Microbial alteration of the hydrogen and carbon isotopic composition of n-alkanes in sediments

Abstract

Long-carbon chain length n-alkanes (n-C 27-n-C 33) are commonly produced by terrestrial plants and preserved in sedimentary archives. The hydrogen (δD) and carbon (δ 13 C) isotope compositions of these compounds provide a record of paleohydrologic and paleoecologic conditions at the time of formation. At present, the role bacteria play in altering the stable isotope composition of biomarker compounds during storage of sediments is unclear. We collected four sediment samples at precise depth intervals from an archaeological site and stored these samples at room temperature for three years. We collected sediment from identical locations (precise to the cm level) three years later and froze these immediately. All stored samples showed high amounts of short-and medium-chain length n-alkanes (n-C 18-n-C 26), which were absent from samples that were immediately frozen. Measured δD values of n-C 29 and n-C 31 n-alkanes increased in stored samples by up to 25‰. δ 13 C values in stored samples are more negative than non-stored samples by up to 4.2‰. DNA analysis of the soil microorganisms showed bacteria containing genes coding for n-alkane degrading enzymes proliferated in samples during their three year storage. In particular, Rhodococcus and Aeromicrobium, genera which contain multiple coding regions for n-alkane degrading enzymes CYP153 and AlkB, increased by an order of magnitude (from 0.7% to 7.5% of bacteria present). These data suggest that bacteria may play a larger role than previously expected in altering measured n-alkane stable isotope values during storage and that analyses conducted on stored samples may lead to erroneous results.

Leaf wax <i>n</i>-alkane patterns and compound-specific <i>δ</i><sup>13</sup>C of plants and topsoils from semi-arid and arid Mongolia

“…However, differences between both n-alkane concentrations are minor, and, thus, we present the n-alkane concentrations including n-C 23 in the Supplement. The OEP was calculated according to Hoefs et al (2002) and serves as a proxy for degradation, with values below 5 indicating enhanced n-alkane degradation (Buggle et al, 2010;Zech et al, 2009Zech et al, , 2010:…”

“…4a). Likewise, the OEP decreases from plants to topsoil and indicates enhanced organic matter degradation (Buggle et al, 2010;Schäfer et al, 2016) and microbial alteration ( Fig. 4b; Schulz et al, 2012).…”

“…However, when interpreting leaf wax n-alkanes and their compound-specific δ 13 C signature several potential pitfalls, such as species-specific and intra-leaf variations (Diefendorf et al, 2011;Gao et al, 2015), the influence of environmental and climatic factors (Carr et al, 2014;Diefendorf and Freimuth, 2017;Farquhar et al, 1982;Hoffmann et al, 2013;Rao et al, 2017;Tipple et al, 2013), the dependency on altitude (Feakins et al, 2018;Hultine and Marshall, 2000), and n-alkane degradation (Brittingham et al, 2017;Buggle et al, 2010;Li et al, 2018a), have to be considered and accounted for. So far, such regional calibration studies on recent leaf wax n-alkane patterns, compound-specific δ 13 C and poten-tial climatic biases on the leaf wax signal do not exist for Mongolia.…”

Leaf wax <i>n</i>-alkane pattern and compound-specific δ<sup>13</sup>C of plants and topsoils from semi-arid Mongolia

“…However, when interpreting leaf wax n-alkanes and their compound-specific δ 13 C isotopes several potential pitfalls such as species-specific and intra-leaf variations (Diefendorf et al, 2011;Gao et al, 2015), the influence of environmental and climatic factors (Diefendorf and Freimuth, 2017;Farquhar et al, 1982;Hoffmann et al, 2013;Rao et al, 2017;Tipple et al, 2013;Carr et al, 2014), and n-alkane degradation (Buggle et al, 2010;Brittingham et al, 2017;Li et al, 2018a) have to be considered and accounted for. So far, such regional calibration studies on recent leaf wax homologue pattern and compound-specific δ 13 C isotopes and potential biases of the vegetational/climatic leaf wax signal are scarce for semi-arid regions and do not exist for…”

From leaf to soil: <i>n</i>-alkane signal preservation, despite degradation along an environmental gradient in the tropical Andes

“…Our results show that only the soil n-alkane signal reflects changes in CPI, which suggests that the processes leading to changes in CPI are related to soil processes. Potentially, such a process could be the de-novo genesis of n-alkanes without an odd-over-even predominance as a result of microbial alteration (Brittingham et al, 2017;Jansen and Wiesenberg, 2017;Rao et al, 2009;Wu et al, 2019), although this should be studied more extensively. Notably, 280 the CPI does not correlate with the ACL and ratio metrics (Fig.…”