Carbohydrate diagenesis in hypersaline environments: application of GC-IRMS to the stable isotope analysis of derivatized saccharides from surficial and buried sediments

Abstract

The stable carbon isotope ratios of individual neutral monosaccharides isolated from microbial mats, mangrove tissue and paleosoils from the Abu Dhabi lagoon-sabkha system were determined by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). Neutral monosaccharides in hydrolysates from surficial and buried “flat” mats dominated by the filamentous cyanobacteria Microcoleus chtonoplastes and Lyngbya aestuarii, have δ 13C (PDB) averaging −5.6 ± 1.3 and −4.6 ± 1.5% respectively. This is the first time such heavy δ 13c values have been reported for individual biosynthetic components, to our knowledge. So-called “blister” mats, formed by decomposition of flat mats by aerobic bacteria, yield slightly lower δ 13C values for individual monosaccharides around −10.4 ± 1.4%. The stable carbon isotope ratios of the monosaccharides isolated from the fresh mangrove leaf ( Avicennia marina) are low with δ 13C around −22.5 ± 2.6%, typical for low latitude C3 plants. Neutral monosaccharides from the mangrove paleosoils from 1250 and 6000 yr BP, however, have δ 13C of −16.0 ± 1.2 and −13.8 ± 2.9% respectively. This is attributed to the presence of anaerobic bacterial contributions in the palesoils. The δ 13C values of saccharides from the paleosoils indicate that galactose, mannose and fucose are mainly derived from bacteria, whereas glucose and arabinose still seem to be largely derived from the mangrove tissues.