A Comparison of Ancient and Modern Antarctic Microbial Mats using Molecular Biosignatures

Abstract:

Earth’s simplest life leaves its trace in the form of molecular biosignatures preserved in the rock record. However, the conditions under which biosignatures preserve alter the composition of these molecules, so it is important to understand the diagenetic processes that occur under contrasting preservational regimes. In this study, we investigated the preservation of intact polar lipids (IPLs), bacteriohopanepolyols (BHPs), and quinones under cold and arid climatic conditions. To this end we investigated the polar lipids from an ancient microbial mat (~10,000 years old) from the meltwater ponds of McMurdo Ice Shelf in Antarctica and compared them to the lipid biosignatures in living mats from the same location. This work will help understand the preservation potential of polar compounds under cold and dry conditions and its relevance to paleobiology and astrobiology.

IPLs, BHPs, and quinones in ancient and modern mat samples were extracted using the modified Bligh-Dyer method and analyzed by chromatography-mass spectrometry (LC-MS). The results revealed significant degradation of cyanobacterial biomarkers in the IPLs and quinones found in the ancient mat samples, due to diagenesis. The relative abundances of glycolipids in the ancient mats, indicative of cyanobacteria, saw a significant decrease in the ancient mat compared to the living ones. The glycolipids also were significantly more degraded than other classes than IPLs, as quantified by a greater decrease in the double bond index. We also found that the relative abundance of the quinones indicative of oxygenic photosynthesis, PQ (9:9) and Vitamin K1, were significantly lower in the ancient mat than other classes of quinones. These findings suggest that IPLs and quinones of inactive microbes, like cyanobacteria, are quickly degraded in a cold, arid setting.