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Project Progress

Marine rock extracts and oils from the Proterozoic to the Paleogene were analyzed for intact saturated and aromatic carotenoid carbon skeletons using new analytical methods. Gas chromatography coupled with tandem mass spectrometry revealed new temporal distribution patterns for chlorobactane, paleorenieratane, and okenane. One interpretation of the revised geologic distributions of these fossilized biomarkers for GSB and PSB, especially okenane and chlorobactane, is that marine photic zone euxinia (PZE) was more intense and frequent in the geologic past, particularly the Phanerozoic, than was previously thought. However, given that okenane and chlorobactane are commonly interpreted to imply hydrogen sulfide at water depths shallower than 24 m, their common occurrence in Phanerozoic marine samples presents several challenges given that atmospheric oxygen levels were near modern values during the time of deposition. Therefore, the prevalence of okenane and chlorobactane in ancient marine systems likely reflects the degree of basin restriction and not global oceanographic redox conditions (Figure 1).

Fossil molecular (biomarker) and stable nitrogen isotopic records in a sedimentary sequence from western Canada provide the first conclusive evidence of PZE and disrupted biogeochemistry on the northeastern Panthalassic Ocean for the end-Triassic. These results indicate the occurrence of increasing water column stratification and deoxygenation across the ETE, leading to PZE in the Early Jurassic. The development of PZE was paralleled by a perturbed nitrogen cycle, ecological turnovers among non-calcifying groups including eukaryotic algae and prokaryotic plankton.

We studied the lipid and C-isotopic composition of s treamer biofilm communities (SBC) within chemosynthetic zones of Yellowstone hot spring outflow channels, where temperatures exceed those conducive to photosynthesis. Nearest the hydrothermal source (75–88◦C) SBC comprise thermophilic Archaea and Bacteria, often mixed communities including Desulfurococcales and uncultured Crenarchaeota, as well as Aquificae, Thermus, each carrying diagnostic membrane lipid biomarkers. We tested the hypothesis that SBC can alternate their metabolism between autotrophy and heterotrophy depending on substrate availability. Feeding experiments were performed at two alkaline hot springs in Yellowstone National Park: Octopus Spring and “Bison Pool,” using various ¹³C-labeled substrates (bicarbonate, formate, acetate, and glucose) to determine the relative uptake of these different carbon sources. Highest ¹³C uptake, at both sites, was from acetate into almost all bacterial fatty acids, particularly into methyl-branched C15, C17 and C19 fatty acids that are diagnostic for Thermus/Meiothermus, and some Firmicutes as well as into universally common C16:0 and C18:0 fatty acids. ¹³C-glucose showed a similar, but a 10–30 times lower uptake across most fatty acids. ¹³C-bicarbonate uptake, signifying the presence of autotrophic communities was only significant at “Bison Pool” and was observed predominantly in non-specific saturated C16, C18, C20, and C22 fatty acids. Incorporation of ¹³C-formate occurred only at very low rates at “Bison Pool” and was almost undetectable at Octopus Spring, suggesting that formate is not an important carbon source for SBC. ¹³C-uptake into archaeal lipids occurred predominantly with ¹³C-acetate, suggesting also that archaeal communities at both springs have primarily heterotrophic carbon assimilation pathways. We hypothesize that these communities are energy-limited and predominantly nurtured by input of exogenous organic material, with only a small fraction being sustained by autotrophic growth.

Other collaborations with numerous colleagues resulted in documentation of lipid and isotopic biosignatures in cultured methanotrophic and planctomycete bacteria.

Lastly, we studied the lipid and isotopic compositions of the microbial communities colonizing ooids and developed a tool to measure, at high resolution, chronologies of their growth using radiocarbon analysis.