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.

Supplement to: Türke, Andreas; Menez, Benedicte; Bach, Wolfgang (2018): Comparing biosignatures in aged basalt glass from North Pond, Mid-Atlantic Ridge and the Louisville Seamount Trail, off New Zealand. PLoS ONE, 13(2)