Living on the Edge: Biodiversity, Adaptation, and Evolution in Extreme Groundwater Habitats

Subterranean ecosystems are characterized by total darkness, low levels of oxygen, relatively stable environmental conditions and limited of energy sources, resulting in “extreme” living conditions compared to those at the surface. These conditions, challenging the existence of life forms, are further exacerbated in peculiar cave ecosystems, namely, those found in sulfidic, volcanic caves with gas emission, and ice caves. The former types have high levels of hydrogen sulphide, methane, and ammonium, while in the latter, the presence of perennial ice deposits causes constantly negative temperatures. In all these caves the apparent reduction of primary production and the lack of photosynthetic basal trophic levels in subterranean food webs constrain overall biodiversity and upper trophic levels (predators) due to food and energy shortages. Consequently, the species able to survive and reproduce have unique morphological, metabolical, physiological and ecological traits. These species, called extremophiles, are recognized as an example of “life at extremes”, being highly adapted to the environment in which they live. 

The unique morphological and ecological traits of these extremophiles result in the peculiar underground habitats they inhabit to be remarkable evolutionary hotspots, thus ideal subjects to answer long-standing questions concerning the interplay between adaptation and evolution. Furthermore, subterranean biodiversity in these systems is described as functionally truncated. Information on such particular ecosystems will enhance our knowledge of life forms living in extreme conditions, establish life limits, and understand the capacity of life to withstand and adapt to change. These ecosystems are more and more increasingly recognized for their unique fauna not found in surface ecosystems and of which special adaptations make them vulnerable to external pressures reflected with time-lag in the subterranean realm. 

In this Research Topic, we welcome studies on biodiversity, adaptation, and evolution of biota and (meta)communities including bacteria and invertebrates from extreme subterranean environments including aquifers, sulfidic, volcanic and ice caves and springs, and lava tubes that can inform about the relationships among the composition, distribution, and diversity of species and communities and the environmental conditions. The studies should aim to document the extent and nature of evolutionary convergence across distinct lineages of organisms and determine to what degree natural selection is the driver of the extreme modifications observed in species and, finally, enhance our knowledge on life forms, to establish life limits and to understand the capacity to withstand and adapt to change.