Fig. 3

Models of microbial carbonate formation by filamentous cyanobacteria and formation of mold holes and crusts. a1 to a3, b1 to b3: Trapping lime micrites and particles; c1 to c3, d1 to d3, e1 to e3: Biologically-induced precipitation of carbonate; f1 to f3: Bio-controlled calcification. c1 to c3: Scattered calcite grains on microbe surface, or as loose sediments depositing on seafloor; d1 to d3: Microbe-induced precipitation formed crusts enclosing the microbes, and the interspaces between adjacent crusts are not fully filled by carbonate; e1 to e3: Microbe-induced precipitation formed crusts enclosing the microbes, and the interspaces between adjacent crusts are fully filled with carbonate; f1 to f3: Carbonate precipitation is limited within the sheaths, forming walls with even thickness, recognized as fossils if preserved in strata. If the microbial role in the sediments in a3 can be recognized, the sediments belong to microbial carbonate; the carbonates in b3, c3, d3 and e3 belong to microbial carbonates; and the carbonate in f3 belongs to both microbial carbonate and microbial reef. In case of b, mold holes form, and in cases of d and e, both mold holes and carbonate crusts form