Evaluation of heterocyte glycolipids with a hexose sugar moiety for tracing terrestrial organic matter in the South China Sea

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Heterocyte glycolipids (HGs) are biomarkers for N2-fixing heterocytous cyanobacteria (HCB). The hexose (hereafter C6) sugar moiety of HGs is produced by free-living HCB dominant in fresh and brackish waters, while the pentose moiety (hereafter C5) of HGs is found in endosymbiotic HCB associated with diatoms in tropical-subtropical marine waters. Therefore, C6 HGs detected in marine environments may indicate terrestrial organic matter (TOM) input, but this may be complicated by the fact that free-living HCB may be active over a wide salinity range. In this study, we evaluate C6 HGs and their fraction in total HGs, referred to as FHG, to trace TOM in the South China Sea (SCS) by analyzing a series of surface sediments from the Pearl River Estuary (PRE) to the open SCS, as well as suspended particular matters (SPM) in the euphotic zone of central SCS and a sediment core 17950-2 in the deep SCS. Our results showed that C6 HGs were abundant in the PRE sediments but almost absent in sediments or SPM from the northern shelf to the central SCS, leading us to conclude that free-living HCB producing C6 HGs are inactive in the modern ocean where salinity is >30. In the sediment core, C6 HGs and FHG showed a variation pattern similar to sea level changes during glacial-interglacial cycles, and the reconstructed sea surface salinity was above 32 throughout the cycles. Such a pattern was also evident in the records of branched to isoprenoid tetraether (BIT) index and detrital content from the same core. Thus, the modern distribution of C6 HGs and FHG, together with their downcore records in this study, demonstrates that C6 HGs and FHG are excellent for tracking TOM in tropical-subtropical marine environments.