Mass spectrometric characterization of 13C-labeled lipid tracers and their degradation products in microcosm sediments

A GC–MS approach was developed to characterize 13C-labeled tracers in microcosm sediments. Three major 13C-labeled fatty acids (16:1ω7, 16:0, and 18:1ω9) and phytol in labeled microalgae (added to sediments as organic substrate) were distinguished from natural sedimentary counterparts. During a series of incubations under various redox conditions (oxic and anoxic) and with and without the presence of macrofauna (i.e. Yoldia limiatula), two newly produced 13C-labeled compounds (iso-15:0 fatty acid and C16 alcohol) were identified from incubated sediments. Analyses of mass spectra showed that the iso-15:0 fatty acid was partially labeled while the C16 alcohol was uniformly labeled with 13C, implying that the formation pathways of these two degradation products may be different. Estimates of degradation rate constants of planktonic lipids during these incubations showed that oxic conditions and the presence of Yoldia significantly accelerated the degradation of planktonic lipids in sediments.