FAST AND SIMPLE DOMAIN-LEVEL DIAGNOSIS OF PROKARYOTIC CELL IN MICROBIAL COMMUNITY USING MICRO-INFRARED SPECTROSCOPY

Domain-level diagnosis of microorganisms in living prokaryotic communities is one of the most fundamental research steps to elucidate natural microbial communities and their ecological/biogeochemical significances. Domain-level diagnosis of microbial cell may be more crucial for geologists and paleobiologists investigating Precambrian microfossils. Identification of ancient microfossils in the framework of potential extant microbial taxa is quite significant because not only existence of certain phylotypes of life but also occurrence of certain phenotypes and metabolisms could be deduced. Nevertheless, it is still based largely on the morphological traits and is not fully justified in many cases. We sought to establish a technical basis for domain-level diagnosis of prokaryotic cells and quantification of abundance in natural microbial communities by using a micro-FTIR spectroscopy. Various prokaryotic cultures (12 species of Bacteria and 9 of Archaea) were tested by micro-FTIR spectroscopic analysis. An aliphatic CH₃/CH₂ absorbance ratio (R_3/2) could be domain-specific signature that is likely attributable to the different cellular lipid compositions. The signatures were preserved even after the chemical cell fixation (formaldehyde) and the nucleic acids staining (DAPI) processes, practically prerequisites for geomicrobiological application. The technique was applied to quantification of Bacteria/Archaea abundance ratio of an active microbial mat community in a subsurface hot aquifer. The R_3/2 signature could be a new powerful tracer for domain-level diagnosis and quantification of extant and ancient microorganisms.