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Research

Our current themes are:

Ancient ocean analogue ecosystems

The overarching aim of our research is to unravel the explicit links between microenvironmental processes and global scale transitions in Earth’s redox landscape during the Precambrian. We therewith work towards a biologically nuanced perspective on Earth’s oxygenation history. What can we learn from modern ecosystems that represent analogues to the ancient Earth? How would microbial metabolic transitions and interactions have impacted oxygen accumulation in the context of changes in the redox state of Earth, evolutionary onset of metabolisms and changes in the Earth-Moon system through deep time? We approach these questions by constraining the potential role of different types of photosynthesis during major Earth oxygenation events by studying natural and artificial analogs of Precambrian ecosystems that have been understudied so far – namely microbial mats and algal blooms in anoxic lakes.

Artistic illustration of the Precambrian oceans including stromatolites, i.e. lithifying microbial mats, covering the coastal oceans and study site Lake Pozo Bravo in the high altitude Andes of Argentina resembling its ancient counterpart.

Artistic illustration of the Precambrian oceans including stromatolites, i.e. lithifying microbial mats, covering the coastal oceans and study site Lake Pozo Bravo in the high altitude Andes of Argentina resembling its ancient counterpart.

Critical in understanding the pattern of Earth’s oxygenation is the regulation of O2 production by cyanobacteria. Reduced sulfur, iron and arsenic are key controls on cyanobacterial photosynthesis, and were pervasive in productive coastal oceans and within microbial mats of the Precambrian. Arsenite or sulfide might have driven the first forms of photosynthesis. We therefore study (1) the kinetics of sulfide and arsenite oxidation dependent on e.g. light, (2) the molecular mechanisms underpinning the transition between oxygenic and anoxygenic photosynthesis and (3) the environmental and microbial controls on the balance between phototrophic modes in cyanobacterial mats and the impact on O2 export.

microsensor measurements in a sulfidic spring close by the cave system of Frasassi, Italy.

Bundles of filamentous sulfur oxidizing bacteria and green cyanobacteria in a sulfidic stream and

Close-up view of green biofilms formed by filamentous cyanobacteria in culture.

Environmental arsenic cycling

Arsenic contamination is of concern around the world due to its ubiquity and short- and long-term effects on human health. We are therefore in urgent need to limit human exposure to arsenic. Understanding its mobility in the environment is, however, hindered by the complex abiotic and biotic interactions of arsenic with e.g. iron, sulfur and organic matter. We therefore aim to develop novel methods that aid to resolve micro-scale processes in soil and sediment – processes that ultimately dictate arsenic redox cycling and mobility. We are particularly interested in using these tools to characterize light-driven arsenic cycling in benthic systems.

Contaminated brook in the Harz Mountains, Germany

Map of arsenic species and iron concentration in the porewater of the soil underneath the brook.

Another important focus is to develop an understanding of arsenic transfer in the larger food web. In high concentrations, arsenic is considered toxic and given its similarity to phosphate, it can easily be incorporated by organisms and may block and interfere with several vital processes. Particularly primary producers, such as cyanobacteria and chlorophytes, incorporate arsenic into carbohydrates and lipids. We aim to (1) characterize the type and origin of organic arsenicals in microbial mats in Andean lagoons and lakes and (2) track their transfer across the food web. Our model organism is Artemia salina, a salt shrimp, feeding on arsenic rich algae, in which we track the fate of organic arsenicals using a combination of classical and image-based mass spectrometry approaches, keeping an eye on its microbiome.

Schematic of arsenic transfer from primary producers to Artemia in the high altitude Andean lakes and lagoons.

Artemia in the field feeding on orange-colored algae.

Miraculous diatoms

Diatoms are eukaryotic oxygenic phototrophs and among the most important primary producers globally. Due to their convoluted evolutionary history, diatoms are characterized by a highly unorthodox combination of genes derived from two secondary endosymbionts and from excessive horizontal gene transfer. Even the organization of organelles is exceptional, with, for instance, mitochondria and plastids in very close proximity within the cell. Thus, diatoms may drive processes that are unknown in other Eukaryotes, especially in the absence of light and photosynthesis. We therefore aim to illuminate the metabolic “dark side” of these phototrophs.

In-situ biogeochemical measurements in tidally influenced sediments off the coast of the island Sylt, Germany.

Confocal of the diatom Craticula cuspidata (120 µm length), a species that migrates vertically down to 4 cm sediment depth on a dial basis and performs nitrate respiration instead of photosynthesis for the majority of the day.

Benthic cycling of reactive oxygen species

Reactive oxygen species (ROS), such as hydrogen peroxide or superoxide, are ubiquitously produced in every aerobic environment and organism. While high concentrations can be cell damaging, ROS were also discovered to be key messenger molecules in cellular pathways and are thus now regarded as “Jekyll and Hyde” molecules. We are particularly interested in the biotic mechanisms of production and scavenging in illuminated sediments and mats. Our favourite study organisms are large benthic foraminifera that thrive in coral reef sediments and substantially shape the sedimentary ROS budget.

H2O2 and O2 microsensors on the surface of the large benthic foraminifera Amphistegina lobifera (diameter ~2 mm). Brownish color arises from the presence of photosynthetic symbionts (diatoms).