The Bosak Lab integrates microbiology, sedimentology, and stable isotope geochemistry into experimental geobiology, seeking to answer questions about early forms of life and their habitats.

shallow water environments store a 3.5 billion year long record of interactions between early life and its habitats. However, it remains an open question what chemicals supported ecosystems in these habitats, how microbes moved and colonized sediments, whether microbes helped form rocks by precipitating rock-forming minerals, and when the early light-harvesting microbes became able to produce oxygen.

Associate Professor Tanja Bosak integrates microbiology, sedimentology, and stable isotope geochemistry into experimental geobiology to help answer questions such as: How do microbes shape sedimentary rocks? How do organisms become fossilized? And how do microbial metabolisms leave biological patterns in sediment? Using these approaches, her group explores modern biogeochemical and sedimentological processes as proxies for what was taking place deep in Earth’s history, to help interpret the record of life on the early Earth, and perhaps suggest how life on other young planets might develop.

Studying water and sediment samples taken from multiple depths of Green Lake—a meromictic lake teeming with anaerobic, sulfur-reducing bacteria—gives us a window into the mechanisms at work in the ancient ecosystems which existed before Earth’s oxygen-rich atmosphere evolved.

Answers to such questions, inferred from the shapes of rocks and traces of chemical signals, require better constraints on the signals that can be produced and preserved in the presence of microbial communities that do not evolve oxygen. To investigate, members of Bosak’s team cultivate communities of light-harvesting microbes that colonize sediments, promote the precipitation of rock-forming minerals, and shape sediments under a range of chemical conditions relevant for the early Earth and other young planets in the lab.

Using analytical techniques at a range of scales, from visible to those smaller than a microbial cell, lab members then investigate the shapes and chemical properties of the microbially produced structures and minerals, applying insights from the laboratory to reconstruct microbial processes in geologic samples.

Before the evolution of oxygenic photosynthesis, anoxygenic photosynthetic organisms using hydrogen, iron, and manganese, reduced sulphur, and organic compounds likely colonized the photic zone, the layer penetrated by sunlight. The sediment-water interfaces of this anoxic world also may have contained microbes that produced and consumed methane. In contrast, modern methane-cycling microbes occur primarily below the photic zone. To better understand life in shallow water environments before the rise of oxygenic photosynthesis, Bosak and her team have been exploring the ecology, morphology, biochemistry and fossilization potential of benthic photosynthetic microbial communities that lack cyanobacteria and grow in the absence of oxygen.

Graduate student Shane O’Reilly pulls up a sample from Green Lake.   
Photo courtesy the researchers

A modern context for this work is provided by Green Lake in Fayetteville in upstate New York. Green Lake is unusual because it is meromictic—that is, there is no mixing between deep and shallow water. In ordinary, holomictic lakes, at least once each year, there is a physical mixing of the surface and the deep waters. Green Lake is also euxinic at depth, that is there is lots of sulphide and low oxygen within and below the photic zone. Evidence in the general rock record suggests that for many periods in early Earth history, much of the global ocean was euxinic, making Green Lake a helpful modern analog for such environments.

Over the course of multiple visits to the site, Bosak and collaborators have collected water and sediment samples for analysis. Among their findings was the discovery of isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) with unique cyclohexane functional groups (moieties). These archaeal tetraethers are named S-GDGTs, where ‘S’ stands for ‘Sulfidic’ and ‘Six-membered-ring’. In contrast to other modifications of archaeal lipids by cyclization, the cyclohexane ring of S-GDGTs is configured in the middle of a C40 biphytane chain, requiring double head-to-head linkage of two isoprenyl groups. Anaerobic deep lake samples also contain S-GDGT derivatives composed of biphytanes modified with double bonds and regular cyclopentane. The intact polar lipid precursors of S-GDGT include compounds with mono- and diglycosyl head groups. The carbon isotopic composition of S-GDGTs and their occurrence in Green Lake (they are also seen in Messel Shale in Germany, Salt Pond in Falmouth, MA and neighboring salt marshes) suggest that they may be produced by chemoautotrophic Archaea that prefer sulfidic conditions.

Read more about the research: http://bit.ly/bosak-lab