Controls on the composition of hydroxylated isoGDGTs in cultivated ammonia oxidizing Thaumarchaeota

Abstract. Membrane lipids of ammonia-oxidizing Thaumarchaeota, in particular isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) and hydroxylated isoGDGTs (OH-isoGDGTs), have been used as biomarkers and as proxies in various environments. Controlled growth experiments have been used to investigate the factors that influence the composition of these lipids, in particular on how these factors affect the TEX₈₆ temperature proxy, which is based on the degree of cyclization of isoGDGTs. Recently, the ring index of OH-isoGDGTs (RI-OH'), based on cyclization patterns of OH-isoGDGTs, and the relative abundance of OH-isoGDGTs (%OH) have emerged as promising temperature proxies. Here, we examined the impact of growth temperature and growth phase on the distribution of OH-isoGDGTs and their associated proxies using cultures of two thaumarchaeotal strains. Analysis of core lipids and headgroup compositions of isoGDGTs and OH-isoGDGTs showed no consistent differences between the mid-exponential and stationary phases for both strains. Nitrosopumilus adriaticus NF5 shows a substantially higher relative abundance of OH-isoGDGTs (~49 %) compared to Nitrosopumilus piranensis D3C (~5 %) and also relative to observations reported for core lipids in the marine environment (< 17 %), indicating large variations in %OH values even among closely related species. Unlike in the marine environment, the %OH did not decrease with increasing temperatures in either of the strains, possibly reflecting a threshold below 15 °C for this response in natural environment. The RI-OH' increases with increasing temperature in cultures of both strains, similar to the ring index of regular isoGDGTs. The relative abundances of the headgroups varied between strains and did not respond to changes in temperature nor growth phase. The %OH and RI-OH' calculated from intact polar lipids with different headgroups revealed large differences between the distinct intact polar lipids, similar to that previously observed for regular isoGDGTs. Together, our findings suggest that growth temperature has a pronounced effect on the degree of cyclization in isoGDGTs and OH-isoGDGTs, in contrast to the relative abundance of OH-isoGDGTs, which mainly exhibits interspecies variability.