Endolith Definition and Characteristics

• Endolith is an organism that survives in various inhospitable environments throughout the world, especially inside rocks, animal shells, coral reefs, and sand particles in the soil.
• Endoliths occupy habitats beneath and between porous and translucent rocks and minerals.
• Rock porosity provides interstitial spaces for microbial colonization and translucence enables photosynthesis to take place.
• Despite their limited water availability, cold temperature, strong winds, and large variations in solar radiation input, cold deserts harbor endolithic microorganisms.
• Microbial life can thrive, and endolithic microbial communities have been intensively studied in the Antarctic region, which is characterized by extreme climatic conditions, with low humidity and precipitation making it practically an inhospitable environment for living beings.
• In terrestrial systems, these microenvironments typically provide protection from intense solar radiation and desiccation, as well as sources of nutrients, moisture, and substrates derived from minerals.
• In marine systems, endolithic communities similarly exploit the rocky seafloors, but also dwell into limestone and mineralized skeleton of a broad range of marine animals.
• Besides tolerating the desiccating conditions and extreme temperatures, microorganisms inhabiting such arid conditions are subjected to osmotic stress due to the high salt concentrations.
• The water content of sandstone colonized by endolithic microorganisms is represented by 0.1–0.2% by weight as a result of little moisture penetrating into the rocks through pores.

Endolith Mode of adaptation

Metabolic activities

• Endoliths are capable of ‘switching’ their metabolic activities on and off in response to rapid changes in environmental conditions.
• In cold areas, the organisms are inactive in a ‘freeze-dried’ state that does not damage cellular structures.
• As the temperature increases, the small quantities of water can maintain a level of humidity that is sufficient for metabolic activity.

Biofilm formation

• Various xerophilic organisms form biofilms as it allows the survival of organisms in habitats with low moisture content.
• These biofilms consist of microbial aggregates and extracellular polysaccharides produced by those organisms.
• The extracellular polysaccharides in the biofilms are hydrophilic, which contributes to rapid water absorption rates and restoration of photosynthetic activity.

Weathering process

• Lichen and algae represent the pioneer species in such environments where the release of various enzymes results in the degradation of rock particles.
• As the weathering process continues, the organic matter of the area increases so that new groups of living microorganisms can thrive at such a climate.

Endolith Examples

• Some examples of endoliths include Leptolyngbya, Helicobacter recurvirostre, Gloeocapsa sanguine, Acaryochloris, Chroococcidiopsis, Anabaena, Spirirestis rafaelensis, etc.

Hypolith Definition and Characteristics

• Hypoliths are organisms or communities of organisms that live on the underside of rocks or at the rock–soil interface.
• Hypoliths are photosynthetic microorganisms that exist in hot and arid climates, usually at the interface between the rocks and the soil.
• The community of microorganisms present in such an area is termed as hypolithon.
• Microorganisms that are present underneath the rocks are protected from the harsh radiations of the sun and the wind.
• The rocks might even trap moisture which can then be used by these microorganisms.
• Different forms of minerals like quartz are found in soil and rocks that also supports different forms of life.
• However, there are different stresses, including low water activity and drastic changes in temperature, which limits the biodiversity of such habitats.
• The most common habitats for hypoliths include the desert lands and polar regions where the climate change is quite drastic with rapid desiccation and rehydration.

Hypolith Mode of adaptation

Biofilm

• One of the most successful means of survival of microorganisms in rock surfaces is by the production of exopolysaccharides that together microbial aggregates form biofilms.
• Biofilms help to retain moisture and also reduce the metabolic activities of these organisms, which help to save energy and nutrients.
• Biofilms also protect hypoliths against the harmful radiation and extreme heat conditions of such habitats.

Dormant stages

• In the case of low water activity, some of the hypoliths modify into dormant stages in the form of spores that help to preserve nutrients and moisture of the surface.
• The dormant stages then convert into the active forms as the climate and nutrient content of the environment becomes sufficient.
• The formation of spores and reduction in metabolic activities provide long periods of survival for many microorganisms as well as larger eukaryotes.

Hypolith Examples

• Some of the common examples of hypoliths include Nostoc, Bryum, Hennediella, Stichococcus mirabilis, Ichthyosporea, etc.