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Chapter 11 - 2HO3.docx

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McMaster University
Life Sciences
Luc Bernier

2HO3 Chapter 11: Effects of Life on the Atmosphere Chapter 11: Effects of Life on the Atmosphere - The prebiotic earth probably has a atmosphere dominated by CO2 and N2 - Life originated 700 mya - A lager change in the atmospheric composition occurred when organisms evolved that were able to perform oxygenic photosynthesis o This lead to the establishment of our modern O2-rich environment Effect of Life on the Early Atmosphere: - We still do not know how life originated but we know when, due to the Apex Chert microfossils - 3.5 bya layers of stromatolites started to form, these stromatolites are the fossilized remains of bacteria o The areas where these stromatolites formed is similar to the Shark Bay on the coast of western Australia - Although we know that the organisms that formed stromatolites were photosynthetic we do not know if they produced O2 as by-product o Oxygenic photosynthesis is carried out by higher plants and algae today  Some bacteria make their living by a related process called anoxygenic photosynthesis  In this process H2S or H2 is used instead of H2O to reduce CO2 to organic carbon  This process does not yield O2 o Cyanobacteria also known as blue-green algae can perform both oxygenic and anoxygenic photosynthesis  Which process they use depends on the composition of the atmosphere in which they are found - The microfossils found in the Apex Chert are in fact not photosynthetic, since the environment in which they were found is similar to modern day hydrothermal vents o It is believed that they instead used the energy produced by chemical reactions to survive  This is called chemosynthetic respiration Production of Methane: - Using the tree of life we can determine the type of organisms living on the primitive earth - Many of the organisms near the base of the tree are hyperthermophiles - Near the hyperthermophiles are the methanogenic bacteria o Methanogenic bacteria produce energy form chemical reactions that generate methane CO2 + 4H2  CH4 + 2H2O o Given that both CO2 and H2 were present in the early environment it is possible that the earliest forms of life on earth were in fact methanogenic bacteria 1 2HO3 Chapter 11: Effects of Life on the Atmosphere - The presence of CH4 in the early atmosphere would have caused it to look much different then it does today o Saturn’s moon Titan is a good example of what early earth may have looked like  Titan has a dense atmosphere and contains 98% N2 and 2% CH4  Contains an orangish haze which makes the surface not visible  The haze consists of hydrocarbon aerosols which are bombarded by CH4 - In the Late Archean a large depletion in 13 C is seen, this is indicative of photosynthesis o 13C produced by oxygenic photosynthesis is typically low Cycling of Atmospheric Nitrogen: - Organisms need nitrogen for making proteins and nucleic acids as well as for other biochemical functions o Most of the functions however cannot use N2, instead they require fixed nitrogen, in which nitrogen atoms are bonded to other types of atoms  NH3 and NO3- o Modern organisms acquire nitrogen in two ways  Lightening, in the high temperature region surrounding a lightening discharge, nitrogen and oxygen react to form nitric oxide (NO) N2 + O2  2NO  Nitric oxide is a radical that plays an important role in ozone photochemistry  The NO is eventually oxidized into nitric acid (HNO3)  HNO3 is soluble in water and thus is rained out of the troposphere  In solution the HNO3 dissociates to form hydrogen and nitrogen ions HNO3  H+ + NO3-  The nitrogen ions can be used directly by organisms as a source of fixed nitrogen o Marine organisms can make their own fixed nitrogen by a process termed nitrogen fixation  Most of the available nitrogen in the oceans is fixed by cyanobacteria  The first nitrogen fixers were probably not cyanobacteria  The ability to fix nitrogen is widely spread among prokaryotes (Bacteria, Archaea and methanogens)  Eukaryotes cannot fix nitrogen o Prokaryotes are thought to be more primitive than eukaryotes because their cell structure is simpler  They are also more resistant to UV radiation, which is consistent with the idea 2 2HO3 Chapter 11: Effects of Life on the Atmosphere that they evolved under low-O2 atmosphere that lacked the pr
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