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

Abstract

The hydrogen (δD) and carbon (δ13C) isotope compositions of long-chain n-alkanes provide a record of paleoenvironmental conditions. At present, the role bacteria play in altering the stable isotope composition of n-alkanes 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. Then, we collected sediment from identical locations (measured to the cm level) three years later and froze these immediately. Samples stored at room temperature showed high amounts of medium-chain n-alkanes (n-C18–n-C26), which were absent from samples that were immediately frozen, and a decrease in the abundance of long-chain n-alkanes (n-C27–n-C33). Measured δD values of n-C29 and n-C31n-alkanes increased in stored samples by up to 25‰. δ13C 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 the 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. We recommend that sediment samples that are exposed to significant changes in oxygen or ambient moisture after collection are extracted as soon as possible or stored refrigerated or frozen.