Could life exist on Mars?What identifies as a living organism?There are seven life processes or characteristics to determine whether something is living or non-living (1):GrowthReproductionRespirationMovementAdaptabilityNutritionExcretionHowever, due to the nature of the topic question, 3 more characteristics can be used to establish a living organism:Cellular OrganisationResponse to Stimuli (2)Energy Use (2)Using scientific evidence from fossils & isotope dating, life on Earth began approximately 3.5 billion years ago (3). To this current day, life on Earth has parted into various types of organism species, which are organised into 6 kingdoms:BacteriaProtozoansAnimalsPlantsFungiArchaeaIn relation to the topic question, the three examples which will be analysed are archaea, bacteria and fungi.Life in extreme places on Earth/Abiotic FactorsEarth’s biosphere has evolved for more than 3 billion years, shielded by the protective blanket of the atmosphere (4) mostly composed of nitrogen [77%] and oxygen [21%] (5).The continuous cycle of life on any planet is dependent upon a stream of energy from a source (in Earth’s case, the Sun); the cycling of critical elements (in Earth’s case, carbon and water); and gravity, preventing atmospheric gases from leaking into space (6).The surface of the Earth is heated utilising the energy from the Sun. This energy is transferred to the atmosphere by means of conduction and radiation (7). Regardless of the time of day, the atmosphere near Earth’s surface keeps absorbing energy. This keeps the temperature of the Earth relatively stable for life.In most places on Earth, the surface and the air above it resides in a temperature range that supports life (7). However, due to Mars’ cold and barren surface, only organisms on Earth that survive extreme abiotic factors could endure Mars’ climate. Types of abiotic factors include (8):SunlightTemperatureMoistureWind & water qualitySoil quality / Nutrient availabilityAcidityHumidityEnvironments that contain hostile abiotic factors are considered to be extreme. Some extremes abiotic factors on Earth include:TemperatureSalinityRadiationpHPressureEnvironmental features of Mars that could impact lifeMars is the fourth planet in our Solar system, characterised as a cold, dry, desert landscape of sand and rocks (9). Average daily temperatures on Mars vary from -60oC to -126oC (10). It has a very thin atmosphere composed mostly of carbon dioxide [95%] and nitrogen [3%] (11). The average atmospheric pressure on Mars is less than 1% of Earth’s (5), simply meaning a human being could not survive the environment without a spacesuit. In addition, Mars’ atmosphere lacks a protective ozone layer, which allows much of the Sun’s UV radiation to affect the planet’s surface (5). Possibly as a result, Mars’ soil pH level was approximately 8-9 (12), slightly higher than Earth’s 6-7.Billions of years ago, researchers believe that Mars had water on its surface. Today, the atmosphere of Mars is too dry for clouds to form but below the surface, a wealth of ice lies intact. These polar caps have been studied since the 17th century and are assumed to be composed of some combination of H2O and CO2 ice (13).[Picture Source: (13)]Despite these adversities; Could archaea, bacteria and fungi still thrive on Mars?ExtremophilesOne organism thought to be adaptable enough for Mars’ conditions is an extremophile. Extremophilic organisms are primarily prokaryotic with few eukaryotic examples (14). Since they are very small, extremophiles fall into the archaea and bacteria kingdom. As a consequence, extremophiles don’t make good eukaryotic organisms. A key feature to their survival is they specifically adapt to unusual limits of one or more abiotic factors in the environment including temperature, pH, high salinity, high levels of radiation and high pressure (15).There are many types of extremophiles (16):NameDescriptionPsychrophilesCapable of growth & reproduction at temperatures ranging from 0-200CHalophilesThrives in high salt concentrationsAlkaliphilesCan survive in high pH environmentsThermophilesCan survive extreme temperatures ranging from 41-1220CBarophilesThrives at high pressures (e.g. 11km below sea level)EndolithsCan survive deep within solid rock; Can survive for hundreds of years by feeding on trace amounts of iron, potassium and sulphur found in the rocks they inhabit (16)Radioresistant MicrobesCan survive in extreme radiation levelsAcidophilesCan survive in extremely low pH environmentsXerophilesCan survive in conditions with low water availabilityLittle is known about extremophile evolution other than tracking the 16S ribosomal RNA sequences:[Picture source: (17)]Using this, scientists were able to identify the first extremophile to have its own genome sequence: Methanococcus jannaschii, a type of barophile and archaeobacterium (18). This could mean that extremophiles are forever adapting to its environment over time.CyanobacteriaCyanobacteria are an essential part of Earth’s long history, having been known as part of the oldest known fossils, more than 3 billion years ago as an aquatic and photosynthetic bacterium. They are a gram-negative bacteria that have no nucleus or internal membrane systems. Being bacteria, they are generally very small and unicellular. However, they can grow in large colonies called Cyanobacterial Blooms which are large enough to be seen from the naked eye. The total estimated number of cyanobacteria species are 2000-8000. They range from 1?m to 40?m in diameter and has a cell wall structure similar to gram-negative bacterium.Cyanobacteria paved the way for life on Earth, being responsible for pumping oxygen into the atmosphere (19). The oxygen atmosphere that life depends on today was generated by various cyanobacteria during the Archaean and Proterozoic Eras (20) around 2.5 million years ago. Before then, the atmosphere surrounding the planet was unsuitable for life. Cyanobacteria is also responsible for endosymbiosis in the origin of plants as well as the origin of the eukaryotic mitochondrion (20). On the contrary, Cyanobacteria were also the cause of the first mass extinction on Earth. Around 2 billion years ago, the majority of Earth’s bacteria were anaerobic, metabolizing their food without the use of oxygen. This changed when a new strand of cyanobacteria appeared; adopting photosynthetic abilities, converting sunlight into energy and produce oxygen as a waste product (21). The Earth’s atmosphere back then didn’t have free oxygen like today’s atmosphere; instead it was locked up in water molecules or bonded to iron in minerals (21). As cyanobacteria thrived, other minerals became saturated and could no longer keep up with the oxygen production, unbalancing Earth’s atmosphere. The free oxygen was toxic to anaerobic bacteria, which resulted in a mass extinction known as the “Great Oxygenation Event”.LichenA lichen is not a single organism, it is a symbiosis between different organisms – a fungus with an algae or cyanobacterium (22). They can colonize various surfaces and can often be found in places where few other macroscopic living things can survive (23). The first recorded existence of Lichen dated back to around 400 million years ago. They range from 1mm to 3m in diameter.Anatomically, the fungus part of the Lichen forms the essential framework that combines the cyanobacteria or algae cell. The fungal cell then provides protection, structure, water storage and mineral-gathering capability to the partnership (24). The cyanobacteria or algae cell provides photosynthetic features. Together, this combination can survive extremely harsh conditions.Lichens are one of three main growth forms:NameDescriptionExampleFolioseA cross between Fruticose and Crustose, they can be very flat, leafy like lettuce, or convoluted and full of ridges and bumps (25)Xanthoparmelia substrigosaFruticosePresented as pendant and hair-like, upright and shrubby, or upright and cup-like (22). Distinctly 3-DUsneaCrustoseThey form a crust over a surface, pressed against their substrates. Distinctly 2-D Caloplaca[Picture Sources: (22)]MethanogensMethanogens are prokaryotic microorganisms that produce methane as part of its metabolic processes. They are anaerobic, unicellular and are part of the archaea kingdom.  Methanogens are made primarily in two shapes, cocci and rods (26). Not needing oxygen, they use hydrogen and carbon dioxide as their energy source, meaning they can often make their homes in extreme environments (10). They are found most frequently in anaerobic freshwater environments but have also been found on multiple occasions in extreme environments, where they thrive at temperatures above 1000C (26).A specific type of methanogen called Methanosarcina soligelidi is said to be one of the toughest microbes in the world. It lives in the permafrost soils of Samoylov Island in Siberia (10), where temperatures drop below -500C, has a very dry climate and the soils remain permanently frozen (10). It can withstand up to 14 times more UV radiation, and 47 times more ionising radiation than any another species of methanogen (10).                                                                                                               [Pic Source: (26)]Could these Earthly organisms exist on Mars?After collecting information about the types of organisms that could potentially inhabit Mars,“Methanogens are perfect candidates for life on Mars, as they don’t need light, oxygen, or organic nutrients to survive” (10).ConclusionWith such unequivocal evidence that Mars was once habitable, the planet is an ideal location for microbes to exist (27).Referencesx1.Science Learning Hub. Characteristics of living things. [Online].; 2007 [cited 2018 10 20. Available from: https://www.sciencelearn.org.nz/resources/14-characteristics-of-living-things.2.Michelangeli D. Phoenix Mars Mission: Properties of Life. [Online].; 2009 [cited 2018 10 21. 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