The Precambrian Volyn biota, Ukraine – example for a continental micro-ecosystem of the deep biosphere

The Volyn biota, a distinct example of 3D-preservation of ~1.5 Ga old Precambrian fossils from huge cavities in granitic rocks as a habitat for microorganisms in the deep biosphere, consist of several different types of organisms. Most common are filaments of variable diameter (10 µm-range to 100 µm-range) and length up to cm. Branching, segmentation, outgrowths, and a central channel are typical. Ends can be simple rounded, thinning out, or with outgrowths. Filaments grew on mineral substrate (?sessile organisms); filaments with two ends indicate growth in soft clay media or floating in water. Spherical objects in the size of tens of µm could represent spores or other types of seeds. For larger objects, some with conical shape, in the 100 µm-range, no recent analogue is obvious. Bi(S,Te) nm-minerals in filaments, either irregularly distributed or in rod-like arrangement, have an analogue in recent fungi, but also in other microorganisms. Objects with flaky to irregular shape are reminiscent of a former biofilm. Filaments are also partly covered with a sheath, interpreted as extracellular protein substance. d 13C/12C-isotope values of approximately -40 ‰ to -50 ‰, typical for methanogenic bacteria, and a range of d 15N/14N-isotope values of +2‰ to +9 ‰ also indicate different organisms.

The crucial fact for the excellent 3D-preservation of the microfossils is their occurrence in cavities of igneous rocks, which preserved them from further diagenetic-metamorphic overprint as common in sediments. These microfossils give an impressive insight into Precambrian life in the deep biosphere.