Contribution of Marine Group II Euryarchaeota to cyclopentyl tetraethers in the Pearl River estuary and coastal South China Sea: impact on the TEX₈₆ paleothermometer

Abstract. TEX₈₆ (TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms) has been widely applied to reconstruct (paleo-) sea surface temperature (SST). While Marine Group I (MG I) Thaumarchaeota have been commonly believed to be the source for GDGTs, Marine Group II (MG II Euryarchaeota) have recently been suggested to contribute significantly to the GDGT pool in the ocean. However, little is known how the MG II Euryarchaeota-derived GDGTs may influence TEX₈₆ in marine sediment record. In this study, we characterize MG II Euryarchaeota-produced GDGTs and assess the likely effect of these tetraether lipids on TEX₈₆. Analyses of core lipid (CL-) and intact polar lipid (IPL-) based GDGTs, 454 sequencing and quantitative polymerase chain reaction (qPCR) targeting MG II Euryarchaeota were performed on suspended particulate matter (SPM) and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (seawater). The results showed that the community composition varied along the salinity gradient with MG II Euryarchaeota as the second dominant group in the mixing water and seawater. qPCR data indicated that the abundance of MG II Euryarchaeota in the mixing water was three to four orders of magnitude higher than the river water and seawater. Significant linear correlations were observed between the gene abundance ratio of MG II Euryarchaeota vs. total archaea and the relative abundance of GDGTs-1, -2, -3, or -4 as well as the ring index based on these compounds, which collectively suggest that MG II Euryarchaeota may actively produce GDGTs in the water column. These results also show strong evidence that MG II Euryarchaeota synthesizing GDGTs with 1–4 cyclopentane moieties may bias TEX₈₆ in the water column and sediments. This study highlights that valid interpretation of TEX₈₆ in sediment record, particularly in coastal oceans, needs to consider the contribution from MG II Euryarchaeota.