2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis

Oxygenic photosynthesis is widely accepted as the most important bioenerget ic process happening in Earth's surface environment(1). It is thought to ha ve evolved within the cyanobacterial lineage, but it has been difficult to determine when it began. Evidence based on the occurrence and appearance of stromatolites(2) and microfossils' indicates that phototrophy occurred as long ago as 3,465 Myr although no definite physiological inferences can be he made from these objects. Carbon isotopes and other geological phenomena( 4,5) provide clues but are also equivocal, Biomarkers are potentially usefu l because the three domains of extant life-Bacteria, Archaea and Eukarya-ha ve signature membrane lipids with recalcitrant carbon skeletons. These lipi ds turn into hydrocarbons in sediments and can be found wherever the record is sufficiently well preserved. Here we show that 2-methylbacteriohopanepo lyols occur in a high proportion of cultured cyanobacteria and cyanobacteri al mats, Their 2-methylhopane hydrocarbon derivatives are abundant in organ ic-rich sediments as old as 2,500 Myr. These biomarkers may help constrain the age of the oldest cyanobacteria and the advent of oxygenic photosynthes is. They could also be used to quantify the ecological importance of cyanob acteria through geological time.