Carbon isotopic composition of branched tetraether membrane lipids in soils suggest a rapid turnover and a heterotrophic life style of their source organism(s)

Abstract

Abstract. Branched Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are membrane spanning lipids synthesised by as yet unknown bacteria that thrive in soils and peat. In order to obtain more information on their ecological niche, the stable carbon isotopic composition of branched GDGT-derived alkanes, obtained upon ether bond cleavage, has been determined in a peat and various soils, i.e. forest, grassland and cropland, covered by various vegetation types, i.e., C 3 -vs. C 4 -plant type. These δ 13 C values are compared with those of bulk organic matter and higher plant derived nalkanes from the same soils. With average δ 13 C values of −28‰, branched GDGTs in C 3 soils are only slightly depleted (ca. 1‰) relative to bulk organic carbon and on average 8.5‰ enriched relative to plant wax-derived long-chain n-alkanes (nC 29 − nC 33 ). In an Australian soil dominantly covered with C 4 type vegetation, the branched GDGTs have a δ 13 C value of −18‰, clearly higher than observed in soils with C 3 type vegetation. As with C 3 vegetated soils, branched GDGT δ 13 C values are slightly depleted (1‰) relative to bulk organic carbon and enriched (ca. 5‰) relative to n-alkanes in this soil. The δ 13 C values of branched GDGT lipids being similar to bulk organic carbon and theirCorrespondence to: J. W. H. Weijers (j.weijers@geo.uu.nl) co-variation with those of bulk organic carbon and plant waxes, suggest a heterotrophic life style and assimilation of relatively heavy and likely labile substrates for the as yet unknown soil bacteria that synthesise the branched GDGT lipids. However, a chemoautotrophic lifestyle, i.e. consuming respired CO 2 , could not be fully excluded based on these data alone. Based on a natural labelling experiment of a C 3 /C 4 crop change introduced on one of the soils 23 years before sampling and based on a free-air CO 2 enrichment experiment with labelled CO 2 on another soil, a turnover time of ca. 18 years has been estimated for branched GDGTs in these arable soils.

Influence of water availability in the distributions of branched glycerol dialkyl glycerol tetraether in soils of the Iberian Peninsula

“…Water saturation was also suggested to play a significant role for brGDGT abundance in African soils (Loomis et al, 2011). BrGDGT source organisms have been suggested to be heterotrophic (e.g., Weijers et al, 2010;Huguet et al, 2012;Opperman et al, 2012;Ayari et al, 2013), which could explain the higher brGDGT abundances coupled to high TOC. Earlier studies showed that brGDGT abundances are usually high in water-saturated soils and peat bogs, thus potentially providing an ideal environment for brGDGT source organisms that have been proposed to be anaerobic (Weijers et al, 2006).…”

Soil pH impact on microbial tetraether lipids and terrestrial input index (BIT) in China

“…The increased supply of ammonia and CO 2 may therefore facilitate the flourish of crenarchaeota, leading to higher abundance of crenarchaeol in organic-rich soils. The bGDGT-producing bacteria were believed to be heterotrophic [31]. Higher abundance of organism in soils may provide more nutrients for bacterial regeneration, leading to a higher production of bGDGTs.…”

Tracing tetraether lipids from source to sink in the Rhône River system (NW Mediterranean)

“…Branched glycerol dialkyl glycerol tetraethers (brGDGTs) (see Schouten et al, 2013 for the review) are membrane-spanning lipids, most likely derived from anaerobic (Weijers et al, 2006a) and heterotrophic (Pancost and Sinninghe Damsté, 2003;Oppermann et al, 2010;Weijers et al, 2010) bacteria that occur ubiquitously in peat (e.g., Weijers et al, 2006a) and soil (e.g., Weijers et al, 2007a) as well as in lacustrine/marine sediments and water column (e.g., Blaga et al, 2009;Peterse et al, 2009a;Tierney and Russell, 2009;Pearson et al, 2011;Loomis et al, 2014;Weijers et al, 2014). The brGDGTs contain four to six methyl groups and up to two cyclopentane moieties formed by internal cyclization (Sinninghe Damsté et al, 2000;Weijers et al, 2006a, see Figure 1 for chemical structures).…”