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Biosignal formation: the role of the sulfur cycle in microbial mats

Final Activity Report Summary - BIOSUMM (Biosignal formation: the role of the sulfur cycle in microbial mats)

Photosynthetic microbial mats provide a good modelling system to study the generation of biosignals in a photosynthetic biosphere. Microbial mats are highly structured, metabolically diverse assemblages of microorganisms comprising a complete ecosystem and complete elemental cycles at a microscale. Capturing such diverse metabolic capabilities within a single system gives us the best chance for assessing a broad range of possible biosignals and understanding the mechanisms of their formation. The small scale of mats provides an opportunity to study the effect of environmental conditions on a wide range of processes within a controlled laboratory setting, but nonetheless at a complete ecosystem scale. Mats have dominated the biosphere for most of Earth history. The chemical evolution of our planet was probably more influenced by microbial mats and related assemblages than by any other type of biological community. Understanding the function and ruling parameters of such mats in the present will aid interpreting the fossil record of life on Earth, hence understanding the means by which we came to this point.

Hypersaline microbial mats from a salina at Guerrero Negro, Baja California, Mexico, were kept at original and reduced sulfate concentrations to reveal the influence of sulfate concentration on S-cycle. The lowest concentrations resembled conditions on early Earth, when sulfate concentrations in oceans were low and photosynthetic microbial mats widespread. With decreasing sulfate concentrations, the rates of sulfate reduction, which were an important step in the modern sulfur cycle, decreased while oxygenic photosynthesic activity remained unchanged. Thus, a larger energy fraction flowed through alternative pathways. The concentration of reduced sulfur species did not vary with sulfate concentration; however the isotopic composition, a potential biomarker that could be preserved within the geologic record, did. With decreasing sulfate concentration, isotopic fractionation between reduced and oxidised pools decreased with no measurable fractionation at around 1 mM. Thus, in these mats, no biosignal in terms of S-fractionation would be preserved under sulfate concentrations as found on early Earth.

Investigations of the oxygenic photosynthesis demonstrated that phototrophic communities were bicarbonate limited under normal light conditions, as well as that photosynthetic guilds competed for this resource. Reducing the light spectrum to visible light only, i.e. to 350 to 700 nm filtering out near infrared greater than 700 nm, increased the availability of bicarbonate for oxygenic phototrophs such as cyanobacteria by shutting down anoxygenic photosynthesis. Therefore, filtering out near infrared light could potentially be used to increase the productivity in photosynthetic bioreactors.