Effects of a short-term experimental microclimate warming on the abundance and distribution of branched GDGTs in a French peatland

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Inglés. Branched glycerol dialkyl glycerol tetraethers (GDGTs) are complex lipids of high molecular weight, recently discovered in soils and increasingly used as palaeoclimate proxies. Their degree of methylation, expressed in the MBT, was shown to depend on mean annual air temperature (MAAT) and to a lesser extent on soil pH, whereas the relative abundance of cyclopentyl rings of branched GDGTs, expressed in the CBT, was related to soil pH. To date, only a few studies were interested in the application of the MBT and CBT proxies in peatlands. In order to validate the applicability of branched GDGTs as temperature proxies in these environments, it is essential to investigate the effect of temperature on branched GDGT-producing bacteria and especially on the speed of adaptation of these microorganisms to temperature changes. The aim of this work was to study the effects of in situ experimental climate warming on the abundance and distribution of branched GDGTs in a Sphagnum-dominated peatland (Jura Mountains, France). Branched GDGTs either present as core lipids (CLs; presumed of fossil origin) or derived from intact polar lipids (IPLs, markers for living cells) were analysed. Air temperature was experimentally increased using a passive warming system consisting of open mini-greenhouses (Open-Top Chamber - OTC). The effect of the OTCs was especially apparent in spring and summer, with (i) an increase in maximal air temperature of ca. 3°C during these two seasons and (ii) an increase in average air temperature of ca. 1°C in summer. Despite the short duration of the climate experiment (26 months), branched GDGT distribution was significantly affected by this temperature rise, with higher MBT values in the OTCs than in the control plots, supporting the empirical relationship between MBT and MAAT established from a large range of soils. The difference in branched GDGT-derived temperatures between control and OTC plots (2-3 °C) was in the same range as the increase in maximal (daytime) temperature induced by the OTCs in spring and summer, suggesting that branched GDGT-producing bacteria might be more active during the warmest months of the year. The OTC treatment had no significant effect on the abundance of branched GDGTs, which were mainly present as "fossil" CLs (70 to 85% of the total branched GDGT pool). Furthermore, no significant differences in branched GDGT distribution were observed between the CLs and IPLs, which both provided higher MBT and MAAT values for the OTCs. This suggests that the fossil pool of branched GDGTs has a very fast turnover (less than the 2 year duration of the experiment) at the peat surface and that branched GDGT distribution may rapidly reflect changes in environmental conditions (at least air temperature) occurring in peat environments.