Early Archean planktonic mode of life: Implications from fluid dynamics of lenticular microfossils

Abstract

Lenticular, and commonly flanged, microfossils in 3.0–3.4 Ga sedimentary deposits in Western Australia and South Africa are unusually large (20–80 μm across), robust, and widespread in space and time. To gain insight into the ecology of these organisms, we performed simulations of fluid dynamics of virtual cells mimicking lenticular forms of variable sizes, oblateness, flange presence, and flange thickness. Results demonstrate that (a) the flange reduces sedimentation velocity, (b) this flange function works more effectively in larger cells, and (c) modest oblateness lowers sedimentation rate. These observations support interpretations that the lenticular microbes were planktonic—a lifestyle that could have been advantageous in an early Earth harsh environment including violent volcanic activities, repeated asteroid impacts, and relatively high UV‐radiation. Although the robustness of these organisms could have provided additional protection on the early Earth, this architecture may have impeded a planktonic lifestyle by increasing cell density. However, our data suggest that this disadvantage could have been compensated by enlargement of cell volume, which could have enhanced the ability of the flange to slow sedimentation rate, especially if coupled with vacuolation. The results of this simulation study may help to explain the unique morphology and unusually large size of these Archean microfossils.

Microfossils with tail-like structures in the 3.4 Gyr old Strelley Pool Formation

“…Previous reports of 3.4-3.0 Ga lenticular microfossils exhibiting a flange were interpreted as demonstrating passive motility of microbial planktons drifting depending on their surrounding environment (House et al, 2013;Sugitani et al, 2015;Oehler et al, 2017;Kozawa et al, 2019).…”

Microbial Diversity and Biosignatures: An Icy Moons Perspective

“…The nearequant morphologies of lenticular microfossils, together with the flange-like appendages that characterise their equatorial regions, have been used as specific evidence for their having a planktonic stage in their life cycle (Sugitani et al 2007(Sugitani et al , 2015Oehler et al 2017). Fluid dynamic modelling of virtual flanged cells has demonstrated both that the presence of the flange reduces sedimentation velocity and enlarges cell volume, two factors increasing their propensity for suspension and dispersion as part of a planktonic lifestyle (Kozawa et al 2018). Dispersion may further be inferred from the widespread distribution of these fossils in space, i.e.…”