Photosythesis and sulfur oxidation in microbial mats: Unravelling the role of versatile cyanobacteria in ancient ocean analogues

The capability to perform oxygenic photosynthesis likely evolved in a cyanobacterial ancestor, probably in microbial mats. The cyanobacterial photosynthetic repertoire is not limited to oxygenic photosynthesis. In fact some specialized cyanobacteria can switch to using H2S as an electron donor instead of H2O in a process termed anoxygenic photosynthesis. Such photosynthetic versatility of cyanobacteria might have been an important adaptation strategy to sulfidic conditions in ancient microbial mats. Furthermore, cyanobacterial anoxygenic photosynthesis might have contributed to sustaining ocean euxinia in Proterozoic oceans. Therefore, studying the activity of modern day sulfide-adapted cyanobacteria and the competitiveness of oxygenic with anoxygenic photosynthesis has broad implications. The aim of this thesis was to gain insights into the activity of sulfide-adapted cyanobacteria in microbial mats that represent ancient Earth analogues. Overall, this thesis highlights the wide spectrum of adaptations to sulfidic conditions among cyanobacteria. A crucial factor determining success in the environment is the specific effect of the local dynamics of light and H2S on activity. The most successful cyanobacteria ancient Earth analogues are photosynthetically versatile. As exposure to sulphidic conditions is like a red line through the history of cyanobacteria, it seems intuitive that cyanobacterial anoxygenic photosynthesis might be an ancient trait. However, there is currently no robust evidence supporting this hypothesis. This thesis highlights that photosynthetically versatile cyanobacteria might, however, have had an important impact on the ancient biogeochemical cycling. Deeper knowledge concerning the timeline of the emergence of anoxygenic photosynthesis among cyanobacteria has the potential to explain major shifts on the global oxygen budgets and redox state of Earth through history.