Temporal variation in community structure and lipid composition of Thaumarchaeota from subtropical soil: Insight into proposing a new soil pH proxy

Crenarchaeol is a unique glycerol dialkyl glycerol tetraether (GDGT) lipid specific to Thaumarchaeota, which play an important role in the global C and N cycles. GDGTs from Thaumarchaeota have been used to develop proxies for paleoclimate or paleoenvironment studies in aquatic environments. However, our understanding of their response to environmental change in soil remains poor. We addressed this question by investigating the change in archaeal lipid composition and community structure in the context of other environmental variables over a period of 12 months in a subtropical soil from Chongming Island, China. The results showed that Nitrososphaera spp. were the dominant archaeal population producing crenarchaeol in the soil. The relative abundance of GDGTs with one and three cyclopentane moieties, and crenarchaeol and its isomer in the core lipid (CL) fraction correlated with seasonal pH change in the soil. We therefore propose a molecular fossil proxy, the thaumarchaeota index (TI), which significantly related to pH in not only Chongming Island soil (R2 0.56, RMSE 0.14, P < 0.05), but also global soils (R2 0.51, RMSE 1.39, P < 0.001) and five thaumarchaeotal enrichments (R2 0.99, RMSE 0.04, P < 0.001). This suggests that TI could be a useful proxy for soil pH, which may corroborate the use of the bacterial GDGT proxy for soil pH, the cyclisation ratio of branched tetraethers (CBT).