Concentrations and TEX86 values of GDGTs present as core lipids and derived from intact polar lipids of station PASOM_10_WS water samples@en

Glycerol dibiphytanyl glycerol tetraether lipids (GDGTs) have proven to be important biomarker lipids for specific archaeal lineages and their distribution is used as a paleotemperature proxy. In this study, we analyzed GDGTs in suspended particles in the water column of the Arabian Sea at different positions above, in and below the oxygen minimum zone (OMZ). GDGTs, both as intact polar lipid (IPL) and as core lipids, were detected throughout the water column but were most abundant at the upper part of the OMZ. Core lipid GDGTs, derived from non-living organic matter, were always much more abundant than GDGTs released by acid hydrolysis of an IPL fraction (IPL-derived GDGTs). Comparisons with 16S rRNA gene abundance showed that likely only 1-14% of total archaeal cells present were caught on the 0.7 lm filter used for lipid analysis. Despite this undersampling, the depth profiles of crenarchaeol core lipid with a phosphohexose or dihexose head group match previously reported profiles of (expressed) genes specific for ammonia-oxidizing Thaumarchaeota, such as 16S rDNA and amoA. In contrast, the crenarchaeol with a hexose head group as well as core lipid and IPL-derived crenarchaeol matched the genetic depth profiles much less, suggesting a contribution of GDGTs from non-living matter. TEX86 values of both core lipid and IPL-derived GDGTs increased from surface waters to the core of the OMZ, below which they decreased again, and did not correlate with in situ water temperature. In contrast, TEX86 values of IPL-derived GDGTs correlated well the relative amount of glycosidic GDGTs and were consistently higher than that those of CL GDGTs. This suggests that selective preservation of glycosidic GDGTs may mask TEX86 values of in situ produced GDGTs in deep marine waters.甘油二植烷甘油四醚脂类（Glycerol dibiphytanyl glycerol tetraether lipids, GDGTs）已被证实为特定古菌谱系的重要生物标志物脂类，其分布特征可作为古温度代用指标。本研究对阿拉伯海水柱中不同位置（低氧区上方、内部及下方）悬浮颗粒物中的GDGTs进行了分析。无论以完整极性脂（intact polar lipid, IPL）形式还是核心脂（core lipid）形式存在的GDGTs，均在整个水柱中被检出，但其丰度在低氧区（oxygen minimum zone, OMZ）上层达到峰值。源自非活体有机质的核心脂GDGTs，其丰度始终远高于通过IPL组分酸水解所释放的GDGTs（即IPL衍生GDGTs）。通过与16S核糖体RNA基因丰度的对比分析发现，本次脂类分析所用的0.7 μm滤膜仅捕获了总古菌细胞的1%~14%。尽管存在采样不足的问题，但带有磷酸己糖或二己糖头部基团的嗜氨古菌醇（crenarchaeol）核心脂的深度分布，与此前报道的氨氧化奇古菌（ammonia-oxidizing Thaumarchaeota）特异性基因（如16S核糖体DNA及amoA（氨单加氧酶亚基A）基因）的表达深度分布相符。与之形成对比的是，带有己糖头部基团的嗜氨古菌醇，以及核心脂形式和IPL衍生形式的嗜氨古菌醇，其深度分布与遗传数据的匹配度则显著较低，这提示非活体物质来源的GDGTs存在一定贡献。核心脂与IPL衍生GDGTs的TEX86值均从表层水体向低氧区核心逐渐升高，随后再次下降，且与原位水体温度无显著相关性。相较而言，IPL衍生GDGTs的TEX86值与糖苷类GDGTs的相对丰度呈现良好的相关性，且其数值始终高于核心脂GDGTs的TEX86值。这表明糖苷类GDGTs的选择性保存作用，可能会掩盖深海水体中原位产生的GDGTs的TEX86信号。