Confounding effects of oxygen and temperature on the TEX86 signature of marine Thaumarchaeota.

Marine ammonia-oxidizing archaea (AOA) are among the most abundant of marine microorganisms, spanning nearly the entire water column of diverse oceanic provinces. Historical patterns of abundance are preserved in sediments in the form of their distinctive glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipids. The correlation between the composition of GDGTs in surface sediment and the overlying annual average sea surface temperature forms the basis for a paleotemperature proxy (TEX86) that is used to reconstruct surface ocean temperature as far back as the Middle Jurassic. However, mounting evidence suggests that factors other than temperature could also play an important role in determining GDGT distributions. We here use a study set of four marine AOA isolates to demonstrate that these closely related strains generate different TEX86-temperature relationships and that oxygen (O2) concentration is at least as important as temperature in controlling TEX86 values in culture. All of the four strains characterized showed a unique membrane compositional response to temperature, with TEX86-inferred temperatures varying as much as 12 °C from the incubation temperatures. In addition, both linear and nonlinear TEX86-temperature relationships were characteristic of individual strains. Increasing relative abundance of GDGT-2 and GDGT-3 with increasing O2 limitation, at the expense of GDGT-1, led to significant elevations in TEX86-derived temperature. Although the adaptive significance of GDGT compositional changes in response to both temperature and O2 is unclear, this observation necessitates a reassessment of archaeal lipid-based paleotemperature proxies, particularly in records that span low-oxygen events or underlie oxygen minimum zones.