1. The Global Phosphorus Cycle
• Phosphorus is also necessary for organic life: It is an
essential component in DNA, cell membranes, and the
two related organic compounds which provide a key
mechanism for the storage and release of energy
• Adenosine diphosphate (ADP) and adenosine
triphosphate (ATP) are made up of adenosine bonded
to either two or three phosphate groups
• When a phosphate group is removed from ATP to
produce ATP, energy is released:
– ATP ADP + energy + phosphate
• When a phosphate group is added to ADP to
produce
ATP, energy must be added:
– ADP + energy + phosphate ATP
“Rechargeable Batteries”
2. The Global Phosphorus Cycle
• The ATP/ADP cycle provides energy for cellular activity
and is a key part of plant productivity
• Photosynthesis, respiration and ATP/ADP are related:
Photosynthesis stores energy, respiration releases it,
and ATP is the central molecule in this process
• Thus, plants require phosphorus to live, and much like
nitrogen, if it is not available in sufficient quantity, it can
be the limiting factor in productivity
• However, the global phosphorus cycle differs from that
from nitrogen in several ways: In particular, the global
phosphorus cycle has no significant gaseous
component
3. Forms of P
• H3PO4 – Phosphate
• H3PO3 – Phosphite
• H3PO2– Hypophosphite
• PH3 – Phosphine
Stable at normal conditions except under
extreme reducing conditions
4. The Global Phosphorus Cycle
Most phosphorus compounds are not very water soluble, thus few chemical
transformations
Significant flow from land to
ocean via rivers (21 Tg/yr)
A small amount of P in dust
6. Soil phosphorus mobilization
and immobilization by bacteria
Bacteria :Micrococcus and some fungi,
Enzymes: Phytases, Nucleases AND Phosphatases
7. The Global Phosphorus Cycle
The largest flow of phosphorus in the global cycle is from rivers to the oceans (21
Tg/yr), and about 10% of this is in reactive form which can be used by marine
organisms
The remainder is strongly bound to soil particles that deposit on the continental shelf.
On a time scale of hundreds of millions of years, these sediments mineralize and
become rock, and are uplifted and subject to rock weathering on land
So while there are significant stores of P on land and in the sea, very little is accessible
to organisms.
Thus, there is significant internal cycling where the available P is reused quite
efficiently in ecosystems
8. The Sulfur Cycle
• Sulfur originates from rocks, oceans, lakes and
swamps.
• Sulfur exists in the elemental form and as
hydrogen sulfide gas, sulfate, and thiosulfate.
• Plants and many microbes can assimilate only
SO4 and animals require an organic source –
amino acids: cystine, cysteine, and methionine.
• Bacteria convert environmental sulfurous
compounds into useful substrates.
9. Forms of S
• SO4 – Sulfate
• SO3 – Sulfite
• S2O3
-2
– Thiosulfate
• S0
– Elemental S
• S2-
– Sulfide
• Sulfatases
• Acid rains –imp in soil buffering
• H2S and CH3SH------- SO2------ H2SO4
• LICHENS Bio indicators of SO2 pollution
10. S Transformations
• Inorganic – both oxidative and reductive
• Organic – both oxidative and reductive,
assimilation, immobilization and
mineralization
• Photosynthetic – reductive as e donar
13. Key processes and prokaryotes in the sulfur cycle
Processes Organisms
Sulfide/sulfur oxidation(H2S→S0
→ SO4
2-
)
Aerobic Sulfur chemolithotrophs
(Thiobacillus, Beggiatoa, many others)
Anaerobic Purple and green phototrophic
bacteria, some chemolithotrophs
Sulfate reduction(anaerobic)(SO4
2-
→ H2S)
Desulfovibrio, Desulfobacter
Sulfur reduction(anaerobic) (S0
→ H2S)
Desulfuromonas, many
hyperthermophilic Archaea
Sulfur disproportionation(S2O3
2-
→ H2S + SO4
2-
)
Desulfovibrio and others
Organic sulfur compound oxidation or reduction(CH3SH→CO2+ H2S)
(DMSO→DMS)
Desulfurylation(organic-S → H2S)
Many organisms can do this
14. The Sulfur Cycle
Oxidative Sulfur Transformations
H2
S + 1/2 O2
S° + H2
O G = -50.1 kcal/mole
S° + 1 1/2 O2
+ H2
O H2
SO4
G =- -149.8
kcal/mole
Thiobacillus species
Reductive Sulfur Transformations
CH3
COOH + 2 H2
O + 4 S° 2 CO2
4 H2
S
Desulfuromonas
H2
+ SO4
2-
H2
S + 2 H2
O + 2 OH-
15. Ion Examples of enzymes containing this ion
Cupric Cytochrome oxidase
Ferrous or Ferric
Catalase
Cytochrome (via Heme)
Nitrogenase
Hydrogenase
Magnesium
Glucose 6-phosphatase
Hexokinase
DNA polymerase
Manganese
Arginase
Molybdenum Nitrate reductase
Nickel Urease
Selenium Glutathione peroxidase
Zinc
Alcohol dehydrogenase
Carbonic anhydrase
DNA polymerase
16. Donors Acceptors Products
H2 S°, S2O3 H2S
H2 CO CH4
H2 O2 , NO3 H2O, NO2
NH4+
, NO2 O2 NO2, NO3
HS, S°, S2O3 O2, NO3 S°, SO4
CH4, CO O2 CO2
Fe 2+
, Mn 2+
O2 Fe3+
, Mn4+
Energetic Base for Chemolithotrophy
at the
Deep Ocean Hydrothermal Vents
S - reduciers
Methanogens
H - oxidizers
Nitrifyiers
S - oxidizers
Methylotrophs
Fe - Mn oxidizers
17. • Bacteria play major roles in both the
oxidative and reductive sides of the sulfur
cycle.
• Sulfur- and sulfide-oxidizing bacteria
produce sulfate, while sulfate-reducing
bacteria consume sulfate as an electron
acceptor in anaerobic respiration, producing
hydrogen sulfide. Because sulfide is toxic and
also reacts with various metals, sulfate
reduction is an important biogeochemical
process.
• Dimethyl sulfide is the major organic sulfur
compound of ecological significance in nature.
18. Iron and manganese
• Iron and manganese cycling revolves
around the transition from oxidized
insoluble forms Fe+3 / Mn+4 to
reduced, soluble oxidation states
Fe+2
/Mn+2
.
19. Oxidation
• Ferrous iron (Fe+2) can be used as an electron donor, but can only be linked with
oxygen reduction. The availability of relatively high levels of Fe+2 is key to this
process. However, under aerobic conditions at near neutral pH iron exists almost
exclusively as solid Fe+3 oxides.
• Bacteria adapted to low pH may encounter higher levels of Fe+2 and thus have
conditions favoring use of Fe+2 as an electron donor. The pH effect on Fe+2
concentrations is reflected in the energy yield:
– Fe+2 + O2 + H+ ---> Fe+3 + H2O DG'o (pH 7) = - 0.25 kJ
• Thiobacillus ferrooxidans is an example of an acidophilic iron-oxidizer , which has
a pH optimum for growth of 2 to 3.
– At near neutral pH, Fe+2 concentrations increase with decreasing oxygen concentration.
The "iron bacteria" (e.g., Gallionella ) have adapted to grow by oxidizing Fe+2 at low O2
concentrations (0.1 - 0.2 mg L-1).
– Because of the low energy yields, microbes must oxidize large amounts of Fe+2 to
sustain growth. A small population of iron bacteria can thus generate a lot of Fe+3. This
is a problem for the well water industry as the resulting FeOOH (hydroxyoxides)
precipitates may clog wells.
20. Dissimilatory reduction
• Heterotrophic bacteria may support growth by coupling
oxidation of organics to Fe+3 reduction. However, as
indicated above, Fe+3 exists in the form of solid FeOOH.
• Thus, use of Fe+3 as an electron acceptor differs from all
other e- acceptors as it is in effect a solid substrate, which
requires physical contact between the bacteria and the FeOOH
and probably receptors or chelators in the cell wall to facilitate
Fe+3 uptake.
• The organisms mediating Fe+3 oxidation are ill-defined as
few iron-reducers have been characterized. These may be
organisms adapted to grow primarily with Fe+3 , that grow
with a variety of electron acceptors (e.g., Shewanella).
21. Fe and Mn transformations
•Primary minerals which contain iron -
biotite, pyroxene, amphibole, and
olivine.
•Iron oxides and hydroxides are
formed by protonation and release of
Fe ions out of primary or secondary
minerals and / or oxidation.
•Their occurence provides useful
information about soil formation.
22. Forms of Fe
• Iron oxides and hydroxides are very stable
under aerobic conditions, but they become more
soluble under anaerobic conditions (low redox
potentials).
• They are able to form metal-organic complexes,
where the metal cations are bonded by
functional groups such as -COOH, =CO, -OH, -
OCH3, -NH2, -SH to organic compounds
resulting in the formation of a ring structure
incorporating the metal ion.
• These complexes are very stable and called
chelates.
23. Iron exists in nature primarily in two oxidation
states, ferrous (Fe2+
) and ferric (Fe3+
), and
bacterial and chemical transformation of these
metals is of geological and ecological
importance. Bacterial ferric iron reduction
occurs in anoxic environments and results in
the mobilization of iron from swamps, bogs,
and other iron-rich aquatic habitats. Bacterial
oxidation of ferrous iron occurs on a large
scale at low pH and is very common in coal-
mining regions, where it results in a type of
pollution called acid mine drainage.
