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

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., C₃ - vs. C₄ -plant type. These δ¹³ C values are compared with those of bulk organic matter and higher plant derived n-alkanes from the same soils. With average δ¹³ C values of -28[per thousand], branched GDGTs in C₃ soils are only slightly depleted (ca. 1[per thousand]) relative to bulk organic carbon and on average 8.5[per thousand] enriched relative to plant wax-derived long-chain n-alkanes ( nC₂₉ -nC₃₃ ). In an Australian soil dominantly covered with C₄ type vegetation, the branched GDGTs have a δ¹³ C value of -18[per thousand], clearly higher than observed in soils with C₃ type vegetation. As with C₃ vegetated soils, branched GDGT δ¹³ C values are slightly depleted (1[per thousand]) relative to bulk organic carbon and enriched (ca. 5[per thousand]) relative to n-alkanes in this soil. The δ¹³ C values of branched GDGT lipids being similar to bulk organic carbon and their 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 CO₂ , could not be fully excluded based on these data alone. Based on a natural labelling experiment of a C₃ /C₄ crop change introduced on one of the soils 23 years before sampling and based on a free-air CO₂ enrichment experiment with labelled CO₂ on another soil, a turnover time of ca. 18 years has been estimated for branched GDGTs in these arable soils.

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