Microbial Community Function at the Extremes: Isotopic Constraints on Carbon Sources and Cycling of Endolithic Communities in Extreme Environments

English Abstract:

The carbon sources and metabolic carbon cycling of microbial communities at the extremes of life are often very difficult to constrain due to low biomass, low metabolic activity and the potential for significant stimulation of the community in response to the addition of tracer compounds. Yet understanding how organisms survive in these environments has implications for our understanding the fundamental capabilities of life, the history of life on Earth, and to the search for evidence of life beyond the Earth. This talk will discuss my research group’s application of natural abundance compound specific isotopic analysis (¹³C and ¹⁴C) of phospholipid fatty acids (PLFA: membrane lipids) and glycolipid fatty acids (GLFA: storage lipids/degradation products) to constrain the carbon sources and cycling of in situ endolithic microbial communities living under conditions of extreme cold and/or aridity.

In the Atacama desert, one of the driest places on Earth, halite endoliths were comprised of carbon derived from the modern atmosphere, while endoliths living within gypsum contained carbon derived impacted by atmospheric weapons testing (“bomb carbon”) indicating that they were cycling carbon on decadal timescales. Only at the driest site, Yungay, were the GLFA significantly depleted in ¹⁴C with respect to the PLFA suggesting evidence of preservation of these compounds. In the Canadian Arctic, a cold, semi-arid environment, endoliths living within gypsum also contained “bomb carbon” and were thus cycling carbon on decadal timescales.

In contrast, in the Antarctic dry valleys, PLFA from three endolithic communities were significantly depleted in ¹⁴C, equivalent to radiocarbon ages of 1060 to 2170 years, while one endolithic community from the adjacent valley had ¹⁴C levels that corresponded to a radiocarbon age of 65 years. Concurrently, GLFA from the Antarctic sites had radiocarbon ages ranging from 500 to 2940 years older than the corresponding PLFA, indicating preservation of GLFA over much longer timescales than observed elsewhere. These observations raise questions regarding the rates of carbon cycling by these communities and the extent to which carbon they are utilizing is recycled within these microbial communities versus new inputs from the atmosphere.