A MODERN ANALOGUE FOR PRECAMBRIAN MICROBIAL JUNGLES

Accruing geological evidence suggests that the continents were colonized by microorganisms long before the evolution of land-based plants in the Phanerozoic. The geological record indeed implicates microbial mats in the formation of Archean paleosols, suggesting a very early origin and proliferation of the continental biosphere. Modern analogues for such terrestrial mats appear missing in the literature to date. We have discovered laminated terrestrial microbial mats growing on wet outcrops of ultramafic rocks in Indonesia. These mats comprise a diverse and stratified assemblage of microorganisms dominated by cyanobacteria in their upper layers. Sections of these mats were excised for controlled growth experiments in illuminated flumes. Oxygen electrode microprofiling over diurnal light cycles revealed dynamic oxygen and carbon cycling with substantial rates of primary production. If similar mats colonized land surfaces in the Precambrian, they would have contributed to the global oxygen and carbon cycles, weathering of the continents, regulation of nutrient fluxes to the oceans, and the formation of early soils.