Earth's earliest and deepest purported fossils may be iron-mineralized chemical gardens

Abstract

Recognizing fossil microorganisms is essential to the study of life's origin and evolution and to the ongoing search for life on Mars. Purported fossil microbes in ancient rocks include common assemblages of iron-mineral filaments and tubes. Recently, such assemblages have been interpreted to represent Earth's oldest body fossils, Earth's oldest fossil fungi, and Earth's best analogues for fossils that might form in the basaltic Martian subsurface. Many of these putative fossils exhibit hollow circular cross-sections, lifelike (non-crystallographic, constant-thickness, and bifurcate) branching, anastomosis, nestedness within 'sheaths', and other features interpreted as strong evidence for a biological origin, since no abiotic process consistent with the composition of the filaments has been shown to produce these specific lifelike features either in nature or in the laboratory. Here, I show experimentally that abiotic chemical gardening can mimic such purported fossils in both morphology and composition. In particular, chemical gardens meet morphological criteria previously proposed to establish biogenicity, while also producing the precursors to the iron minerals most commonly constitutive of filaments in the rock record. Chemical gardening is likely to occur in nature. Such microstructures should therefore not be assumed to represent fossil microbes without independent corroborating evidence.

Keywords: astrobiology; biomorphs; chemical gardens; fossil bacteria; fossil fungi.

Figure 1.

Photomicrographs and scanning electron micrographs of experimental iron-mineralizing chemical gardens. (a) Numerous straight and irregularly curved siliceous filaments attached to the knob-like remnants of iron sulfate seed grains less than 63 µm in diameter. (b) Siliceous filament showing rough iron (oxyhydr)oxide-coated interior with hollow central cavity. (c) Siliceous filament with the central cavity filled by iron (oxyhydr)oxides. (d) Siliceous filament with laminated wall; overlaid energy-dispersive X-ray (EDX) spectroscopy data show iron-rich innermost layers (red/yellow). (e) Multiple siliceous branching filaments radiating from a seed grain remnant; the brown colour is contributed by ferric iron. (f) Branching siliceous tube with minimal inner coating. (g) Siliceous filaments with variable yellow/brown inner coating showing branch (arrowed). (h) Anastomosing siliceous filaments; arrows indicate direction of growth. (i) Broken siliceous filament (arrowed) on the interior of a larger tube. (j) Siliceous filament with discrete swellings (arrowed). (k) Curving filaments produced from iron sulfate seed grains in sodium carbonate solution. Scale bar: (a) 200 µm; (b) 95 µm; (c) 85 µm; (d) 45 µm; (e) 300 µm; (f) 115 µm; (g) 55 µm; (h) 55 µm; (i) 73 µm; (j) 70 µm; (k) 350 µm. (Online version in colour.)

Figure 2.

Composition of chemical garden filaments showing iron (oxyhydr)oxides. (a) Raman spectra showing characteristic peaks for hematite obtained on repeat analysis of Raman laser-damaged filaments. The additional peaks at approximately 1000 cm⁻¹ in the uppermost spectrum are due to the underlying plastic Petri dish. The hematite standard shown for comparison is RRUFF 040024 [37]. (b) XRD traces showing the occurrence of diffraction peaks at angles consistent with goethite, ferrihydrite, hematite, and feroxyhyte (corresponding reference sample numbers in the Crystallographic Open Database are indicated). The low signal-to-noise ratio is due to poor crystallinity and iron fluorescence. (Online version in colour.)

Figure 3.

Photomicrographs of individual filaments grown in sodium carbonate solutions acidified to pH 12–7. The filaments illustrated are of lengths 200 µm (pH 12), 120 µm (pH 11), 120 µm (pH 10), 220 µm (pH 9), 105 µm (pH 8), and 120 µm (pH 7). (Online version in colour.)