Biogeochemistry of microbial mats under Precambrian environmental conditions: a modelling study

Abstract

Microbial mats have arguably been the most important ecosystem on Earth over its 3.5 Gyr inhabitation. Mats have persisted as consortia for billions of years and occupy some of Earth's most hostile environments. With rare exceptions (e.g. microbial mats developed on geothermal springs at Yellowstone National Park, USA), today's mats do not exist under conditions analogous to Precambrian habitats with substantially lower oxygen and sulphate concentrations. This study uses a numerical model of a microbial mat to investigate how mat composition in the past might have differed from modern mats. We present a numerical model of mat biogeochemistry that simulates the growth of cyanobacteria (CYA), colourless sulphur bacteria (CSB), and purple sulphur bacteria (PSB), with sulphate‐reducing bacteria (SRB) and heterotrophic bacteria represented by parameterized sulphate reduction rates and heterotrophic consumption rates, respectively. Variations in the availability of light, oxygen, sulphide, and sulphate at the upper boundary of the mat are the driving forces in the model. Mats with remarkably similar biomass and chemical profiles develop in models under oxygen boundary conditions ranging from 2.5 × 10−13 to 0.25 mm and sulphate boundary concentrations ranging from 0.29 to 29 mm, designed to simulate various environments from Archean to modern. The modelled mats show little sensitivity to oxygen boundary conditions because, independent of the overlying oxygen concentrations, cyanobacterial photosynthesis creates similar O2 concentrations of 0.45–0.65 mm in the upper reaches of the mat during the photoperiod. Varying sulphate boundary conditions have more effect on the biological composition of the mat. Sulphide generated from sulphate reduction controls the magnitude and distribution of the PSB population, and plays a part in the distribution of CSB. CSB are the most sensitive species to environmental change, varying with oxygen and sulphide.

Microaerobic steroid biosynthesis and the molecular fossil record of Archean life

“…Even if the total flux were far below the lower limit considered by Zahnle et al (22), dissolved O 2 concentrations in higher-productivity hot spots ("oxygen oases"), would likely have been high enough to allow steroids to be made. Such hot spots probably included phototrophic microbial mats, which could have had daytime O 2 concentrations as high as 450-650 μM within the mat layers (46). The presence of microaerobic regions of the euphotic ocean by the late Archean, and their persistence over long periods of time, is consistent with geologic and geochemical evidence for an anoxic atmosphere at that time.…”

The rise of atmospheric oxygen

“…So additional proxies must be sought. Fossilized microbial mats might hold the clue to the early origin of oxygen photosynthesis if it can be demonstrated that the expected strong redox gradients 18 existed and produced isotopic or compositional variations that can be recovered from Archaean rocks.…”

Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis

“…In other words, as today, there is no reason to assume that the gas concentrations at weathering interfaces are directly reflective of atmospheric conditions. Indeed, 1D modeling with oxygen and sulfate boundary conditions encompassing those hypothesized from the Archean to present indicates that oxygen production and redox zonation within ancient microbial mats should have been independent of the chemistry of their environment (45). It is important to note that the attendant effects of BSCs and FMMs in the Archean would have been multiple, and would also have included enhanced mineral dissolution of underlying sediments (46) and increased flux of dissolved organic carbon to overlying waters, which in turn, facilitates transport of normally insoluble metals (e.g., Fe, Al) downstream (47).…”