Endolithic Microorganisms and Their Habitats

Abstract

Endolithic microorganisms are widespread in desert biomes, where hostile environmental conditions limit the majority of life to rock habitats. In these habitats, microorganisms receive light for photosynthesis, moderated and warmer temperatures, protection from UV radiation, and prolonged exposure to liquid water. In general, these microbial communities are composed of phototrophic microorganisms as well as fungi and heterotrophic bacteria. Microbial composition is distinct from soil communities, suggesting these habitats select for microorganisms best suited to this environment. The habitat is not nutrient limited, which explains why these microbial communities colonize a wide range of lithic substrates with different mineralogies; however, greater environmental pressures select for those able to tolerate increasingly harsh conditions. Growth rates vary primarily as a function of moisture availability, resulting in long-lived communities in the driest deserts. While most microorganisms require liquid water for growth, some lichens with an algal phycobiont can photosynthesize with water vapor alone, a significant advantage in these water-limited biomes. Additional strategies against stress include synthesis of pigments, EPS, and osmoprotectants, which significantly offsets the growth of biomass. Microbial activity leads to physical and geochemical weathering, but can also result in stabilization of the lithic habitat. Identification of endolithic biosignatures and microbial fossils has resulted in their study from an astrobiological perspective in the search for life on other planets.