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Our current themes are:

Ancient ocean analogue ecosystems

Our research seeks to unravel the explicit connections between microenvironmental processes and global-scale transitions in Earth's redox landscape during the Precambrian era. The evolution of oxygenic photosynthesis in cyanobacteria was a milestone on the way to today's oxygen-rich atmosphere. However, much remains unknown about the biogeochemical and ecological interactions surrounding ancient cyanobacteria. These interactions not only influenced atmospheric oxygen levels but also played a crucial role in the fluctuations of greenhouse gases like CO2 and CH4, thereby driving substantial climate changes. Critical in understanding the pattern of Earth’s oxygenation is thus a comprehensive understanding of the regulation of key microbial processes in the ancient carbon cycle. Here, we focus on the regulation of primary production by reduced sulfur, iron and arsenic. These compounds likely facilitated the emergence of the first forms of anoxygenic photosynthesis, exerted crucial control over cyanobacterial oxygenic photosynthesis, and were widespread in productive coastal oceans and microbial mats during the Precambrian.

We therewith work towards a biologically nuanced perspective on Earth’s oxygenation history. How did microbial activity impact the accumulation of oxygen within the context of changes in Earth's geochemistry, the evolutionary onset of different metabolisms, and the evolution of the Earth-Moon system through deep time? To address these questions, we employ two main approaches. First, we study natural analogs of Precambrian ecosystems, such as microbial mats in extreme environments, to constrain the potential role of different types of photosynthesis during major Earth oxygenation events. Second, we simulate conditions in artificial ecosystems within a laboratory setting, enabling us to investigate and understand the dynamics of these processes in depth.

Drawing of a hypothetical view into the Archaean coastal ocean with active volcanoes in the background, and geyser-like environments in the front. The coastline is characterized by abundant stromatolites.
Photo: Peter Sawyer, Smithonian
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.
Flat mat and microbialites in a lake. Inactive volcanoes are in the background. Scientists set up experiments and a camp at the coast.
Photo: Judith Klatt

Scientist working in a river. In the forefront, there are microsensor setups for measurements in green biofilms connected to analytical tools via many cables. The scientist in the background is collecting water samples.
Photo: Judith Klatt
microsensor measurements in a sulfidic spring close by the cave system of Frasassi, Italy.
Green and white feather-like structures attached to wood sticks in a stream of water. Leaves cover parts of the microbially formed feathers.
Photo: Institution/Lubos Polerecky.
Bundles of filamentous sulfur oxidizing bacteria and green cyanobacteria in a sulfidic stream and
Green cyanobacterial biofilms form variable structures including pinnacles lifted up by oxygen bubbles.
Photo: Judith Klatt
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.

Brook with orange-colored water in the forefront and tree without leaves in the background.
Photo: Judith Klatt
Contaminated brook in the Harz Mountains, Germany
On the left, a map of the distribution of arsenic spezies and phosphate in the porewater of contaminated soil is shown. The solutes occur in horizontal bands, except for a central patch that is enriched in all analytes. On the right, a close-up view of the central patch is shown highlighting the microscale variability.
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.

Sketch of the cycling of arsenic by primary producers and salt shrimps in high altitude hypersaline lakes. A spring carrying water enriched in arsenite emerges into a salt lake. Algae redox cycle arsenic and convert it to organic arsenic species. Artemia shrimps feed on the algae.
Photo: Judith Klatt
Schematic of arsenic transfer from primary producers to Artemia in the high altitude Andean lakes and lagoons.
Artemia shrimps swimming in hypersaline water above a salt crust harboring orange algae.
Photo: Judith Klatt
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.

Coastal lagoon surrounded by sandy sediment. A measuring device is operating in the middle of the lagoon.
Photo: Judith Klatt
In-situ biogeochemical measurements in tidally influenced sediments off the coast of the island Sylt, Germany.
Microscopic image of a diatom cell. Chloroplasts surrounding the cell are visible due to their autofluorescence. The nucleus is located in the center of the cell.
Photo: Benedikt Geier
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.

Microscopic image of a large benthic foraminifera. The tips of two microsensors are positioned on the foraminifera surface.
Photo: Katharina Neumüller
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).

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