated that regions favorable for photocalcification occur in travertine‐encrusted cyanobac‐terial mats and may initiate carbonate precipitation. However, the bulk of the carbonate precipitated in such travertines is known to be formed through inorganic processes.