Prokaryotic life has dominated most of the evolutionary history of our planet, evolving to occupy virtually all available environmental niches. Extremophiles, especially those thriving under multiple extremes, represent a key area of research for multiple disciplines, spanning from the study of adaptations to harsh conditions, to the biogeochemical cycling of elements. Extremophile research also has implications for origin of life studies and the search for life on other planetary and celestial bodies. In this article, we will review the current state of knowledge for the biospace in which life operates on Earth and will discuss it in a planetary context, highlighting knowledge gaps and areas of opportunity.

The ability of microorganisms to withstand long periods with extremely low energy input has gained increasing scientific attention in recent years. Starvation experiments in the laboratory have shown that a phylogenetically wide range of microorganisms evolve fitness-enhancing genetic traits within weeks of incubation under low-energy stress. Studies on natural environments that are cut off from new energy supplies over geologic time scales, such as deeply buried sediments, suggest that similar adaptations might mediate survival under energy limitation in the environment. Yet, the extent to which laboratory-based evidence of starvation survival in pure or mixed cultures can be extrapolated to sustained microbial ecosystems in nature remains unclear. In this review, we discuss past investigations on microbial energy requirements and adaptations to energy limitation, identify gaps in our current knowledge, and outline possible future foci of research on life under extreme energy limitation.

Abstract In the past few years, surface drillings have expanded our perspective of the Earth’s deep biosphere from the terrestrial and oceanic realms to include deep dark subterranean and subseafloor environments. Conditions in the deep biosphere approach the limits for life, but scientific expeditions undertaken for deep subsurface life suggest that these biospheres are hosting a large fraction of microorganisms; living in water, occupying pore space, or colonizing mineral and rock surfaces. Microbes in the deep biosphere are phylogenetically different from most cultured taxa and represent novel microbial assemblages with novel metabolic capabilities. Thus, investigating these subsurface terrains, which are the deepest and potentially the largest accessible ecosystem on Earth, provides a unique opportunity for obtaining direct insights into the deep microscopic world. The insights obtained from deep biosphere expeditions are also likely to facilitate and inform exobiological exploration. This chapter describes current understanding of the subsurface depth and environmental limits of our planet’s life processes and factors that potentially define diversity/distribution of life in the deep terrestrial crustal system. The chapter also highlights the microbiological studies from the selected deep terrestrial biospheres such as hydrothermal vents, oil reservoirs, caves, and subsurface mines.

Seafloor hydrothermal system and deep subsurface are of great interest for microbiologists as paradise of unusual lives so-called “Extremophiles” in this planet. Such peculiar microorganisms have been believed to be minority in the earth throughout the long history after the early evolution of life. Recent investigations for microorganisms present in the active hydrothermal seafloor and subsurface have revolutionized the concept. Ubiquity, predominance and diversity of extremophiles in the present and past global environments signify the unresolved, but significant role in the co-evolution of earth and life. In this article, we summarize the expeditions for the microbial world in the seafloor hydrothermal system and deep subsurface and shed light on the foci of the future investigation.

including soils. The organisms in this zone are sometimes referred to as intraterrestrials. A subset of the deep biosphere found at depths where pressure and The deep biosphere is the part of the biosphere that resides below the first few meters of the land surface and seafloor. It extends 10 km (6.2 mi) below the continental surface and 21 km (13 mi) below the sea surface, at temperatures that may reach beyond 120 °C (248 °F) which is comparable to the maximum temperature where a metabolically active organism has been found. It includes all three doma T…

and the first multicellular organism found at deep subsurface levels. A previously known species found at similar depths in the same study was Plectus Halicephalobus mephisto is a species of nematode, among a number of other roundworms, discovered by geoscientists Gaetan Borgonie and Tullis Onstott in 2011. It was detected in ore recovered from deep rock fracture water in several gold mines in South Africa 0.9 km (0.56 mi), 1.3 km (0.81 mi), and 3.6 km (2.2 mi) under the surface of the Earth. Onstott said that "it scared the life out of me when Halicephalobus mephisto is a species of nematode, among a number of other roundworms, discovered by geoscientists Gaetan Borgonie and Tullis Onstott in 2011. It was detected in ore recovered from deep rock fracture water in several gold mines in South Africa 0.9 km (0.56 mi), 1.3 km (0.81 mi), and 3.6 km (2.2 mi) under the surface of the Earth. Onstott said that "it scared the life out of me when I first saw them moving", and explained that "they look like black little swirly things". The finding is significant because no other multicellular organism had ever been detected farther than 2 km (1.2 mi) below the Earth's surface. It is named after Mephistopheles, the Lord of the Underworld in the Faust story, and alludes to the fact it is found so deep under the Earth's surface. Research has shown that Halicephalobus mephisto possesses special heat-shock and stress-response genes that allow it to survive over 3 kilometers below Earth's surface, in high temperatures and low oxygen conditions. Halicephalobus mephisto is resistant to a temperature as high as 37 °C (higher than most terrestrial nematodes can tolerate), it reproduces asexually, and feeds on subterranean bacteria. According to radiocarbon dating, these worms live in groundwater that is 3,000–12,000 years old. The worms are also able to survive in waters with extremely low levels of oxygen, lower than one percent of the level of most oceans. This nematode is able to thrive in such extreme conditions due to its adaptations to the environment including changed cellular respiration pathways to facilitate survival in low levels of oxygen, an expansion in their stress-response gene families such as Hsp70 to aid in the protections against thermal damage, and its modified cytochrome c oxidase enzyme that helps stabilize respiration and elevated temperatures. It is the deepest-living animal ever found, able to withstand heat and crushing pressure, and the first multicellular organism found at deep subsurface levels. A previously known species found at similar depths in the same study was Plectus aquatilis. Borgonie said that the worm was similar to the detritus-feeding species found on the surface, and probably…

Artificial lakes formed from past mining activities represent unique but underexplored ecosystems that support diverse microbial communities. This study examined how seasonal variation and depth influence bacterial, archaeal, and microeukaryotic assemblages in the stratified water column of the Blackburn mine (Outaouais, Quebec, Canada). Water and biofilm samples were collected by technical divers from the surface to 52 m during spring, summer, and autumn of 2021-2022, and analysed by 16S/18S rRNA gene sequencing. Seasonal changes had little effect on physicochemical parameters but strongly shaped microbial community composition, together with depth. Archaeal taxa displayed greater stability across depths compared to bacteria and eukaryotes. Oxygen profiles defined three ecological zones: an oxic layer dominated by Actinobacteria and the methanogen Methanosarcina; a transition zone enriched in Chlorobium and methanogens such as Methanospirillum and Methanosaeta; and an anoxic layer containing sulfur-reducing (Desulfomonile and Desulfobacca), sulfur-oxidizing (Sulfuricurvum), and methane-cycling archaea. Eukaryotic communities included algae, particularly Chrysophyceae, and diverse protists. These findings suggest that microbial communities in the mine are integral to sulfur and carbon cycling, emphasizing the ecological significance of such stratified, mining-associated aquatic systems. The Blackburn mine provides valuable insight into how anthropogenic legacies shape microbial diversity and ecosystem functioning in artificial aquatic environments.