Nutrient Cycles, Bioremediation, and Symbioses

1. Chapter 24 Nutrient Cycles, Bioremediation, and Symbioses

2. I. The Carbon and Oxygen Cycles • 24.1 The Carbon Cycle • 24.2 Syntrophy and Methanogenesis

3. 24.1 The Carbon Cycle • Carbon is cycled through all of Earth’s major carbon reservoirs • i.e., atmosphere, land, oceans, sediments, rocks, and biomass

4. Figure 24.1

5. Reservoir size and turnover time are important parameters in understanding the cycling of elements

6. Major Carbon Reservoirs on Earth

7. CO2 in the atmosphere is the most rapidly transferred carbon reservoir - CO2 is fixed primarily by photosynthetic land plants and marine microbes - CO2 is returned to the atmosphere by respiration of animals and chemoorganotrophic microbesas well as anthropogenic activities • Microbial decompositionis the largest source of CO2 released to the atmosphere

8. The carbon and oxygen cycles are intimately linked • Phototrophic organisms are the foundation of the carbon cycle • Oxygenic phototrophic organisms can be divided into two groups: Plants and microorganisms - Plants dominant phototrophic organisms of terrestrial environments - Phototrophic microbes dominate aquatic environments

9. Photosynthesis and respiration are reverse reactions - Photosynthesis CO2 + H2O (CH2O) + O2 - Respiration (CH2O) + O2 CO2 + H2O

10. Redox Cycle for Carbon Figure 24.2

11. The two major end products of decomposition of carbon-containing materials are CH4 and CO2

12. 24.2 Syntrophy and Methanogenesis • Methanogenesis is central to carbon cycling in anoxic environments - Most methanogens reduce CO2 to CH4 with H2 as an electron donor; some can reduce other substrates to CH4 (e.g., acetate) - Methanogens team up with partners (syntrophs) that supply them with necessary substrates

13. Anoxic Decomposition Figure 24.3

14. Rxns of Anoxic Conversion of Organic Compounds to CH4

15. Methanogenic symbionts can be found in some protists (e.g. protozoa) - Believed that endosymbiotic methanogens benefit protists by consuming H2 generated from glucose fermentation

16. Termites and Their Carbon Metabolism Hindgut and termite larva Fluoresces due to F420 Methanogens in symbiotic protozoa in hindgut Figure 24.4

17. On a global basis, biotic processes release more CH4 than abiotic processes (Table 24.3)

18. Estimates of CH4 Released into the Atmosphere

19. Acetogenesis is a competing H2-consuming process to methanogenesis in some environments • e.g., termite hindgut, permafrost soils - Methanogenesis from H2 (-131 kJ) is energetically more favorable than acetogenesis (-105 kJ) - Acetogens position themselves in the termite gut nearer to the source of H2 produced from cellulose fermentation (by protozoa) than methanogens - Acetogens can ferment glucose and methoxylated aromatic compounds from lignin whereas methanogens cannot

20. Sulfate-reducing bacteria outcompete methanogens and acetogens in marine environments

21. II. Nitrogen, Sulfur, and Iron Cycles • 24.3 The Nitrogen Cycle • 24.4 The Sulfur Cycle • 24.5 The Iron Cycle

22. 24.3 The Nitrogen Cycle • Nitrogen • A key constituent of cells • Exists in a number of oxidation states

23. Redox Cycle for Nitrogen Figure 24.5

24. N2 is the most stable form of nitrogen and is a major reservoir • The ability to use N2 as a cellular nitrogen source (nitrogen fixation) is limited to only a few prokaryotes • Denitrification is the reduction of nitrate to gaseous nitrogen products and is the primary mechanism by which N2 is produced biologically • Ammonia produced by nitrogen fixation or ammonification can be assimilated into organic matter or oxidized to nitrate

25. Anammox is the anaerobic oxidation of ammonia to N2gas (NH4+ + NO2- → N2 + 2 H2O) • Denitrification and anammox result in losses of nitrogen from the biosphere

26. 24.4 The Sulfur Cycle • Sulfur transformations by microbes are complex • The bulk of sulfur on Earth is in sediments and rocks as sulfate and sulfide minerals (e.g., gypsum, pyrite) • The oceans represent the most significant reservoir of sulfur (as sulfate) in the biosphere

27. Redox Cycle for Sulfur Figure 24.6

28. Hydrogen sulfide is a major volatile sulfur gas that is produced by bacteria via sulfate reduction or emitted from geochemical sources - Sulfide is toxic to many plants and animals and reacts with numerous metals

29. Sulfur-oxidizing chemolithotrophs can oxidize sulfide and elemental sulfur at oxic/anoxic interfaces

30. Organic sulfur compounds can also be metabolized by microbes - The most abundant organic sulfur compound in nature is dimethyl sulfide (DMS) • Produced primarily in marine environments as a degradation product of dimethylsulfoniopropionate (an algal osmolyte) - DMS can be transformed via a number of microbial processes

31. 24.5 The Iron Cycle • Iron is one of the most abundant elements in the Earth’s crust - On the Earth’s surface, iron exists naturally in two oxidation states • Ferrous (Fe2+) • Ferric (Fe3+)

32. Redox Cycle of Iron e.g. Fe + CuSO4 FeSO4 + Cu (Fe + Cu2+ Fe2+ + Cu) Figure 24.7

33. Fe3+ can be used by some microbes as electron acceptors in anaerobic respiration • In aerobic acidic pH environments, acidophilic chemolithotrophs can oxidize Fe2+ (e.g., Acidithiobacillus)

34. Oxidation of Ferrous Iron (Fe2+) Figure 24.8a

35. A Microbial Mat Containing High Levels of Ferrous Iron (Fe2+) Figure 24.8b

36. Pyrite (FeS2) • One of the most common forms of iron in nature • Its oxidation by bacteria can result in acidic conditions in coal-mining operations

37. Pyrite and Coal Figure 24.9

38. Role of Iron-Oxidizing Bacteria in Oxidation of Pyrite Figure 24.10a

39. Acid Mine Drainage • An environmental problem in coal-mining regions • Occurs when acidic mine waters are mixed with natural waters in rivers and lakes • Bacterial oxidation of sulfide minerals is a major factor in its formation

40. Acid Mine Drainage from a Bituminous Coal Region Figure 24.11

41. Ferroplasma acidarmanus Streamers of F. acidarmanus, an extremely acidophilic iron-oxidizing archaeon Figure 24.12

42. Ferroplasma acidarmanus Scanning electron micrograph of F. acidarmanus Figure 24.12

43. III. Microbial Bioremediation • 24.6 Microbial Leaching of Ores • 24.7 Mercury and Heavy Metal Transformations • 24.8 Petroleum Biodegradation • 24.9 Biodegradation of Xenobiotics

44. Bioremediation • Refers to the cleanup of oil, toxic chemicals, or other pollutants from the environment by microorganisms • Often a cost-effective and practical method for pollutant cleanup

45. 24.6 Microbial Leaching of Ores • Microbial Leaching • The removal of valuable metals, such as copper, from sulfide ores by microbial activities • Particularly useful for copper ores

46. Effect of the Acidithiobacillus ferrooxidans on Covellite CuS Figure 24.13

47. In microbial leaching, low-grade ore is dumped in a large pile (the leach dump) and sulfuric acid is added to maintain a low pH • The liquid emerging from the bottom of the pile is enriched in dissolved metals and is transported to a precipitation plant • Bacterial oxidation of Fe2+ is critical in microbial leaching as Fe3+ itself can oxidize metals in the ores

48. The Microbial Leaching of Low-Grade Copper Ores A Typical Leaching Dump Effluent from a Copper Leaching Dump Figure 24.14a

49. Small Pile of Metallic Copper Removed from the Flume Recovery of Copper as a Metallic Copper Figure 24.14c

50. Arrangement of a Leaching Pile and Reactions Figure 24.15