Assessing the metabolism of sedimentary microbial communities using the hydrogen isotopic composition of fatty acids

The hydrogen isotopic composition of fatty acids (FAs) has previously been shown to reflect the metabolism of microorganisms in pure culture, but has rarely been tested in the environment. Here, we report the abundance and hydrogen isotopic composition of polar lipid derived FAs extracted from surface sediments of the saline Lake Grevelingen (The Netherlands), at two different stations and during two seasons with oxic bottom water conditions during spring and hypoxic to anoxic conditions during late summer. These data are compared with the bacterial diversity revealed by 16S rRNA gene amplicon sequencing. All measured FAs were depleted in deuterium relative to the bottom water by 103‰ to 267‰. FAs associated with heterotrophic bacteria (i-15:0 and ai-15:0) showed the smallest fractionation (−103‰ to −185‰) while those derived from pelagic photoautotrophic phytoplankton (20:5) showed the largest fractionation (−230‰ to −267‰). Overall, the hydrogen isotope fractionation reflected in the majority of the more commonly occurring FAs (14:0, 16:0, 16:1ω7) is relatively large (−172‰ to −217‰). Together with the high relative abundance of the 20:5 FA, this suggests a substantial contribution from dead pelagic biomass settling from the water column to the sedimentary polar lipid derived FA pool and not from the in situ microbial communities. Therefore, the majority of the isotope signal in the fatty acids from surface sediments might not represent the general metabolism of the active sedimentary communities. Therefore, the input of pelagic biomass into sedimentary environments may bias the information contained in the hydrogen isotopic composition of FAs.