MATHEMATICAL MODELS OF ANCIENT AND MODERN MICROBIAL MATS

Microbial mats have arguably been the most important ecosystem on Earth over its 3.5 Gyr inhabitation. Although mats have persisted as a type of consortium for billions of years and occupy some of the most hostile environments on Earth, none of today’s mats exist under conditions analogous to Precambrian habitats with substantially lower O₂ and sulfate concentrations. This study uses a numerical model of a microbial mat to investigate how the composition of microbial mats in the past might have differed from modern microbial mats.

We present a numerical model of microbial mat biogeochemistry that simulates the growth of cyanobacteria (CYA), colorless sulfur bacteria (CSB), and purple sulfur bacteria (PSB), with sulfate-reducing bacteria (SRB) represented by parameterized sulfate reduction rates. Light, oxygen, sulfide, and sulfate are the driving forces in the model. Mats with different biomass distributions developed in models under oxygen boundary conditions ranging from 2.5x10^-13 to 0.25 mM and sulfate boundary concentrations ranging from 0.29 to 29 mM, designed to simulate various environments from Archean to modern. Under Archean and Proterozoic type boundary conditions, some versions of the modeled mats did not include CSB.

The modeled mats showed little sensitivity to varying oxygen boundary conditions because, independent of the overlying oxygen concentrations, cyanobacterial photosynthesis created similar O₂ concentrations of 0.6 to 0.75 mM in the upper reaches of the mat during the photoperiod. The primary variations with O₂ concentration occurred in the distribution of CSB and the biomass of CYA, with CYA biomass somewhat reduced under higher oxygen conditions. Varying sulfate boundary conditions had more effect on the biological composition of the mat. Sulfide generated from sulfate reduction controlled the magnitude and distribution of the PSB population, and played a part in the distribution of CSB. When included, CSB proved the most sensitive species to environmental change, varying with oxygen and sulfide. In mats modeled without CSB, overall biomass and CYA biomass were lower than in runs with CSB.