Microbial mats and the evolution of photosynthesis on the early Earth – ARCHAEMAT

This project is aimed at tracing the early evolution of the photosynthetic metabolisms of microorganisms, as preserved in the geological rock record, collected in ~3.5 to 2.1 Ga- old rock formations from South Africa and Canada. Anoxygenic photosynthesis is believed to have been the precursor oxygenic photosynthesis and it is hypothesised that the further evolution of more complex life forms was dependent upon the oxygenation of the environment (mostly through the activity of oxygenic photosynthesisers). Thus, it is of utmost importance to determine the appearance and early evolution of photosynthesis in order to (1) better understand the oxygenation of the environment of the primitive Earth, (2) to constrain models relating to this important phenomenon, and (3) to trace the further evolution of life,. Objectives: Our specific objectives are to (1) identify the biogeochemical signatures (including isotopes) of microbial mats cultured in microaerobic and anoxic conditions (Archaean-Palaeoproterozoic) conditions; (2) make geochemical and isotopic characterisation of microbial mats from 3.4-1.9 Ga formations whose biogenicity has been firmly established; (3) evaluate the structural, geochemical and isotopic evidence for types of photosynthesis in modern mats; and (4) establish if possible whether there is firm or ambiguous evidence for oxygenic photosynthesis in the Archaean biolaminated sediments based on the criteria obtained from modern microbial mats (Gautret et al., 2008) and this project. Context: The appearance of photosynthetic metabolisms is one of the key factors in the evolution of life. The ability to use the unlimited energy from the sun enabled early life forms to become more efficient and to develop more rapidly. The first photosynthetic metabolisms are presumed to have been initially anaerobic since they are simpler than those of oxygenic photosynthesis. However, the evolution of oxygenic photosynthesis was critical to the development of more sophisticated life forms and, thus, for evolution in general. The remains of microbial mats occur in the oldest, well-preserved sediments on Earth in the Early Archaean terrains (3.5-3.2 Ga) of the Pilbara in Australia and Barberton in South Africa. Initial studies of Early Archaean stromatolites-like structures suggested that they were formed by oxygenic photosynthesisers since carbonaceous remains in some of the associated Early Archaean were interpreted as fossil cyanobacteria. However, these interpretations have been subjected to recent controversy. It has since been shown that the so-called microfossils are hydrothermal kerogen filamentous deposits and that chemical precipitation can also create finely laminated, stromatolite-like structures. These problems underline the necessity of multi-disciplinary, global studies using both basic and state-of 'the-art equipment. Such investigations have recently been used to prove the biogenicity of some of the stromatolites and other biolaminated sediments. These latter studies suggest that the Early Archaean fossil mats were formed by microorganisms that may have already used photosynthesis. The evidence that the photosynthesis was anaerobic is circumstantial. The rapid rise of oxygen in the atmosphere took place at around 2.4 Ga, but the organisms that caused it must have evolved much earlier. The critical question is when did this event occur and is it traceable' The evidence for oxygen in the biosphere in the Late Archaean period starting around 2.8-2.9 Ga is also circumstantial. It is based on the existence of stromatolites forming large carbonate banks in Canada, South Africa and Australia, carbon isotope signatures (N.B. overlap anoxygenic isotope signatures), a decrease in the mass-independent isotopic fractionation of sulphur in 2.6 Ga-old sediments, and the presence of molybdenum in these formations. All these lines of evidence could be explained by other processes but their combination is strongly supportive of oxygenic photosynthetic activity that resulted in at least a small amount of oxygen. Establishment of reliable biogeochemical (including isotopic) criteria for oxygenic or anoxygenic photosynthesis will require the study of mats cultured in Palaeoproterozoic to Archaean conditions since no modern mats are entirely one or the other. With these criteria we will be able to trace the evolution of photosynthesis in the rock record, recognising that the signature in the 3.4-2.8 Ga time period may not be clear because of transition and/or poor preservation.