GDGTs of Marine Group II Archaea in the Pearl River Estuary: Toward a Better Understanding of the Bias of TEX86

Abstract:

TEX₈₆, calculated based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) from planktonic Thaumarchaeota, is widely used for paleo sea surface temperature reconstruction. Although increasing evidence shows that TEX₈₆ can be affected by multiple environmental variables, little is known about the additional contribution of TEX₈₆-related GDGTs from other planktonic archaeal groups. In this study, we test the hypothesis that GDGTs potentially derived from MG II Archaea affect TEX₈₆ calculations. Analysis of core (C) and intact polar (IP) GDGTs and 454 sequencing and quantitative PCR (qPCR) targeting MG II were performed on suspended particulate matter (SPM) collected along a salinity gradient from the lower Pearl River, its estuary, and the northern South China Sea. The results showed that the community structure varied along the salinity gradient with MG II as the second dominant group in the mixing water and seawater. qPCR data indicated that the abundance of MG II in the mixing water was four to five orders of magnitudes higher than the fresh water and seawater. The Ring Index of archaeal lipids was notably elevated in the Pearl River estuary, which may be attributed to the additional export of cyclopentane moiety-enhanced GDGTs from MG II in the mixing water. Furthermore, a linear correlation was observed between qPCR of MG II and IP-GDGTs derived from phosphate head groups, suggesting that MG II actively produces GDGTs in the water column. These results show strong evidence that MG II synthesizes GDGTs with more cyclopentane moieties, which may bias TEX₈₆ signal derived from GDGTs of Thaumarchaeota. This study highlights that valid interpretation of TEX₈₆ in the geologic record, particularly in coastal oceans, needs to consider the paleo-community structure of planktonic Archaea.