S. Pantoja . S. Wakeham
equally distributed between terrigenous and marine systems (55% and 45%, respectively). On land, most organic carbon becomes incorporated into soils, while about
0.2 Gt yr- l each is transported as POC and DOC to the ocean. Marine photosynthesis,
dominated by the nano- and picoplankton, fixes about 50 Gt yr- l . However, since the
reservoir of marine biomass is relatively small, most OC is rapidly recycled in surface
waters. About 6 Gt yr- l is converted to DOC, and 4 Gt yr- l sinks as particulate matter
out of the upper ocean. The remaining 40 Gt yr- l is efficiently remineralized back to
inorganic nutrients and eventually re-enters the cycle via photosynthesis. Sinking OC
is also efficiently degraded in the interior of the ocean, with the result that 0.15 Gt yr- l
(or <1% of primary productivity), is buried in surface sediments and a fraction of this
is converted into kerogen. Tectonic uplift and weathering of sedimentary rocks returns
OC from kerogen to the contemporary OC cycle at a rate of 0.15 Gt yr- l .
Carbon undergoes transitions from highly oxidized to reduced states, ranging from
+4 for CO2 and HCO; to -4 for CH4 as it passes through the carbon cycle (Fig. 2.3).
Photosynthetic fIxation of inorganic carbon, whether CO2 or HCO;, involves a 4 electron reduction to produce organic matter with the generic formula CH20, in which C is in
the zero-oxidation state. Subsequent degradation of organic matter by a sequence of
terminal electron acceptors further reduces the oxidation state of C. In aerobic respiration, O2 is the electron acceptor for the reaction that converts glucose to CO2 and
H 2 0, with an energy yield of 686 kcal mOrl. In fermentation, a much less efficient process producing about 57 kcal mor l , energy is transferred by oxidation of part of the
organic molecule and reduction of another part. The low molecular weight organic
products are available to be used as substrates by other groups of microbes. Once oxygen has been depleted, bacteria use compounds other than oxygen as terminal electron acceptors, beginning with NO; (nitrate reduction; 650 kcal mor l ), and progressing
to soi- (sulfate reduction; -10 kcal mor l ) and CO2 (methanogenesis; -40 kcal mor l ).
Low molecular weight substrates, such as lactate, acetate, and H2 serve as electron
donors in sulfate reduction and methanogenesis, and H2S, CO2, H20 and CH4 are products. Since low molecular weight compounds are used in sulfate reduction and
methano-genesis, there is a close coupling of activities of fermenters, sulfate reducers,
and methanogens.
2.1.2
Nitrogen Cycle
Most of the nitrogen in earth is present as molecular dinitrogen in the atmosphere
(4000000 Gt). The oceanic nitrogen reservoir is also dominated by N 2 (20000 Gt),
followed by nitrate (570 Gt), which comprises about 6% of seawater N. The world ocean
appears to be experiencing a net loss of nitrogen. The magnitude of the input to the
sea from rivers, biological fIxation and precipitation accounts for about 70% of the
losses by denitrifIcation plus burial (Schlesinger 1997). Sediment burial removes nitrogen from seawater mainly as organic matter. Organic nitrogen trapped in sedimentary rocks is a major reservoir for nitrogen in the earth (2000000 Gt, Wada and Hattori
1991). During diagenesis, the CIN ratio of organic matter does not increase with sediment age as fast as between the surface of the ocean and the sediment surface. The
result is that 1 atom of nitrogen out of ca. 10000 produced in the photic zone is preserved in sediments (0.1%), similar to carbon burial efficiency (Hedges 1992).
equally distributed between terrigenous and marine systems (55% and 45%, respectively). On land, most organic carbon becomes incorporated into soils, while about
0.2 Gt yr- l each is transported as POC and DOC to the ocean. Marine photosynthesis,
dominated by the nano- and picoplankton, fixes about 50 Gt yr- l . However, since the
reservoir of marine biomass is relatively small, most OC is rapidly recycled in surface
waters. About 6 Gt yr- l is converted to DOC, and 4 Gt yr- l sinks as particulate matter
out of the upper ocean. The remaining 40 Gt yr- l is efficiently remineralized back to
inorganic nutrients and eventually re-enters the cycle via photosynthesis. Sinking OC
is also efficiently degraded in the interior of the ocean, with the result that 0.15 Gt yr- l
(or <1% of primary productivity), is buried in surface sediments and a fraction of this
is converted into kerogen. Tectonic uplift and weathering of sedimentary rocks returns
OC from kerogen to the contemporary OC cycle at a rate of 0.15 Gt yr- l .
Carbon undergoes transitions from highly oxidized to reduced states, ranging from
+4 for CO2 and HCO; to -4 for CH4 as it passes through the carbon cycle (Fig. 2.3).
Photosynthetic fIxation of inorganic carbon, whether CO2 or HCO;, involves a 4 electron reduction to produce organic matter with the generic formula CH20, in which C is in
the zero-oxidation state. Subsequent degradation of organic matter by a sequence of
terminal electron acceptors further reduces the oxidation state of C. In aerobic respiration, O2 is the electron acceptor for the reaction that converts glucose to CO2 and
H 2 0, with an energy yield of 686 kcal mOrl. In fermentation, a much less efficient process producing about 57 kcal mor l , energy is transferred by oxidation of part of the
organic molecule and reduction of another part. The low molecular weight organic
products are available to be used as substrates by other groups of microbes. Once oxygen has been depleted, bacteria use compounds other than oxygen as terminal electron acceptors, beginning with NO; (nitrate reduction; 650 kcal mor l ), and progressing
to soi- (sulfate reduction; -10 kcal mor l ) and CO2 (methanogenesis; -40 kcal mor l ).
Low molecular weight substrates, such as lactate, acetate, and H2 serve as electron
donors in sulfate reduction and methanogenesis, and H2S, CO2, H20 and CH4 are products. Since low molecular weight compounds are used in sulfate reduction and
methano-genesis, there is a close coupling of activities of fermenters, sulfate reducers,
and methanogens.
2.1.2
Nitrogen Cycle
Most of the nitrogen in earth is present as molecular dinitrogen in the atmosphere
(4000000 Gt). The oceanic nitrogen reservoir is also dominated by N 2 (20000 Gt),
followed by nitrate (570 Gt), which comprises about 6% of seawater N. The world ocean
appears to be experiencing a net loss of nitrogen. The magnitude of the input to the
sea from rivers, biological fIxation and precipitation accounts for about 70% of the
losses by denitrifIcation plus burial (Schlesinger 1997). Sediment burial removes nitrogen from seawater mainly as organic matter. Organic nitrogen trapped in sedimentary rocks is a major reservoir for nitrogen in the earth (2000000 Gt, Wada and Hattori
1991). During diagenesis, the CIN ratio of organic matter does not increase with sediment age as fast as between the surface of the ocean and the sediment surface. The
result is that 1 atom of nitrogen out of ca. 10000 produced in the photic zone is preserved in sediments (0.1%), similar to carbon burial efficiency (Hedges 1992).