In Lagoa Vermelha, Brazil, a lagoonal stromatolite and a saltpan microbial mat are investigated to understand the influence of environmental changes on the decomposition of microbial carbonates. The lagoonal stromatolite, composed mainly of magnesian calcite and aragonite, is developed on a dolomite-containing carbonate crust. While most stromatolites are eroded to the water surface level, some smaller, green stromatolites below the water surface retain a domal shape. The domal stromatolite surface is dominated by endolithic cyanobacteria with conspicuous microborings. In addition, microbial aerobic respiration causes carbonate dissolution in darkness, and metazoans grazing the inner surface of the stromatolite excrete fecal pellets. This suggests that the formational stage of lagoonal stromatolites has ceased and they are now decomposing, most likely because of environmental changes in recent years. The microbial mat, which is about 3 cm thick, developed in a saltpan pond precipitating carbonate and gypsum, and it contains quartz, magnesian calcite, aragonite, and gypsum. At the time of the investigation, the population of oxygenic phototrophs is low at the mat surface, and carbonate dissolution, rather than precipitation, is occurring by microbial metabolism deeper in the mat. This suggests that the formation of carbonate in the mat has ceased and is decomposing, probably due to the progressive salinity increase in the salt pan. This examination of two carbonate deposits in Lagoa Vermelha suggests that microbial metabolism is an important process for decomposing microbial carbonates in addition to grazing and microboring, and that environmental changes may alter microbial compositions from carbonate-constructive to carbonate-destructive communities.