A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

Abstract

Diverse micro‐organisms populate a global deep biosphere hosted by rocks and sediments beneath land and sea, containing more biomass than any other biome except forests. This paper reviews an emerging palaeobiological archive of these dark habitats: microfossils preserved in ancient pores and fractures in the crust. This archive, seemingly dominated by mineralized filaments (although rods and coccoids are also reported), is presently far too sparsely sampled and poorly understood to reveal trends in the abundance, distribution, or diversity of deep life through time. New research is called for to establish the nature and extent of the fossil record of Earth's deep biosphere by combining systematic exploration, rigorous microanalysis, and experimental studies of both microbial preservation and the formation of abiotic pseudofossils within the crust. It is concluded that the fossil record of Earth's largest microbial habitat may still have much to tell us about the history of life, the evolution of biogeochemical cycles, and the search for life on Mars.

Microbe‐Mediated Extracellular and Intracellular Mineralization: Environmental, Industrial, and Biotechnological Applications

“…Organic components (biosignatures) associated with biominerals are thought to play important roles in prebiotic and biotic reactions. [ 374 ] In astropaleontology, planetary sites with enriched silica are believed to be the most appropriate locations to search for extraterrestrial lifeforms. The opaline silica related with microbe silicification has become a promising target to identify potential habitability beyond what has been identified on earth, such as in Mars.…”

The fossil record of igneous rock

“…The establishment of the fossil record of the igneous crust as a relevant research field within paleobiology, geobiology, and oceanography is of prime significance. A number of current publications highlight the importance and previous neglect of this fossil record (Ivarsson et al, 2015a, 2015c, 2015d, McMahon and Ivarsson, 2019Onstott et al, 2019) but more work is needed to increase the awareness of the scientific community. More research is crucial to understand the full complexity of these fossils, and their global significance.…”

“…nuclear waste repository research, or the establishment of international deep drilling programs such as ICDP and DCDP, later ODP and IODP (Table 1). A number of reviews cover both the continental and the oceanic deep realm, the latter with emphasis on sediments (Orcutt et al, 2011;Heim, 2011;Colwell and D'Hondt, 2013;McMahon and Ivarsson, 2019). Abundance and diversity is usually described as high, and both prokaryotes and eukaryotes including fungi, protists and nematodes have been identified (Orcutt et al, 2011;Heim, 2011;Colwell and D'Hondt, 2013;Sohlberg et al, 2015;Borgonie et al, 2015;Lopez-Fernandez et al, 2018a).…”

“…Whereas the latter is based on heterotrophy and exploitation of refractory organic matter, the former is based to a much greater extent on autotrophy (Magnabosco et al, 2018;Bar-On et al, 2018). This makes the fossil record of the igneous crust unique within paleobiology and highly significant to understand past life (Onstott et al, 2019;Ivarsson et al, 2019a;McMahon and Ivarsson, 2019). We are convinced that deep igneous settings can provide crucial clues to the early evolution, past and present biogeochemical cycles, and abundance and diversity of life on Earth but also beyond.…”

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

“…These microstructures range from 1 to approximately 50 µm in diameter and up to several millimetres in length, and show complex morphological features taken to indicate a high probability that they are mineral-encrusted microorganisms, including strongly curved growth trajectories, circular cross-sections, discrete spore-like swellings, true bifurcate branching and anastomosis (cross-linking or convergence of adjacent branches), hollowness, and nestedness [1,2,4,[6][7][8][9][10][11][12][13][14][15]. These structures have long been of palaeobiological and astrobiological interest, particularly as evidence to inform the search for ancient subsurface life on Mars [7,8,[16][17][18]. Precambrian examples have recently been presented as candidates for Earth's oldest fossils [1] and Earth's oldest fossil eukaryotes (fungi) [6].…”