تيرصنتسولا تعهاجلا 
                                      ةايحلا مولع نسق/مولعلا تيلك 
                           تيحابصلا تساردلا/تيرهجولا ءايحلاا عرف/ تثلاثلا تلحرولا 
                   Lec (5):   Extreme Environments 
              There are two definitions of extreme Environments ,The first characterizes 
         an environments as extreme if  the environmental conditions are at one or two 
         extreme  (  high  or  low).these  environmental  conditions  can  include  pH, 
         temperature , salinity ,pressure and nutrients . The second definition refers to 
         Environments in which conditions select for extremely low microbial diversity. 
         Extremophile:  The  organisms  that  have  successfully  adapted  to 
         environments where it is difficult or impossible for other organisms to 
         survive.Extreme environments are important to environmental microbiologists 
         because  there  is  much  speculation  that  such  environments  harbor  unique 
         microorganisms with activities that are not only of scientific interest but also 
         that have commercial potential. 
         Microbial  communities  in  extreme  environments  have  adapted  to  amazing 
         levels of stress. These adaptations are of interest for development of  
            remediation approaches for some contaminated sites including  
            acid mine drainage sites and radioactive waste sites. 
            They also are of interest for applications of novel enzymes adapted to 
            temperature or pH extremes. 
             Finally, they are of interest for understanding evolutionary history and 
            possible impacts of future climate change. 
         Types of extreme environments 
             Extremophiles have been selected over time for characteristic that allow 
           them  to  grow  and  multiply  in  a  variety  of  extreme  environments,will 
           describe  some  of  environments  and  the  physiological  adaptions  use  by 
           extremophiles to compete or survive in their particular niche. 
          
                                 يجرعلاا نشاه نظاك . د.أ : ةداولا ذاتسأ 
                                 يركسعلا يلع ءارفع .د.م                   
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                 1.  Low Temperature Environments  
                      The  McMurdo  Dry  Valleys  in  Antarctica  represent  one  of  the  driest  and 
                    coldest ecosystems known. The average mean annual surface air temperature is  
                    -27.6°C and the average surface soil temperature is -26.1°C. This ecosystem 
                    has  the  only  permanently  ice-covered  lakes  on  Earth,  varying  in  ice-cover 
                    thickness  from  3  to  5  m  .Researchers  studying  the  site  found  a  diverse 
                    community  of  phototrophic  purple  bacteria  ,  sulfur  chemoautotrophs,  and 
                    heterotrophic sulfate-reducers. For example, cell numbers of sulfur-oxidizing 
                    bacteria were found to peak at 200 cells per ml at a depth of 9.5 m , this is 
                    where both dissolved oxygen and sulfide coexist in the water column. Three 
                    sulfur-oxidizers were cultured from lake water samples, all most closely related 
                    to    Thiobacillus  thioparus  .Sulfate-reducing  bacteria  were  also  found  In 
                    addition a methane gradient begins at a depth of 12   m   .This gradient implies 
                    the presence of a methane cycle and the presence of both methanogenic and 
                    methanotrophic microorganisms .  
                      cold-adapted microorganisms: Their ability to survive and grow in the cold 
                    requires specialized adaptations For example, these microorganisms synthesize 
                    cold-adapted enzymes which have had to evolve specific structural features 
                    that  make  them  highly  flexible  in  comparison  to  their  warm  temperature 
                    equivalents .This flexibility, particularly around the active site of the enzyme 
                    (the site where the enzyme interacts with its substrate) means that the enzyme 
                    can  operate  efficiently  at  low  temperatures.  This  also  means  that  at  high 
                    temperatures the enzyme becomes unstable.  
                    In  fact,  it  is  these  two  properties  of  coldactive  enzymes  that  makes  them 
                    suitable for biotechnological application: their high activity at low temperature 
                    and their low stability at elevated temperatures. Low temperature enzymes that 
                    have been examined or used in industry include  α- amylase ,cellulase , β- 
                    galactosidase ,lipase proteases , xylanase . 
                     
                     2.         Desiccation and UV Stress  
                       The deserts of the world represent both hot and semi- cold and hyperarid 
                    environments  where  extreme  conditions  severely  limit  primary  productivity 
                    and thus the diversity of life. Factors limiting microbial life in the arid deserts 
                    include water availability, temperature, and the intensity of UV radiation. Arid 
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         deserts are characterized by mean annual rainfall levels of 25–200 mm while 
         hyperarid deserts have mean annual rainfall of  < 25    mm. Water availability 
         in a desert is determined not only by mean annual rainfall, but also by the 
         combined effects of precipitation (P) and potential evapotranspiration (PET). 
         Hyperarid areas are defined as those with a P/PET ratio less than 0.05 Due to 
         the lack of available moisture, plants are sparse or completely absent, creating 
         soil  conditions  with extremely low soil organic carbon and nitrogen levels, 
         further limiting the potential diversity of microbial life.  
              commonly  studied  feature  in  arid  environments  is  the  lithic  microbial 
         communities, those that inhabit rock surfaces and subsurface rock pores. These 
         communities are dominated by photoautotrophic nitrogen-fixing cyanobacteria 
         and are capable of colonizing a diverse group of minerals including, granite, 
         gypsum, halite, , quartz, and sandstone. These communities have been found in 
         a range of hot and cold deserts. These hypolithic (inhabit rock surfaces) and 
         endolithic (inhabit pore spaces within the rocks) communities are believed to 
         exploit the protection offered by rock surfaces that scatter UV radiation and 
         presumably trap limited water supplies. The dominant photoautotroph found in 
         the  majority  of  these  communities  is  the  desiccation  and  radiation  tolerant 
         cyanobacterium  Chroococcidiopsis .  
         Adaptation  to  desiccation  is  unique  among  the  extremes  experienced  by 
         bacteria  induce  survival  strategies  for  the  cells  rather  than  the  ability  to 
         function under extreme conditions.  
         The survival strategies identified include the following:    
          ● The ability to protect and repair DNA exposed to UV radiation 
          ●  Maintenance of protein stability in the dehydrated state   
          ●  Maintenance of membrane integrity  
             The primary adaptative mechanism of the cyanobacteria is the production of 
         an extracellular polysaccharide (EPS) sheath. This sheath regulates the uptake 
         and loss of water, serves as a matrix for immobilization of cellular components 
         produced by the cell in response to desiccation, and may protect cell walls 
         during shrinking and swelling . 
          
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                    3-  Air-Water Interface   
                 The air –water interface is a unique habitat that is often considered an extreme 
                 environment for many reasons, including 
                       high levels of solar radiation 
                       accumulation of toxic substances(e.g. heavy metals,pesticides). 
                       Large temperature 
                       pH 
                       salinity fluctuations 
                       competition 
                 The  air–water  interface,  also  referred  to  as  the  neuston,  contains  higher 
                 concentrations  of  organisms  than  other  layers  of  the  water  column  ,most 
                 reports  suggest  that  the  numbers  may  be  higher  but  the  ratio  of  metabolic 
                 activity to total counts is lower in the neuston than in the planktonic habitat. 
                    The neuston accumulates nutrients and especially attracts nonpolar organic 
                 and inorganic molecules, in addition to the nutrients accumulation the neuston 
                 tends  to  accumulate  toxins  .Among  these  toxins  are  nonpolar  organic 
                 molecules,including pesticides such as DDT and petroleum hydrocarbons,as 
                 well as metals such as Cd,Cu,Mn,Hg,Pb,Se and Cr .The microorganisms that 
                 inhabit the neuston have developed unique metabolic,genetic and functional 
                 strategies that allow them to survive the extreme environments.These strategies  
                 include 
                      Use of pathways that catabolize toxic compounds and provide resistance 
                        to metals that accumulate at the interface. 
                      Some microbes have developed efficient  DNA  repair  mechanisms  to 
                        combat DNA damage caused by exposure to ultraviolet radiation. 
                    4- High Temperature 
                      There  are  many  examples  of  environments  with  extreme  temperature. 
                  Environments  with  high  temperature  (>70  oC)  include  terrestrial  and 
                  submarine hot springs, some of which can reach temperature of 100 oC and 
                  hydrothermal vents, which can reach temperature  in excess of 300 oC. Such 
                  high temperature are inhospitable for most forms of life except for certain 
                  bacteria and archaebacteria. Genera commonly found in these environments 
                  include    Thermus,  Methanobacterium,  Sulfolobus,  Pyrodictium,  and  
                  Pyrococcus. temperatures of up to 100°C.   
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