Extremophiles: applications and roles in environmental sustainability

Abstract

Microorganisms are the most ubiquitous living entities on Earth. They are also the most diverse organisms, present in about every nook and corner of the blue planet. As per estimates only about 1% of the total microorganisms present on Earth have been discovered so far and there still are unexplored habitats where these omnipresent organisms may be present. Extremophilic microorganisms are a largely unexplored group that have the abilities to thrive in extreme conditions. These microbes have made useful adaptations in their genetic and metabolic machinery to thrive in the hostile conditions. Some of these microbes represent the very ancient life forms which must have thrived on Earth billions of years ago when the conditions were very different. In fact studying these microbes helps to find out how the evolution of life occurred on Earth and what type of conditions prevailed when life came on the planet. Extremophiles are categorized according to conditions in which they grow. Thermophiles/hyperthermophiles can be found in habitats with high or very high temperatures such as hydrothermal vents, volcanic sites, hot springs; psychrophiles flourish in cold habitats such as polar regions, on the mountains at high altitudes, deep inside the oceans; barophiles which love high pressure conditions are mainly found deep inside the oceans or sea; acidophiles at pH less than 5.0 or even near to 0.0, including habitats such as acid mine drainage sites, acidic lakes; alkalophiles thrive at highly alkaline pH such as in sodic lakes; halophiles love very high salt concentrations such as in sea, saline alkaline lakes; xerophiles can prosper in conditions with very low water availability which include deserts; metallophiles can tolerate and grow in presence of high concentrations of metals/heavy metals and there also are some other extremophilic forms which can even survive and grow in presence of high concentration of nuclides, pollutants such as poly hydroxyl alkanoates (PAHs), pesticides etc., and can be termed as pollution loving microbes which have evolved mainly due to anthropogenic activities. Sometimes, multiple stresses are present in the niche simultaneously and extremophiles which are able to thrive in such habitats are defined as polyextremophiles (such as thermoacidophiles, haloalkaliphiles and so on). There are innumerous examples of these fascinating organisms which have been discovered now and these include primarily prokaryotes (bacteria and archaea) and some eukaryotes (algae, yeast and fungi). Hyperthermophilic archaea Pyrolobus fumarii and Geogemma barossii have been reported with ability to tolerate temperature up to 121 °C. Another hyperthermophilic archaeon Methanopyrus kandleri strain 116 from deep sea hydrothermal field has been reported to withstand highest recorded temperature of 122 °C for any life form on Earth. Amongst psychrophiles, Psychrobacter cryopegellain isolated from Polar Regions of Siberian permafrost, Russia can survive up to − 20 °C. Thermophilic methanogens, Methanocaldococcus jannaschii and Methanothermococcus thermolithotrophicus are examples of barophilic microbes which have been isolated from high pressure niches of deep sea beds. Likewise, halophiles have been isolated from Great Salt Lake and Dead Sea. Halarsenatibacter silvermanii from a salt lake in USA is reported amongst the most halophilic microbe discovered and can survive in salt concentration of approximately 35%. Thiobacillus, Sulfolobus and Thermoplasma are some common genera that are considered to be very acidophilic in nature. Recently, Picrophilusoshimae and Picrophilustorridus have been included as the most acidophilic archaea from hot spring in Noboribetsu, Japan, which can grow at pH as low as 0.06. Haloalkalophilic Halomonas campisalis reported from Soap Lake, USA can grow at pH up to 12. Metallophiles which are able to resist high concentrations of heavy metals such as cadmium, cobalt, copper, lead, mercury, nickel, zinc and so on, have been isolated from volcanic areas, geothermal and hydrothermal vents and from industrially polluted sites. Xerotolerant and radiation * Naveen Kumar Arora nkarora.bbau@gmail.com