Hydroxylated GDGTs-0 in marine methane seep environments: A putative indicator for archaeal methanogenesis

Marine methane seeps are environments with a high microbial diversity, but are known for one biogeochemical key process, the sulfate-driven anaerobic oxidation of methane (SD-AOM) performed by anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). SD-AOM is also the dominant process at methane seeps in the South China Sea based on the lipid biomarker inventory of authigenic seep carbonates characterized by crocetane, a high sn2-hydroxyarchaeol over archaeol ratio, low contents of glycerol dibiphytanyl glycerol tetraethers (GDGTs), and δ¹³C_{lipid-methane} offsets of −52 ± 4‰. Combined with the dominance of aragonite over other carbonate minerals, such pattern suggests high seepage intensity with a predominance of ANME-2/SRB consortia. Interestingly, the studied seep carbonates also yielded some uncommon biomarkers for these consortia, which may derive from methanogenic archaea. Methylotrophic methanogenesis has been shown to be the dominant mode of methanogenesis in seep environments where non-competitive substrates like methanol or trimethylamine are abundant. The presence of methylotrophic methanogens is possibly indicated by high contents (more than 50% of all GDGTs) of hydroxylated GDGTs-0 (OH-GDGT-0 and 2OH-GDGT-0) with extreme ¹³C-depletion (−128‰ to −116‰); this unique pattern is recognized in only some of the studied seep carbonates, while other carbonates are dominated by typical distributions of ANME-2 lipids, also comprising GDGTs with 0 to 4 rings, but lacking high contents of OH-GDGTs. The overall lack of crenarchaeol, the specific biomarker of planktonic Thaumarchaeota, agrees with the tentative assignment of the highly abundant OH-GDGTs-0 to methanogenic archaea. Such interpretation is necessarily circumstantial considering that the compound and carbon isotope composition of the membranes of ANME and methanogenic archaea is similar. Although production of OH-GDGTs has been previously reported for both planktonic Thaumarchaeota and ANME, OH-GDGT-0 as sole and highly abundant OH-GDGT has only been recognized in one culture of methanogenic archaea. Therefore, the high abundance of ¹³C-depleted hydroxylated GDGTs-0 compared to only minor contents of regular, ANME-derived GDGTs 1–4 with similar ¹³C-depletions can possibly be used as an indicator for methylotrophic methanogenesis in seep environments. Future experimental work is needed and should test if ¹³C-depleted hydroxylated GDGTs-0 are indeed biomarkers of methylotrophic methanogenesis at seeps.