Identifying microbial life in rocks: Insights from population morphometry

Abstract

The identification of cellular life in the rock record is problematic, since microbial life forms, and particularly bacteria, lack sufficient morphologic complexity to be effectively distinguished from certain abiogenic features in rocks. Examples include organic pore-fillings, hydrocarbon-containing fluid inclusions, organic coatings on exfoliated crystals and biomimetic mineral aggregates (biomorphs). This has led to the interpretation and re-interpretation of individual microstructures in the rock record. The morphologic description of entire populations of microstructures, however, may provide support for distinguishing between preserved micro-organisms and abiogenic objects. Here, we present a statistical approach based on quantitative morphological description of populations of microstructures. Images of modern microbial populations were compared to images of two relevant types of abiogenic microstructures: interstitial spaces and silica-carbonate biomorphs. For the populations of these three systems, the size, circularity, and solidity of individual particles were calculated. Subsequently, the mean/SD, skewness, and kurtosis of the statistical distributions of these parameters were established. This allowed the qualitative and quantitative comparison of distributions in these three systems. In addition, the fractal dimension and lacunarity of the populations were determined. In total, 11 parameters, independent of absolute size or shape, were used to characterize each population of microstructures. Using discriminant analysis with parameter subsets, it was found that size and shape distributions are typically sufficient to discriminate populations of biologic and abiogenic microstructures. Analysis of ancient, yet unambiguously biologic, samples (1.0 Ga Angmaat Formation, Baffin Island, Canada) suggests that taphonomic effects can alter morphometric characteristics and complicate image analysis; therefore, a wider range of microfossil assemblages should be studied in the future before automated analyses can be developed. In general, however, it is clear from our results that there is great potential for morphometric descriptions of populations in the context of life recognition in rocks, either on Earth or on extraterrestrial bodies. | 283 ROUILLARD et AL.

An Alternative Approach for Assessing Biogenicity

“…In one of our recent studies, we explored the potential of population morphometry for evaluating biogenicity (Rouillard et al, 2019). Morphometry is the quantitative study of morphology.…”

“…4), and it may be attributed to the closest system (interstitial spaces in this example). We note that in this specific decision space, due to the small number of populations considered in the work of Rouillard et al (2019), the data available as of now for each system are plotted as one (filled) symbol, which means that it is not possible to give numeric probabilities for the sample to belong to each of the defined hypotheses/ systems. By characterizing distinct groups of populations from these systems (coming, e.g., from distinct natural locations/ experimental settings), each system could be represented by a cloud of points (see hollow symbols in Fig.…”

“…This indicates that the training sets do not encompass the heterogeneity of the systems. Data used to make the analyses are from Rouillard et al (2019). Color images are available online.…”

“…(a-c) Example populations from each of the three systems (natural microbial communities, biomorphs, and interstitial spaces in clastic rocks) compared inRouillard et al (2019). (a) Stromatolite-dwelling microbial community from Alchichica lake (Mexico).…”

“…Note the different types of correlation distinguishing the three systems. All data fromRouillard et al (2019). SD, standard deviation.…”