BIOL0129_Lecture 11

BIOL0129_Lecture 11 - The Gaia hypothesis Introductory...

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1 Introductory Microbiology Lecture 11 - Microbial ecology and biogeochemical cycling The Gaia hypothesis • James Lovelock (1995) proposed that living organisms and their natural environment are tightly coupled • He envisaged the Earth as a ‘super- organism’ where physicochemical conditions were self-regulated to support life Biogeochemical Cycles (BGC) • Biogeochemical cycling/nutrient cycling – Transformation between biotic and abiotic forms of chemical elements • This is achieved MAINLY by microorganisms – Most notably because they have the metabolic plasticity to exist in a wide range of niches and transform a vast range of substrates • It is IMPOSSIBLE to appreciate how ecosystems function without considering the role of microorganisms Evolution of BGC • BGC were instrumental in creating the biosphere we know today – Oxygenation of the atmosphere and creation of the ozone layer by cyanobacteria • Recently mans activities have impacted ‘normal’ BGC – Global warming, acid rain, algal blooms
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2 • Microorganisms are involved in the BGC of most elements key to life –E.g. C, N, O 2 , metals • We will focus on two examples –Carbon cycle –Nitrogen cycle • Many BGC are intimately linked: –E.g. C and O 2 cycle • CO 2 Fxation by oxygenic phototrophs releases O 2 and much organic matter is oxidized back to CO 2 by aerobic respiration The Carbon Cycle • On a global basis, carbon (C) is cycled through all the Earth’s major C reservoirs : – atmosphere (the air) – land (soils) – water (oceans and freshwater) – sediments and rock (beneath soil horizons and marine and freshwater sediments) – biomass (living organisms) The Carbon Cycle Earth’s crust Soil formation Rock formation Mineralization in sediments Humus Fossil fuels animals Microbes Atmospheric CO 2 Dissolved CO 2 Aquatic microbes, algae, plants Aquatic animals Human activities
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3 Where is the carbon ? • The largest reservoir of C on Earth is in the sediment and rock of the Earth’s surface • However, turnover of this C is insigniFcant on a human scale • In terms of a living scale, the largest amount of C is in land plants – ±orests and grasslands, the major site of CO 2 Fxation Where is carbon turnover greatest ? • CO 2 (CH 2 O) • (CH 2 O) CO 2 Photosynthesis (CO 2 fixation by land plants) Decomposition (humus degradation by microbes) The importance of photosynthesis in C cycling • CO 2 is Fxed by photosynthetic organisms and by chemolithotrophic bacteria • Photosynthesis accounts for most of this
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BIOL0129_Lecture 11 - The Gaia hypothesis Introductory...

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