A New Macrofossil Built by Microbial Biofilms: Lithoautotrophs Construct Iron-Oxide-Cemented Columns in Sandstone Aquifers

Compared to microbes in liquids, those within biofilms are crowded and socially interactive. Although they remain flat in moving media, microbial biofilms aggregate to form towers in still media under large diffusion... [ view full abstract ]

Compared to microbes in liquids, those within biofilms are crowded and socially interactive. Although they remain flat in moving media, microbial biofilms aggregate to form towers in still media under large diffusion gradients. Here we show that, in dark, still water that filled interconnected pores of a Jordanian aquifer, lithoautotrophic microbes cemented close-packed, decimeter-scale columns as they grew toward diffusing ferrous iron. Prior studies of non-columnar, iron-oxide-cemented structures in sandstones, using SEM and stable carbon isotopes, have shown that the cement is the product of iron-oxidizers, including a type resembling modern Gallionella. These microbes lived within subsurface biofilms and exploited the mineral siderite (FeCO3) as a source of carbon and energy. The columns in Jordanian sandstones also contain Gallionella-like twisted stalks, and their overall, exterior form is convergent with that of columnar stromatolites— products of photosynthesizing microbes that have now lived under Earth’s clear surface waters and bright skies for more than 3 Gy. Like most columnar stromatolites in the rock record, Jordanian structures initially comprised small-diameter columns that nucleated on horizontal surfaces. Most iron-cemented columns grew downward; faster-growing columns expanded and branched as they elongated, crowding out thinner neighboring columns. Because they formed within a porous, lithified framework, their morphogenesis is simpler than that of stromatolites. Biofilm growth rates, distribution of dissolving siderite crystals and precipitating cement likely influenced tower morphology and spacing, but the subsurface setting precluded binding of transported sediment, shearing by swift fluid flows, and burial. Following recent insights into stromatolite morphogenesis and microbial behavior, we conclude that Jordanian columns were products of biogeochemical self-organization mediated by microbes that competed for diffusing iron. These macrofossils formed in the shallow subsurface beneath an uplifting, eroding landscape—an excellent setting for both fossilization and subsequent discovery.