4.Life in ExtremeConditions IntroductionThe Early Earth had an environment with severe conditions withsurface temperatures of 70-80 degrees Celsius, low pH levels, little 02in the surrounding atmosphere, high salinity and high UV radiation exposure. Asa result, extremophile lifeforms were the only living organisms that were ableto survive the conditions. (Frances Westall, 2002) Since then, thelifeforms have evolved into multicellular organisms in an environment that sustainsa wider variety of organisms, however extremophiles still inhabit remotelocations around the world.
What are extremophiles?Extremophile lifeforms are organisms that are able to residein extreme conditions considered inhospitable to many other organisms. Oftenthese environments will have drastic temperatures, pH , pressure or salinitylevels uninhabitable for other lifeforms. Of these extremophiles there areseveral types including: thermophiles, psychrophiles, acidophiles,alkaliphiles, barophiles and halophiles. (Rampelotto, 2013)Types of ExtremophilesThermophilesThermophiles are lifeforms that are able to thrive in high temperatureconditions, with there being 3 main types of thermophiles: thermophiles,extreme thermophiles and hyperthermophiles. (Francesco Canganella, 2014) In generalthermophiles have optimal growth occurring in the temperature range of 60-108degrees Celsius.
Unlike other organisms with enzymes that denature at high heat,Thermophiles contain unique enzymes known as “extremozymes” which continue tofunction. This is due to the enzyme’s ability to preserve their 3D structure inthose temperatures. (Beal, 2018) These lifeforms areusually found in environments such as volcanic sites, hydrothermal vents, hotsprings and even man-made environments such compost piles. (Francesco Canganella, 2014) An example of a hyperthermophilewould be the “Methanopyrus kandleri” whichthrives in vents on the ocean floor at a temperature of 122 degrees Celsius. (Rampelotto, 2013)PsychrophilesOn the contrary to Thermophiles, Psychrophiles are lifeforms that residesin low temperature conditions, ranging from 20 degrees to sub zero temperatures.
For psychrophiles, the optimal growth is at a temperature of 15 degrees Celsiusand below, while minimal growth temperature would be sub zero. As a result ofthis, many adaptions are required to ensure the survival of the lifeform suchas membrane fluidity, antifreeze proteins, cold adapted enzymes, etc. The capabilityof the membrane to continue to be fluid at low temperatures is an importantcharacteristic as it allows the cells to intake nutrients. Furthermore, antifreezeproteins along with cold adapted enzymes also help Psychrophiles survive in lowtemperatures as the effects of ice crystallization is reduced along with thelifeform’s optimal temperature. (Craig L Moyer, 2017) Imagesource: http://embor.embopress.org/content/15/5/508Locations in which Psychrophiles can be found include the articpermafrost, super cooled cloud droplets and rocks within the Antarctic dryvalleys.
(Feller, 2013) Some examples ofthis type of lifeform would be Psychromonas ingrahamii and Psychrobacter arcticuswhich both have growth temperatures at -12 and -10 degrees Celsiusrespectively. (Craig L Moyer, 2017)AcidophilesAcidophilesare lifeforms that are able to settle in acidic environments, often with a pHof approximately 3 being optimal for growth. However, these organisms are notable to tolerate a low pH within their cell membrane, as a result pHhomeostasis is a crucial part of its adaption in this environment. One of it’smain adaptions would be its highly impermeable cell membrane to protons. Thiswill help maintain the internal pH of the cell as a low pH is caused by a highconcentration in protons, in this case the hydrogen atoms. Additionally,another method used to maintain a balanced pH is through pumping out excessprotons from within the cell, thus increasing the pH level. Furthermore, achemical gradient is also utilised to prevent a large intake of the hydrogenprotons. (Craig Baker-Austin, 2007) (Below is a visualrepresentation of some of these adaptations)These acidophilic lifeforms are found in acidic environments such ashydrothermal vents, sulfuric pool and geysers.
One example of this would be Picrophilaceae,an organism which has an optimal pH level of approximately zero. (Zinni, 2017)AlkaliphilesConversely, Alkaliphiles are lifeforms which are adapted to a basicenvironment with high pH levels of approximately 9. Similar to Acidophiles, theyhave a cell membrane and a chemical gradient which assists in regulating arelatively neutral pH range of 7-8.5 within the cell. Alkaliphiles are found inbasic environments such as soda deserts and lakes with one example being “Natronobacterium magadii” inLake Magadi, Kenya which has an optimal pH level of 10.
(Eissa, 2017)Barophiles Barophiles are organisms with the capability of thriving in high pressureenvironments, usually at pressures greater than 400 atm. There are two maintypes of Barophiles known as obligate barophiles for pressures of 400-500 atmand extreme barophiles for pressures greater than 500atm. (Prasanna Kondepati) Consequently, adaptationsare required for these high pressure environments, in order for these organismsto survive. A main adaptation is the reduced amount of amino acids within theproteins of Barophiles, resulting in more rigid proteins that are less likelyto deform under pressure.
Furthermore, the cell membrane is found to containgreater amount of poly unsaturated fatty acids (PUFAs), hence membrane fluidityis increased allowing for cell to function normally at these great pressures. Theselifeforms are found in areas of deep seas such as the Marianis Trench in thePacific Ocean. (Munn, 2003) An example of a Barophilewould be the bacteria “Colwellia MT41” which is able to survive in temperaturesof 8 degrees and a pressure of 103 MPa. (Prasanna Kondepati)HalophilesHalophiles are a form of extremophiles that are able to survive inenvironments of high salinity which would have otherwise cause detrimentaldamage to other lifeforms.
Because of this, certain adaptations are requiredwith there being two main strategies called the “salt in” and “osmolyte” strategy. The “salt in” involves potassiumions being pumped into the cell, as a result increasing the salt concentration.Thus this leads to a balanced concentration inside and outside the cell,preventing water loss from occurring. In contrast, the “osmolyte” methodinvolves rejecting salt from the cell but instead intaking compatible solutesthat do not affect the functions of the organisms as a high soluteconcentration is maintained. (Munn, 2003) Halophiles are foundin areas of high salt concentration such as salterns (ie Dead Sea) brine poolsand natural salt lakes . One example of a Halophile is “Haloquadratum” which isknown to colour saltern ponds pink-red.
(Yanhe Ma, 2010)Possibility of Extra-terrestrial LifeExtremophiles are lifeforms that are able to reside in theharshest conditions, that otherwise normal organisms cannot. Thus, theseadaptable organisms open up various possibilities for extra-terrestrial life. Marsbeing one of the planets that once had an environment similar to early Earth,has a considerable potential of being a habitat to extra-terrestrial life thatare similar to extremophiles.
(Frances Westall, 2002) Additionally, one ofJupiter’s moon “Europa” is also a candidate for extra-terrestrial life as itcontains all the essential elements for life, including water, organic moleculesand energy despite its harsh icy environment. (Redd, Jupiter’s Icy Moon Europa: Best Bet for Alien Life?, 2014) MarsEarlyMarsThe early conditions in Mars were hypothesised to be similar to earlyEarth with there being an abundant source of water throughout the planet whichis essential for life. However, there were also many differences, one of whichwas the high CO2 concentrationwithin the atmosphere compared to earth. This would have caused the water tobecome more acidic and theoretically could have supported acidophilic lifeforms.Additionally, due to the large amounts of shallow water that existed along withsalts, it was hypothesised the water sources had high salinity which would havebeen a habitable environment for halophiles. Moreover, early Mars was alsothought to have volcanic activity, hot springs and a greater temperature rangethan earth. Consequently, hyperthermophiles and hyperpsychrophiles may haveexisted.
From this, we are able to conclude that early Mars had the potentialto accommodate for extra-terrestrial life that were similar to the likes ofextremophiles. (Frances Westall, 2002)Present-dayMarsSince the early conditions, Mars has developed into a dry and barren planet.However, in 2015 “recurring slope lineae” also known as RSL were discovered tobe caused by salty water that had run down the slopes (as shown in diagrambelow).
(Redd, Water on Mars: Exploration & Evidence, 2017)Along with the study by the Space telescope Science Institute and University ofMaryland, where they were able to conclude halophiles are able to grow withinthe temperature range of present day Mars, we can deduce that present-day Mars maypresent a suitable environment for halophiles. This is due to the presence ofhigh salinity water and a temperature range halophiles are able to thrive in. (Could microbes survive on Mars?, 2006) Image Source: https://www.jpl.nasa.gov/spaceimages/images/largesize/PIA17727_hires.jpg EuropaJupiter’s moon, Europa has also recently been subject to theories that itwould support extra-terrestrial life.
Although it does not seem like a placefor life to thrive with a surface temperature of -160 degrees Celsius, it containsall the essential elements for life. (Howell, 2016) With the surfacebeing a frozen layer of ice, water lies below protected from dangerousradiation. Furthermore, with NASA’s Hubble Space Telescope revealing geysers inthe moon’s southern hemisphere and the theory of vents under the surface ofEuropa, the possible existence of extra-terrestrial life similar to Barophiles,Thermophiles and Psychrophiles is suggested from the high pressure environmentwith a vast range of temperatures. (Redd, Jupiter’s Icy Moon Europa: Best Bet for Alien Life?, 2014) Supplementaryevidence that supports the theory of possible extra-terrestrial life includesthe surface with similarities to the theory plate tectonics and the productionof 10 times the oxygen compared to hydrogen, both of which are conditionssimilar to Earth.
(Howell, 2016)Image Source: http://www.bbc.com/news/science-environment-38925601ConclusionExtremophiles are organisms that have been thriving since the existenceof early Earth, marking the extremities organism are able to survive in. As aresult, given our understanding of extremophiles, we have broadened ourunderstanding on the endless possibilities of extra-terrestrial life in ouruniverse.