Distribution of branched glycerol dialkyl glycerol tetraethers in soils on the Northeastern Qinghai-Tibetan Plateau and possible production by nitrite-reducing bacteria

Branched glycerol dialkyl glycerol tetraethers(b GDGTs) are ubiquitous and abundant in soils, but their sources remain elusive. Recent studies demonstrate that the distributions of b GDGTs are sensitive to various environmental factors. In an effort to understand how and to what extent soil moisture(expressed as soil water content(SWC) or mean annual precipitation(MAP), p H and temperature may impact the distribution of b GDGTs, and to shed more light on the biological sources of b GDGTs in cold and arid regions, we investigated the distribution of b GDGTs as well as bacterial 16 S r RNA gene and functional genes involved in the N cycle(including amo A, nir S and nir K) in 41 surface soil samples from around Lake Qinghai and east of Qaidam Basin on the Northeastern Qinghai-Tibetan Plateau. We found that lower soil moisture reduced the fractional concentrations of cyclic b GDGTs and thus the cyclisation ratio of branched tetraethers(CBT) index correlated negatively with SWC and MAP, suggesting that soil moisture is an important factor controlling b GDGT distributions in soils in this arid and semi-arid region. Two subgroups of b GDGTs were assigned on the basis of cluster analysis, and b GDGT indices behaved differently in the two groups, hinting at different biosynthetic mechanisms for b GDGTs under different environmental conditions. Real time PCR results showed that nir S and nir K genes correlated significantly with the concentration of b GDGTs, suggesting that the nir S- or nir K-encoding bacteria involved in denitrification might potentially be an additional biological source for soil b GDGTs(besides Acidobacteria). Moreover, our results also support the application of new indices based on 5-methyl b GDGTs and 6-methyl b GDGTs in reconstructing past temperature and p H variations in this region.