24.1 The Carbon Cycle Carbon is cycled through all of Earth’s major carbon reservoirs –Includes...

24.1 The Carbon Cycle

• Carbon is cycled through all of Earth’s major carbon reservoirs– Includes atmosphere, land, oceans,

sediments, rocks, and biomass

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

Figure 24.1

Humanactivities

Biological pump

Death andmineralization

Respiration

Landplants

Aquaticplants and

phyto-plankton

Animals andmicroorganisms

Fossilfuels

Soil formation

Earth’s crust Rock formation

Aquaticanimals

• CO2 in the atmosphere is the most rapidly transferred carbon reservoir

• CO2 is fixed by photosynthetic land plants and marine microbes

• CO2 is returned to the atmosphere by respiration as well as anthropogenic activities– Microbial decomposition is the largest source of

CO2 released to the atmosphere

• Carbon and oxygen cycles are linked • Phototrophic organisms are the foundation of the

carbon cycle– Oxygenic phototrophic organisms can be divided

into two groups: plants and microorganisms• Plants dominant organisms of terrestrial

environments• Microorganisms dominate aquatic environments

• Photosynthesis and respiration are part of redox cycle (Figure 24.2)

• Photosynthesis

CO2 + H2O (CH2O) + O2

• Respiration

(CH2O) + O2 CO2 + H2O

• The two major end products of decomposition are CH4 and CO2

Figure 24.2

Organic matter

Syntrophassisted

Oxygenic photosynthesis

Chemolithotrophy

Respiration

Methanotrophy

Methanogenesis

Acetogenesis

Anoxygenicphotosynthesis

Anoxic

Anaerobicrespirationandfermentation

(CH2O)n

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

24.3 The Nitrogen Cycle

• Nitrogen– A key constituent of cells

– Exists in a number of oxidation states

Figure 24.7

Nitrification

Nitrogenfixation

Anoxic

Anammox

groupsof protein

Denitrification

Assimilation Assimilation

Assimilation

Ammonification

24.3 The Nitrogen Cycle

• N2 is the most stable form of nitrogen and is a major reservoir

– Only a few prokaryotes have the ability to use N2 as a cellular nitrogen source (nitrogen fixation)

– 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

II. Biodegradation and Bioremediation

• 24.7 Microbial Leaching• 24.8 Mercury Transformations• 24.9 Petroleum Biodegradation and

Bioremediation• 24.10 Xenobiotics Biodegradation and

Bioremediation

24.7 Microbial Leaching

• 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

• Microbial leaching– The removal of valuable metals, such as

copper, from sulfide ores by microbial activities (Figure 24.15)

– Particularly useful for copper ores

• In microbial leaching, low-grade ore is dumped in a large pile (the leach dump) and sulfuric acid is added (Figure 24.16)

• 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

Figure 24.16Low-gradecopper ore(CuS)

Sprinkling of acidic solution on CuS

Soluble Cu2

Precipitationpond

Copper metal (Cu0)

Acidic Fe2-rich solution

Acidic solutionpumped back totop of leach dump

additionRecovery of copper metal (Cu0)

Fe0 Cu2 Cu0 Fe2

(Fe0 from scrap steel)

Oxidation pond

Leptospirillum ferrooxidansAcidithiobacillus ferrooxidans

1. CuS 2 O2 Cu2 SO42

Cu2 8 Fe2 SO42 8 H

2. CuS 8 Fe3 4 H2O

Copper ore can be oxidized by oxygen-dependent (1) and oxygen-independent(2) reactions, solubilizing the copper:

• Microorganisms are also used in the leaching of uranium and gold ores (Figure 24.17)

24.9 Petroleum Biodegradation and Bioremediation

• Prokaryotes have been used in bioremediation of several major crude oil spills (Figure 24.20)

Figure 24.20

• Diverse bacteria, fungi, and some cyanobacteria and green algae can oxidize petroleum products aerobically

• Oil-oxidizing activity is best if temperature and inorganic nutrient concentrations are optimal

• Hydrocarbon-degrading bacteria attach to oil droplets and decompose the oil and disperse the slick (Figure 24.21)

Figure 24.21

Oil droplets Bacteria

• Gasoline and crude oil storage tanks are potential habitats for hydrocarbon-oxidizing microbes (Figure 24.22)

• If sufficient sulfate is present, sulfate-reducing bacteria can grow and consume hydrocarbons

Figure 24.22

• Some microorganisms can produce petroleum– Particularly certain green algae

24.10 Xenobiotics Biodegradation and Bioremediation

• Xenobiotic compound– Synthetic chemicals that are not naturally

occurring• Examples: pesticides, polychlorinated

biphenyls, munitions, dyes, and chlorinated solvents (Figure 24.23)

– Many degrade extremely slowly

Figure 24.23

DDT, dichlorodiphenyltrichloroethane(an organochlorine)

Malathion, mercaptosuccinicacid diethyl ester(an organophosphate)

Site of additionalCl for 2,4,5,-T

2,4-D, 2,4-dichlorophenoxy-acetic acid

Atrazine, 2-chloro-4-ethylamino-6-isopropylaminotriazine

Monuron, 3-(4-chlorophenyl)-1,1-dimethylurea(a substituted urea)

Chlorinated biphenyl (PCB), shown is 2,3,4,2,4,5-hexachlorobiphenyl

Trichloroethylene

• Pesticides– Common components of toxic wastes

– Include herbicides, insecticides, and fungicides

– Represent a wide variety of chemicals• Some can be used as carbon sources by

microorganisms• Some can be used as electron donors

• Some xenobiotics can be degraded partially or completely if another organic material is present as a primary energy source (cometabolism)

• Chlorinated xenobiotics can be degraded anaerobically (reductive dechlorination) or aerobically (aerobic dechlorination)

• Reductive dechlorination is usually a more important process as anoxic conditions develop quickly in polluted environments

• Plastics of various types are xenobiotics that are not readily degraded by microorganisms (Figure 24.25)

• The recalcitrance of plastics has fueled research efforts into a biodegradable alternative (biopolymers)

Figure 24.25

Polyethylene Polypropylene Polyvinyl chloride(PVC)

Polystyrene Polyurethane Teflon

PHV PHB

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