Life Preserved In The Extreme Environment And Their Astrobiological Implications

Astrobiology is an interdisciplinary scientific field which is to study of the origin,evolution,and distribution of life in the context of cosmic evolution.It includes to study the habitability in the Solar System and beyond.Besides Earth,Mars is the best candidate where life exists or once existed in the Solar system.Evaporites including sulfates,phyllosilicates,carbonates,and putative chlorides have been detected on modern Mars.Generally,hypersaline and arid environment on Earth are evolved from continuing evaporation of lakes which contributes to evaporites sedimentation.It records the regional geological and climate history and provides habitats from microbial life.Evaporites on the Earth are proved to be good materials for life traces’preservation.In this consideration,the detection of Martian evaporites is significant on exploring Mars potential life.Similar to Mars evaporites condition,hyper saline and arid environment,such as playa and desert,are suitable for Mars analogue study.Land forms in Qaidam Basin are similar to kinds of those on Mars,including playa,gullies,Yardangs and sand dunes.It has been listed as research site for comparative study with Mars.Dalangtan Playa in north-western Qaidam Basin is evolved from ancient lakes which were dried out because of climate changing and tectonic movement.Tertiary and Quaternary sedimentary strata occur in Dalangtan playa.This region is covered by salt（sulfates and chlorides）crusts.The elevation there is between 2700 and2900 m.The average monthly relative humidity（RH%）ranges from 23 to 36%in this region.The Rio Tinto river in south of Spain is acidic because of microbial mediated activities since 6 million years ago.A large mount of iron oxides and sulfates（including jarosite）deposited.Similar minerals are found in Meridiani Planum on Mars which is also thought to have an acidic water history in Noachian.In this study,Dalangtan playa is studied as modern and arid Mars analogue,whereas the Rio Tinto River is studied as ancient and wet Mars analogue.In one side,we collected salt crusts in the Playa as well as sediments from two profiles with depths6-7 m.In order to clarify the geological evolution in recent history,mineral analysis and physio-chemical parameters measurement was carried out.Meanwhile,we studied the microbial diversity using both culture and culture independent methods.This is helpful to explore potential life in modern Mars evaporitic condition.In another side,ToF-SIMS（Time-of-Flight Secondary Ion Mass Spectrometry）technique was used for analyzing organic compounds of ancient sediments from the Rio Tinto river.The sediments are 2.10 million years old and are mainly composed of hematite and goethite.The distribution and origination of the preserved organic compounds were described for explain life forms living under acidic condition.It helps for the studying of life preservation under ancient Mars acidic watery environment.The recent progress of this study is as follows:1)Based on the physical and chemical properties of surface sediments and profile samples of Dalangtan Playa,it is confirmed as analogue for Mars evaporitic environment.According to mineralogical analysis,there are mainly two kinds of sedimentary composition on the surface of Dalangtan playa:1)salt crusts composed of chlorides,gypsum and a small amount of thenardite,and 2)a mixture of evaporative minerals and detrital particles such as feldspar and quartz.Surface sediments are alkaline with a pH ranging from 7.95 to 8.90.The mineral composition of the two subsurface profiles（P1,5.90 m and P2,6.85 m）reflects more than one lake sedimentary sequence under different climatic conditions,which shows cyclic sediments with clay-carbonate-sulfate and chloride as the main components.The sediments in the subsurface are more alkaline than the surface sample.The pH values of P1 and P2 are 7.93-9.05 and8.25-9.48,respectively.Surface of Dalangtan playa is extremely arid and is exposed to strong ultraviolet radiation.The moisture content of samples below the surface varies greatly at different depths.The moisture content of P1 and P2 is 0.33-5.14%and0.08-12.45%,respectively.Due to the different proportion of soluble salts,the salinity of samples at different depths also changed significantly,which is 0.4-50%and 5-50%respectively.2)47 strains of bacteria and 5 strains of fungi were isolated from Dalangtan playa.This enriched the diversity of culturable microorganisms in Dalangtan playa and provided valuable experimental materials for further studying about the adaptability of microorganisms to extreme environments.By means of culture method,47 strains of bacteria and 5 strains of fungi were obtained from 22 underground samples（23 in total）,and 3 strains of bacteria and 2strains of fungi were obtained from 3 surface samples（7 in total）by using three kinds of media and a series of salinity gradients.The bacteria isolated from the profile sample mainly belonging to Bacillus of Firmicutes which contains 25 strains and is potential dominant species.Other strains include:Oceanobacillus,Halobacillus,Gracilibacllus,Thalassobacillus,Sediminibacillus,Ornithinibacillus and Pontibacillus beloing to Firmicute phylum,and Microbacterium and Nocardiopsis genera affiliated within Actinomycetales and Streptosporangiales orders respectively belonging to the Actinobacteria phylum.Three moderate halophilic bacteria were isolated from surface samples,belonging to Oceanobacillus,Gracilibacllus and Thalassobacillus.Fungal isolates from subsurface sample showed high homology with members of the Aspergillus and Penicillium genera of the Eurotiales order and the Cladosporium genera affiliated within the Capnodiales order,all of them of the Ascomycota phylum,which is the most representative of the xerophilic fungi.The two fungal strains isolated from surface samples were belong to Aspergillus genus.3)CAtalyzed Reporter Deposition-Fluorescent in situ Hybridization（CARD-FISH）and laser confocal microscopy were firstly used to analyze the subsurface samples of Dalangtan playa.Firmicutes account for 76.6%and 75.7%of the total bacteria in the studied two profiles,i.e.P1 and P2 respectively.CARD-FISH is carried out as a way to reveal more microbial diversity and to verify the results from culturing.According to CARD-FISH:in the subsurface sediments of profile P1,bacteria accounts for（1.54±0.49）×10⁵ g^-1,and in the profile P2,it accounts for（3.22±0.95）×10⁵ g^-1.Among all the bacteria,Firmicutes account for 76.6%and 75.7%,respectively.Confocal microscopy images of bacterial colonies were obtained.The total DNA was extracted from the environmental samples,with an extremely low DNA concentration which is between 1.0-5.0 ng/μl.DNA amplification were conducted with bacterial primers（27F/1492R and s-d-bact-0341-b-s-17/s-d-bact-0785-a-21）,archaeal primers（Arch1R/Arch1F）and fungal primers（563F/1132R）,however no effective amplification products were obtained.The extreme harsh condition of Dalangtan is very critical for the survival of microorganisms.The study of microbial diversity reveals the resistance of spore-producing microorganisms to high salinity,ariity and even high radiation,which also provides enlightenment for the exploration of life on Mars.Surface of modern Mars is arid,of high radiation and lack of oxygen,conditions for the emergence of life maintain huge challenge.However evaporitic regions,especially below the surface of Mars,still has certain potentiality for searching life trace.Because 1)translucent surface of salt crust can filter part of the solar radiation,protects UV damage from（possible）organic life;2)Deliquescent salt is easy to absorbe water vapor to produce a microenvironment suitable for life.As a result,the search for life on Mars could focus on shallow subsurface of Mars enriched in salt minerals.4)Abundant organic compounds have been identified in the sediments of Rio tinto terrace,providing an important reference for studying molecular fossils in similar environments on Mars.The ancient terrace sediments（2.10 Ma）in the acidic environment of the Rio Tinto River preserved many molecular fossils.The relative abundance and origin of chain hydrocarbons,aromatic compounds,terpenoids and fatty acids were identified by ToF-SIMS.Molecular fragments from terpenoid compounds may be produced by higher eukaryotes（such as plants）.Short chain n-alkanes come from the degradation or post-deposition process of microbial films.Olefin groups may be involved in various biochemical processes,such as the degradation of fatty acids dueing to decarboxylation from plant and microbial waxes.The diagenetic process of terrace deposition might be involved in hydrothermal activity,or there might be temperature increasing events in the geological history.Many molecular fossils have been deposited in the ancient terraces of the acidic river Rio Tinto,which proves the diversity of life forms under extremely acidic conditions and the ability of diagenesis to preserve organic compounds for a long time（2.10 Ma）.Meridiani Planum may have acidic watery history during Noachian.Due to the similarity between Meridiani Planum and the Rio Tinto River on minerals and physicochemical properties,if life ever existed in the Meridian Planum,the diagenetic process was likely to recard the information about the morphology and composition of life as evidence.The study of the Rio Tinto River as an analogue to Meridiani provides support for the detection of life traces in similar environments on Mars.It helps accumulate experience in experimental design and operation for analysis of future returning samples from Mars.