Chapter 6
Organic Chemical Reaction Rates in the Ocean:
Molecular Approaches to Studying Extracellular
Biochemical Processes
S. Pantoja
6.1
Introduction
The recycling efficiency of organic matter produced in situ or supplied externally to
the ocean is so high that continuous regeneration of recycled nutrients allows levels of
production that would not be possible otherwise. In practice, photosynthetic production of organic matter in the ocean (simplified as H20 + CO2 + nutrients ~ organic
matter + electron acceptor) is almost balanced by the reverse reaction, respiration (organic matter + electron acceptor ~ H20 + CO2 + nutrients), therefore, more than
99% of the organic matter produced is recycled (Hedges 1992). The remaining organic
matter is buried in sedimentary rocks (see Chapter 5). Uplifting and weathering of sedimentary rocks provide a link between biologically and geologically-mediated processes.
A simplified model of organic matter recycling is shown in Fig. 6.1. In this model,
organic matter produced in the photic zone, mainly by photosynthetic organisms, is
degraded to inorganic compounds, a process called mineralization. In the open ocean,
only 1 % of the organic matter produced by photosynthesis escapes mineralization and
reaches the bottom waters (Henrichs and Reeburgh 1987). In sediments, production
and degradation continue, leaving only a small fraction of the organic matter produced
by photosynthesis to be preserved in the sedimentary record (ca. 0.1%, Hedges 1992).
Degradation of organic matter has been studied in sea water and sediments in order to understand cycling of chemical compounds in the environment. Furthermore,
degradation supplies nutrients for photosynthesis and labile! molecules for bacterial
consumption. Organic matter is continuously removed from surface waters by the
downward sinking of dead organisms and faecal pellets. Degradation provides a
mechanism to release nutrients from the organic matrix by transformation into the
inorganic form. These nutrients, released at depth due to degradation maybe transported up to the photic zone by mixing, and support new production. Eppley and
Peterson (1979) showed that new production approximates the sinking flux of particulate organic matter.
Analysing small concentrations of individual organic compounds in seawater has
been a challenge to chemical oceanographers, yet reactions involving these compounds
are critical links within biogeochemical processes. For instance, it has been recognized
that the chemical structure of organic matter may determine the extent and rates of
degradation (e.g. Henrichs and Doyle 1986; Canfield 1994). Understanding how molI Labile molecules are defined here as the ones that easily degrade via biochemical reactions.
Organic Chemical Reaction Rates in the Ocean:
Molecular Approaches to Studying Extracellular
Biochemical Processes
S. Pantoja
6.1
Introduction
The recycling efficiency of organic matter produced in situ or supplied externally to
the ocean is so high that continuous regeneration of recycled nutrients allows levels of
production that would not be possible otherwise. In practice, photosynthetic production of organic matter in the ocean (simplified as H20 + CO2 + nutrients ~ organic
matter + electron acceptor) is almost balanced by the reverse reaction, respiration (organic matter + electron acceptor ~ H20 + CO2 + nutrients), therefore, more than
99% of the organic matter produced is recycled (Hedges 1992). The remaining organic
matter is buried in sedimentary rocks (see Chapter 5). Uplifting and weathering of sedimentary rocks provide a link between biologically and geologically-mediated processes.
A simplified model of organic matter recycling is shown in Fig. 6.1. In this model,
organic matter produced in the photic zone, mainly by photosynthetic organisms, is
degraded to inorganic compounds, a process called mineralization. In the open ocean,
only 1 % of the organic matter produced by photosynthesis escapes mineralization and
reaches the bottom waters (Henrichs and Reeburgh 1987). In sediments, production
and degradation continue, leaving only a small fraction of the organic matter produced
by photosynthesis to be preserved in the sedimentary record (ca. 0.1%, Hedges 1992).
Degradation of organic matter has been studied in sea water and sediments in order to understand cycling of chemical compounds in the environment. Furthermore,
degradation supplies nutrients for photosynthesis and labile! molecules for bacterial
consumption. Organic matter is continuously removed from surface waters by the
downward sinking of dead organisms and faecal pellets. Degradation provides a
mechanism to release nutrients from the organic matrix by transformation into the
inorganic form. These nutrients, released at depth due to degradation maybe transported up to the photic zone by mixing, and support new production. Eppley and
Peterson (1979) showed that new production approximates the sinking flux of particulate organic matter.
Analysing small concentrations of individual organic compounds in seawater has
been a challenge to chemical oceanographers, yet reactions involving these compounds
are critical links within biogeochemical processes. For instance, it has been recognized
that the chemical structure of organic matter may determine the extent and rates of
degradation (e.g. Henrichs and Doyle 1986; Canfield 1994). Understanding how molI Labile molecules are defined here as the ones that easily degrade via biochemical reactions.