Tracing Aridity: Sedimentary Preservation of Scytonemin from Microbiotic Desert Soil Crusts

Baylor University, Waco TX

Investigators

Abstract

(1) Technical description Deserts are a critical end-member in biome reconstructions used to help constrain our understanding of paleoclimatology. Yet given the lower fossilization potential for desert plants and animals, direct paleontological evidence of climate change is much less abundant from arid environments than from other terrestrial ecosystems. However, following the model of wind-blown pollen, potentially continuous sedimentary records can be derived from small eolian particles carrying molecular biomarkers. We posit that one such molecule is scytonemin, a pigment that is produced by cyanobacteria in desert microbiotic soil crusts to protect against UV exposure. We have recently published a mid-Holocene record of sedimentary scytonemin from the Black Sea, and stable isotope analysis supports a desert microbiotic soil source for these molecules. Furthermore, the Black Sea record coincided with 7-methylheptadecane, which is also enriched in desert microbiotic soil cyanobacteria. In a pilot study, we showed that scytonemin is preserved in Great Salt Lake sediments as old as 11,000 years, and its highest concentrations appear to be associated with other sedimentary signs of arid conditions, suggesting that scytonemin may be a representative tracer of cyanobacteria in desert microbiotic soil crusts. Thus, we hypothesize that scytonemin is a biomarker of aridity, with increased accumulation when deserts expanded and cyanobacteria-dominated microbiotic soil crusts became more prevalent. We propose developing scytonemin and 7-methylheptadecane as molecular biomarkers to reconstruct records of past aridity in the late Quaternary and deeper into the Phanerozoic by addressing four overarching questions: (1) Does scytonemin degradation yield molecular fragments that are chemically recognizable and analytically tractable in ancient geological samples? (2) What is the physiological relationship between scytonemin and 7-methylheptadecane production in desert microbiotic soil cyanobacteria? (3) Do scytonemin and 7-methylheptadecane distributions in the Great Salt Lake and the Gulf of California reflect changes in aridity since the Last Glacial Maximum? (4) Were the Late Permian and Early Triassic characterized by increased aridity? The proposed research incorporates biochemical, microbial physiological, organic geochemical, paleoclimatological, and geological components. We anticipate showing that two types of compounds, one a pigment associated with exposure to ultraviolet radiation and the other a lipid associated with desiccation, produced by photosynthetic cyanobacteria in desert soils can be used to trace the spread of arid conditions in the recent (Holocene; the last 12,000 years) and distant (Permian-Triassic; ca. 252 Ma). This work requires a combination of liquid and gas phase chromatography to separate the target compounds from other organic molecules that are also found in modern and ancient sediments. A variety of mass spectrometric techniques, including high resolution mass spectrometry and stable isotope ratio mass spectrometry, will help identify the target compounds and determine their ecological source, for example if they were derived from desert soils or other potential sources such as microbial mats. Ultimately, development of these biomarkers should help us decipher past climate change using a new organic geochemical approach that may modify or enhance our current understanding of aridity. (2) Broader significance and importance Understanding the past climate record (paleoclimate) is arguably of great utility in enabling us to predict future climate scenarios. Often, we can look to the fossil record to find information about past climates. However, this leaves a gap in our knowledge about arid environments, where it is less likely for a strong fossil record to exist. The purpose of this project is to develop a method to inform us about past conditions in dry places. Cyanobacteria, which are microorganisms found in desert soil crusts, produce molecules in response to climactic conditions--one is a pigment, scytonemin, that acts as a "sunscreen" to protect the cyanobacteria from UV radiation, and the other is 7-methylheptadecane, a lipid associated with dessication. In this study, the research team will examine the production of these molecules by cyanobacteria in response to environmental conditions, examine their breakdown products, and determine the utility of these molecules as markers of aridity in past environments. This work has implications for enabling us to build a more complete understanding of the past behavior of dry environments, which could help us to better model future climate in the arid environments of today.