Spatial and temporal variations in core- and polar- isoprenoid tetraether lipids along a salinity gradient from the lower Pearl River to its estuary

Abstract:

Isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) are major archaeal lipids that widely occur in nature and have been used to study paleo-climate and paleo-environments. Estuaries are a dynamic ecosystem that links or divides terrestrial and marine processes. However, factors controlling changes in iGDGTs in estuaries are poorly known. In this study we performed monthly sampling of sediments between July 2012 and May 2013 at four sites from the lower Pearl River to the estuary. Total lipids were extracted to obtain core- and polar-GDGTs, which were identified and quantified using liquid chromatography-mass spectrometry (LC-MS). The average concentrations of archaeal lipids were highest at the seawater station (C-iGDGTs: 1804.0 ng/g; P-iGDGTs: 986.9 ng/g) and lowest at the brackish water station (C-iGDGTs: 364.1 ng/g; P-iGDGTs: 324.4 ng/g) with C-iGDGTs being more abundant than P-iGDGTs in most samples. The composition of iGDGTs varied widely from station to station, but was generally dominated by GDGT-0 or crenarchaeol in both C- and P-iGDGTs. At the freshwater station in the lower Pearl River, the relative abundance of C- and P-iGDGTs varied from month to month, whereas such temporal changes were much less at stations in the brackish and marine environments. These results suggest that Archaea living in freshwater might be more sensitive to environmental variation. A significant positive correlation (R²>0.80, p<0.01) was found between the ratio of Cren./(Cren.+GDGT-0) or ring index and salinity in both C- and P-iGDGT fractions, indicating that salinity is the most important factor affecting the composition of iGDGTs in the estuary environment.