Abstract
Oxygen isotope thermometry has been traditionally based on the ratio of 18O/16O in oxyanions of minerals such as carbonate (CaCO3) in shells/tests, and phosphate in bioapatites (Ca5(PO4)3OH) of marine invertebrates/vertebrates (for example, fish) and mammals. The requirement of mineral biomass, however, has restricted the application of oxygen isotope thermometry to only those organisms possessing biomineral hardparts. This has completely omitted from study, not only organisms lacking hard mineral tissues, but two entire Domains of life: Bacteria and Archaea. Prokaryotic organisms in the domains Bacteria and Archaea comprise the majority of earth's biodiversity and also inhabit the most extreme environments on earth. This calls for a thermometry based on a more rudimentary component of biomass that is present in all organisms such as DNA. Our previous studies of the ubiquitous intracellular enzyme inorganic pyrophosphatase (PPase), which catalyzes oxygen isotope exchange between dissolved inorganic PO4 (Pi) and water inside of cells, suggest that DNA may contain even more information than the blueprint for life. Here we show that PO4 moieties in DNA record the temperature at which life forms and evolves. Our results demonstrate that the 18O/16O ratio of PO4 (δ18OP) in DNA as well as in bulk biomass, reflects the temperature-dependent exchange of oxygen isotopes between PO4 and intracellular water. Thus, δ18OP values of DNA may serve as both a new soft-tissue bio-thermometer and probe of intracellular PO4 or water during DNA synthesis and cellular growth.
Results presented here of the first direct measurements of the oxygen isotopic composition of PO4 in microbial DNA, demonstrate systematic variation in δ18OP of DNA from several different strains of microorganisms as a function of temperature, and also extend the PO4-water O-isotope thermometer to >70 °C. A composite calibration curve based on several strains of bacteria is presented for DNA-PO4–water –temperature relations between 12 and 75 °C and suggests a “universal” DNA-based oxygen isotope thermometry for microorganisms that may further extend to all organisms. Our results open the possibility of connecting temperature with taxonomy and also expand the range of investigations of habitat temperatures and limits to life in extreme and diverse environments, from Earth's subsurface deep biosphere to Antarctic ice sheets and extraterrestrial systems where life may have originated at extreme temperatures.
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