FTIR raw data of organic molecules in basalt samples from IODP Hole 336-U1383C and 330-U1376A

Microbial life can leave various traces (or biosignatures) in rocks, including biotic alteration textures, biominerals, enrichments of certain elements, organic molecules, or remnants of DNA. In basalt glass from the ocean floor, microbial alteration textures as well as chemical and isotopic biosignatures have been used to trace microbial activity. However, little is known about the relationship between the physical and chemical nature of the habitat and the prevalent types of biosignatures. Here, we report and compare strongly variable biosignatures from two different oceanic study sites. We analyzed rock samples for their textural biosignatures and associated organic molecules. The biosignatures from the 8 Ma North Pond Region, which represents young, well-oxygenated, and hydrologically active crust, are characterized by little textural diversity. The organic matter associated with those textures shows evidence for the occurrence of remnants of complex biomolecules like proteins. The biosignatures from the older Louisville Seamount Trail (~70 Ma), for which archaeal origin is suggested, are much more texturally diverse and the associated organic molecules are also more degraded. We hypothesize that microbial communities change significantly during crustal evolution and aging, and suggest that microbes that are associated with older and severely altered crust are not responsible for the biotic alteration textures commonly found in subseafloor basalt glass. We suggest that biotic alteration textures are related to microbially-catalyzed oxidation of Fe2+, Mn2+, and S compounds and form predominantly within the first ~10 Ma of crustal evolution. In older crust with less glass and decreased permeability, other metabolic pathways may dominate which only leave molecular biosignatures. We propose that diverse biosignatures in oceanic crust may form during different stages of crustal evolution.

微生物生命可在岩石中留下多种痕迹（即生物标志物（biosignatures）），包括生物蚀变结构、生物矿物、特定元素富集现象、有机分子乃至DNA残体。在海底玄武岩玻璃中，微生物蚀变结构以及化学与同位素类生物标志物曾被用于示踪微生物活动。然而，学界对于栖息环境的物理化学性质与常见生物标志物类型之间的关联仍知之甚少。本研究报道并对比了两处不同海洋研究点位中差异显著的生物标志物。我们针对岩石样本的结构类生物标志物与伴生有机分子开展了分析。来自距今8百万年（8 Ma）北塘区的生物标志物，对应年轻、富氧且水文活跃的洋壳，其结构多样性较低。与这些结构伴生的有机质显示存在蛋白质等复杂生物分子残体的痕迹。来自更为古老的路易斯维尔海山链（~70 Ma）的生物标志物被认为起源于古菌，其结构多样性显著更高，且伴生有机质的降解程度也更强。我们提出假说：微生物群落会随洋壳演化与老化发生显著改变；同时认为，与老旧且蚀变严重的洋壳伴生的微生物，并非海底玄武岩玻璃中常见生物蚀变结构的成因。我们认为，生物蚀变结构与微生物催化的亚铁离子（Fe²+）、锰离子（Mn²+）与硫化合物的氧化过程相关，且主要形成于洋壳演化的前~10百万年阶段。在玻璃质含量更少、渗透性降低的老旧洋壳中，其他代谢途径可能占据主导，仅会留下分子类生物标志物。我们提出，洋壳中多样的生物标志物可能形成于洋壳演化的不同阶段。