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Coupled autotrophic and heterotrophic processes cycle matter between the inorganic and organic domains of the biosphere Element Cycling in the Ecosystem CO2 •Energy flow through ecosystems drive matter cycles •The water cycle and the role of evapotranspiration •The carbon cycle •N assumes many oxidation states as it cycles •The Phosphorus cycle and eutrophication •The sulfur cycle and the role of microorganisms Autotrophic process (assimilatory) Nutrients, N, P, etc Light Heterotrophic process (dissimilatory) Physical energy heat biomass O2 Energy flows through the biosphere and drives material cycles within it We say that energy flows because it enters in a short wave useful form that can be captured and biochemically used and leaves in a long wave form that cannot be recaptured—but it does help keep us warm Living processes sometimes provide the energy source to drive other material transformations and physical reactions Photosynthesis by land plants CO2 The Global Water Cycle is driven mainly by the sun’s energy •involves mainly state changes not chemical changes #’s are teratons (TT), (=1012T), and TT/yr Evapotranspiration Nutrients, N, P, etc Light Water from soil [P─E sea] heat Wind energy [P─E land] biomass What is the residence time of water in the atmosphere? O2 Water vapour The water cycle is mostly a physical phenomenon, but plants contribute toward “making it rain” through transpiration Heat energy is captured and stored by the state change from liquid to vapour. This potential energy is released upon condensation •an example of a living process enhancing a physical process that is energetically favourable. In this way terrestrial photosynthesis is an important driver to the global water cycle a huge water pump that is driven by the sun—evaporation requires energy, condensation releases it 1 Carbonate precipitation can also be driven by photosynthesis and sometimes animal metabolism Photosynthesis Most of the earth’s C is contained in sedimentary rocks CaCO3 (s) HCO3─ Nutrients, N, P, etc These massive limestone deposits were produced by coralline algae and reef building corals. H+ Light heat biomass O2 CaCO3 (solid) OH─ These coralline algae are red algae that deposit CaCO3 on their cell walls as a result of their photosynthesis. Intracellular pH rises during photosynthesis and helps carbonates ppt Much of the Carbon in the world is stored in limestone which is being formed by reef building corals and algae. The Carbon cycle involves many chemical reactions driven by organisms #’s are gigatons (GT), (=109 T), and GT/yr Plant growth, especially trees, can also facilitate weathering, which releases dissolved substances from bedrock Ca++, K+, Mg++ HCO3- , SiO3- , PO4-3, clay minerals etc. Limestone, granite, shales Plant growth What is the residence time of atmospheric C? •the atmospheric pool is growing rapidly due to fossil fuel burning •Most of the earth’s C is stored in rocks, fossil fuels and limestone—long residence time Deeper rooting zone Weathering is the physical and chemical breakdown of rocks (lithosphere), and is the most important input to the biosphere for many elements. •P & R plus ocean exchange rapidly cycle C through the atmosphere—short residence time? •CO2 in air and carbonates and bicarbonates dissolved in water control pH. 2 Atmospheric CO2 levels were historically much higher than today Land plants evolved near the end of the Paleozoic era, and huge forests began to cover a once bare landscape Retallack, 1997 Soil clay content increased More C sequestered as coal and ocean CaCO3 Plants become larger, more trees Driving down atmospheric CO2 concentrations Where did all this C go? Studies on fossil soils show great changes as plant evolution progressed C-transformations in aerobic and anaerobic environments The Nitrogen cycle is driven by microbes especially N-fixing organisms 4 x 106 Oxidation state Where do we find methanogens? •The residence time of atmospheric N is about 16,000 yr -4 0 •Humans have increased N-fixation rates about 3X through fertilizer use +4 •Denitrification tends to balance N-fixation on the global scale •Under anaerobic conditions organic molecules break down to methane instead of CO2 —This process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria. •They utilize the energy released from 2H2 + Organic C (CH2O)→CH4 + H 20 to build their biomass. •The biggest N pool is the atmosphere and it is virtually absent from rock •N pollution through industry is also significant source of fixed N 3 Many microorganisms can reduce N2 to NH3 Plants such as legumes have N-fixing Rhizobium nodules on their roots Azolla, an aquatic fern used in rice culture •The leaves of this aquatic fern have cavities that harbour filamentous cyanobacteria Anabaena azollae •These nodules maintain an anaerobic environment and are nourished by plant sugars to pay the energy cost of N-fixation •Most of the N used by plants in natural ecosystems comes indirectly from N-fixation (nitrogenase) •The same reduction reaction N2 + 3H2 → 2NH3 is used to make chemical fertilizers which supplies most of the N for modern agriculture •N-fixing crops are often used to restore N content of depleted soils •Use of N-fertilizers depresses N-fixation rates? Why? •The large cells (heterocysts) are specialized for N-fixation •Traditional rice farming in many countries involve planting Azolla to build up N concentrations in rice paddy. The Nitrogen cycle involves many different oxidation states, and the redox processes are facilitated by plants and wide variety of bacteria Anthropogenic influences on the N-cycle Chemoheterotrophs (CH) -3 CH PA 0 +1 Nitrite +3 •3-fold increase in N-fixation—agriculture •Formation of NOx emissions—auto engines, acid rain •N-losses from clearcut forests •Eutrophication of waterways through fertilizer & sewage pollution No detectable impact on atmospheric N2 content CH +5 Photoautotrophs (PA) Chemoautotrophs(CA) 4 The P cycle is involves no redox reactions •Weathering liberates large amounts of PO4-3 from apatite rocks The S cycle is affected by similar microbially mediated redox reaction as the N-cycle—no stable gaseous phase •Most S is found in the reduced form in rocks eg pyrites •Stored and cycled in forests, but released in large amounts by land clearing—eutrophicati on of waterways •Oxidized and released by volcanic activity, mining smelting, fossil fuel burning (→SO2 ), or microbial activity eg hydrothermal vents, or weathering •P-fertilizers used extensively in agriculture. •Cycled between the dissolved and the particulate phase (plankton) in lakes and oceans •Phosphorus tends to accumulate in sediments except when they become anaerobic (→SO4 -2) •A volatile gas DMS is produced by ocean plankton—affects cloud formation •SO2 is rained out of the atmosphere as H2SO4 •Taken up by biota (proteinsSH) and released as H2S Chemoheterotrophs (CH) -2 0 +4 +6 CH PA Photoautotrophs (PA) Chemoautotrophs(CA) Streams draining mine tailings are extremely acidic—the effect of Thiobacillus oxidizing pyrites and iron. 5 Thiobacillus ferrooxidans oxidizes both the iron, Fe(+2) to Fe(+3) and the sulphur in the pyrites, S(-1) to S(+6) using molecular oxygen. Hydrothermal vent fauna: a community founded on Thiobacillus a chemoautotrophic bacteria This reaction splits water to produce a great deal of acid. How do you suggest that mine tailings should be stored? 2 FeS 2 + 7.5O 2 + 4 H 2O ! 4 SO 4 "2 + Fe 2O 3 + 8 H + + Energy The pogonophoran tube worms harbour nodules of Thiobacillus inside their body cavities as mutualistic symbionts 6