Lipid biomarker patterns reflect seepage activity and variable geochemical processes in sediments from the Haima cold seeps, South China Sea

The anaerobic oxidation of methane (AOM), aerobic methane oxidation and the oxidation of non-methane hy-drocarbons have been demonstrated to occur in seepage ecosystems, but when they occur during seepage evolutionary is lack of constraints. Here, lipid biomarker patterns combined with total organic carbon (TOC), delta C-13(org), delta C-13(TIC) and delta O-18(TIC) of a sediment core (similar to 3 m long) from the active Haima cold seep were analyzed. The observed lipid inventories reflected the prevailing of anaerobic methane oxidizing archaea (ANME)/Desulfo-sarcina/Desulfococcus (DSS) and ANME-2/DSS dominated over the whole sediments. Seepage fluids originated from local gas hydrate dissociation as indicated by the pronounced delta C-13(TIC) and delta O-18(TIC) values, while a mixture of microbial and thermogenic methane (-65 parts per thousand to-55 parts per thousand) was corroborated by back-calculation based on the predominant consortia and delta 13C values of ANME-specific biomarkers. AOM occurred over the whole sediments and was strengthened between depths 40 cm and 120 cm as revealed by the extremely low delta(13)(CTIC), delta(13)(Corg) values, and abundant lipid biomarkers of ANME such as crocetane, 2,6,10,15,19-pentamethylicosane (PMI), archaeol and sn(2)-OH-archaeol with strong C-13 depletions (delta C-13 values as low as-118 parts per thousand). For sediment below the depth of 120 cm, molecular fossils of aerobic methanotrophs 4 alpha-methylcholesta-2,8(14),24-triene and C-34-3 beta-methyl tetracyclic-ketone occurred and yielded their delta C-13 values of-64 parts per thousand to-61 parts per thousand, indicating the prevailing of oxic condition and aerobic methane oxidation. Compared to the sediment between 40 cm and 120 cm, the abundant iso-/anteiso-C-15:0 (i-/ai-C-15:0) fatty acids with relatively enriched delta C-13 values in the bottom sediment were better explained by contribution of sulfate-reducing bacteria (SRB)-driven oxidation of non-methane hy-drocarbons. The distinct biomarker patterns and the respective geochemical processes at various sediment depths revealed the microbial community succession sequence in seepage evolutionary, since the different growth rates may have led to a successive colonization of aerobic methanotrophs, SRB, and ANME in an emerging cold seep ecosystem.

Automated summary

The study of a sediment core from the Haima cold seep revealed the presence of anaerobic oxidation of methane, aerobic methane oxidation, and oxidation of non-methane hydrocarbons at different sediment depths. The distinct biomarker patterns and geochemical processes indicate a microbial community succession sequence in seepage evolutionary.