Environmental factors shaping the archaeal community structure and ether lipid distribution in a subtropic river and estuary, China.

Archaea are widespread and abundant in aquatic and terrestrial habitats and play fundamental roles in global biogeochemical cycles. Archaeal lipids, such as isoprenoid glycerol diakyl glycerol tetraethers (iGDGTs), are important biomarkers tracing changes in archaeal community structure and biogeochemical processes in nature. However, the linkage between the archaeal populations and the GDGT distribution in the natural environment is poorly examined, which hindered the application and interpretation of GDGT-based climate or environmental proxies. We addressed this question by investigating changes in archaeal lipid composition and community structure in the context of environmental variables along the subtropical Jiulong River Watershed (JRW) and Jiulong River Estuary (JRE) in southern China. The results showed that both the archaeal cells and the polar GDGTs (P-GDGTs) in the JRW and JRE were mostly autochthonous rather than exogenous input from surrounding soils. We further found that only five (Methanobacteriales, Ca. Bathyarchaeota, Marine Benthic Groups A (MBGA), Marine Benthic Groups B (MBGB), and Marine Benthic Groups D (MBGD)) out of sixteen lineages showed significant impacts on the composition of P-GDGTs, suggesting the significant contribution of those archaea to the changes of P-GDGT compositions. Salinity and total phosphorus (TP) showed significant impact on the distribution of both genetic and P-GDGTs compositions of archaea; whereas, sand and silt contents only had significant impact on the P-GDGTs. MBGD archaea, which occur widely in marine sediments, showed positive correlations with P-TEX86 in the JRW and JRE, suggesting that uncultivated MBGD might also contribute to the variations in TEX86 signals in marine sediments. This study provided insight into the sources of P-GDGTs and the factors controlling their distributions in river-dominated continental margins, which has relevance to applications of GDGT-based proxies in paleoclimate studies.