Community Structure

Lecture 40: Nutrient Cycles

Reading: Economy of Nature, pp. 167-188

Phosphorus Cycle
Reservoir is phosphate rocks, sedimentary rocks containing phosphate apatites, such as calcium fluorophosphate (Ca₅FP₃O₁₂)

Phosphate Weathering

sedimentary phosphorus>>>>> PO₄^-

Phosphate apatites are decomposed by abiotic weathering yielding phosphate ions (PO₄^-) dissolved in water. Phosphate is most readily in solution (ionized) at pH values between 6 and 7. At both higher and lower pH, phosphate becomes insoluble in water, forming compounds with iron, aluminum, and calcium.

Phosphate Assimilation

PO₄^->>>>>>organic phosphorus>>>>>>>organic phosphorus (energy requiring)

Phosphate ions in soil or water are utilized directly by plants, but as noted above, the environmental pH determines phosphate availability. Heterotrophs consume autotrophs to get phosphorus containing compounds.

Organic Phosphorus Decomposition

organic phosphorus>>>>>> PO₄^- (energy releasing)

Organic phosphate compounds (such as nucleic acids and nucleotides) are decomposed by phosphatizing bacteria to phosphate ion.

Phosphorus Sedimentation

Inorganic phosphate precipitation from solution, inorganic phosphates in bone and exoskeletons, and organic phosphates in fecal wastes (guano) are sources of phosphorus removal to sedimentary reservoirs.

Sulfur Cycle

Reservoir for sulfur is sedimentary deposits, consisting of elemental sulfur, sulfides (such as pyrite FeS2), and sulfates ( compounds like gypsum CaSO4 + 2H2O).

Sulfur Weathering

sulfur rock>>>>>>>(sulfate)
sulfur rock (and fossil fuels)>>>>>>(sulfur dioxide)
sulfur rock>>>>>> (sulfides)

Abiotic weathering of sulfur rocks releases sulfate ions to water. Volcanic eruptions also result in the thermal decomposition of sulfur rocks and the release of sulfur dioxide () gas and dust. Deep ocean hot water vents (hydrothermal vents) are often rich sources of dissolved sulfides and the combustion of fossil fuels (specifically coal and oil) is another source of sulfur dioxide gas. Combustion sulfur dioxide () gas is an important pollution problem because it returns to terrestrial and aquatic environments as sulfuric acid () in acid precipitation.

Transformation relationships in the sulfur cycle and the importance of bacterial sulfur reactions are shown. The potential energy of sulfur compounds is indicated by their oxidation state (Ricklefs, 1996, p 161, Fig. 7.11).

Sulfate Assimilation

>>>>>>organic sulfur>>>>>>>organic sulfur (energy requiring)

Sulfate ions are reduced by plants and microorganisms and incorporated in organic compounds (proteins and amino acids). Heterotrophs consume these organisms and obtain sulfur containing organic compounds.

Organic Sulfur Decomposition

The decomposition of organic sulfur compounds is performed by bacteria under either aerobic conditions (yielding sulfate ions), or anaerobic conditions (desulfhydration yielding sulfide ions). In either process, energy is released to the bacteria.

Aerobic Sulfur Decomposition

organic sulfur>>>>>> (sulfate) (energy releasing)

Anaerobic Desulfhydration

organic sulfur>>>>> (sulfide) (energy releasing)

Sulfate reduction

>>>>>>> (energy requiring, but linked to oxidation of carbon)

Another anaerobic process performed by bacteria is the reduction of sulfate which yields sulfides. Like all reductions, this requires an energy input, but these reactions are linked to the oxidation of carbon compounds which release energy. Sulfur is used as an electron acceptor in these linked reactions (as is oxygen under aerobic conditions) so energy can be extracted from carbon compounds in excess of that necessary to reduce sulfur.

Sulfide Oxidation

>>>>>>S (elemental sulfur) (energy releasing)

Photoautotrophic bacteria under anaerobic conditions use sulfides as an electron donor (instead of water molecules) in this form of photosynthesis.

Sulfide and Sulfur Oxidations

>>>>>> (sulfite) (energy releasing)
S >>>>>> (sulfite) (energy releasing)
>>>>>> (sulfate) (energy releasing)

Under aerobic conditions bacteria can oxidize sulfide, elemental sulfur, or sulfite to extract energy. This is energy capture from inorganic (non-carbon) materials, without the use of sunlight, and without consumption of other organisms, so these bacteria are termed chemoautotrophic. These oxidations are common phenomena in the runoff from sulfur containing mining wastes. Sulfate in water forms sulfuric acid () which can significantly lower the pH of the water.

Sulfur Sedimentation

The formation of new sedimentary sulfur containing rock results from the sedimentation of organic sulfur (with incomplete decomposition), elemental sulfur, and sulfides.

Carbon Cycle

Reservoir pool (sink) is sedimentary rock (limestone, dolomite, and fossil fuels). Cycling pool is the atmosphere and oceans with carbon in the form of carbon dioxide.

Most carbon in largely inaccessible sinks, sedimentary rocks and fossil fuels.

As much as 66% of terrestrial carbon is in soil organic matter, 2-3x more carbon than is present in the atmosphere (these 1996 data are not reflected in the carbon flux figure that follows).

Carbon cycle pools and fluxes are shown with units (in parentheses) in billions of metric tons (1015g) and billion metric tons per year (Ricklefs, 1996, p 155, Fig. 7.5).

Carbon Weathering

CaCO3 >>>>> +

+ 2 < > + < >H2CO3

H2CO3 << >> H2O + CO2

Abiotic weathering of carbonate containing rocks (limestone) results from acids in water causing carbonate ionization (), bicarbonate (), and carbonic acid (H₂CO₃) formation. Carbonic acid will dissociate to water and carbon dioxide (dissolved in water) which makes carbon available for fixation from water (or the atmosphere).

Volcanic decomposition of carbonate rocks occurs when molten rock (magma) melts carbon containing sedimentary rocks as the magma moves toward the earth’s surface. This results in the direct release of carbon dioxide in the gases that are emitted from a volcanic eruption. Fossil fuel combustion (natural gas, coal, and petroleum) also results in the direct release of carbon dioxide to the atmosphere. Fossil fuel carbon combustion is dependent on human activities and has changed significantly in the past 200 years.

Carbon Fixation

CO₂>>>>>carbohydrates (energy requiring)

Carbon dioxide is removed from the atmosphere (or water) in the process of photosynthesis.

Respiration and Heterotrophic Consumption

carbohydrates >>>>> CO₂ (energy releasing)

The respiration of autotrophs and the consumption of carbohydrates by consumers and subsequent combustion in cellular respiration returns carbon dioxide to the atmosphere (or water).

Biological Decomposition

carbohydrates >>>>> CO₂ (energy releasing)

The carbon containing remains of all organisms are consumed by decomposer bacteria and fungi that release carbon dioxide from cellular respiration.

Carbon Sedimentation

Carbon is returned to the sedimentary reservoir in both marine and terrestrial environments. Terrestrial burial of vegetation with incomplete decomposition results in the formation of the fossil fuels coal and methane (natural gas). Marine sedimentation of photosynthetic protozoa with incomplete decomposition results in the formation of petroleum and methane. The marine sedimentation of the calcium carbonate exoskeletons of protozoa results in the formation of limestones, and similar carbonate rocks form as a result of photosynthesis induced carbonate precipitation in marine waters.

In marine systems at equilibrium:

CaCO3(insoluble) + H2O + CO2 << >> + 2 (bicarbonate, soluble)

Photosynthesis removes carbon dioxide (left side of equilibrium) from water so the equilibrium shifts to calcium carbonate formation and precipitation. This is most pronounced in warm shallow marine environments where the rates of photosynthesis can greatly exceed the rates of cellular respiration. The exoskeletons of corals (the rock of a coral reef) is formed in this manner.

Methanogenesis

carbohydrate >>> CH4 + CO2 + H2O or CO2 + 4H2 >>> CH4 + 2 H2O

Under anaerobic conditions, some bacteria and Archaea (non-bacterial prokaryotes) form methane (CH₄) in coupled reactions that enable cells to extract energy from carbon compounds. Methane is formed as carbon acts as the final electron acceptor (carbon is reduced) while other carbon compounds are oxidized and energy is released. This is similar to the coupled reactions seen in the sulfur cycle in which sulfate is reduced as carbon compounds are oxidized under anaerobic conditions. The anaerobic conditions in which methane production occurs include freshwater swamps, acid bogs, salt marshes, tundra and taiga environments, deep ocean and lake environments, and the ruminant gut .

Methane Combustion

CH4 + 2O2 >>>>> CO2 + 2H2O

Methane combustion results fossil fuel use for human energy needs.

Methane Loss

Both fossil fuel methane (that escapes) and recent methane production disperse in the atmosphere once released.

Atmospheric Carbon Dioxide Concentration

Daily and Yearly Cycles

Since photosynthesis cannot occur in the absence of sunlight, during a growing season (summer) carbon dioxide is removed from the atmosphere in the daytime, but not at night. The graph below shows carbon dioxide concentration increases each night and decreases each day.

Similar variation is seen on a yearly scale where there is seasonality in the rate of photosynthesis, such as the temperate zone winter and summer seasons.

Global Carbon Dioxide Change

Global atmospheric carbon dioxide concentration has increased during the past 40 years as shown in the graph below. The fluctuation is due to seasonal variation in photosynthesis at the site where these data were collected (Hawaii), but the trend is increasing concentration.

This is part of a long-term carbon dioxide increase since 1750. Atmospheric carbon dioxide concentration has increased from 280ppm to 360ppm at present. This change is caused mainly by fossil fuel combustion (fuel of the Industrial Revolution) and deforestation.