CHAPTER 6 . Organic Chemical Reaction Rates in the Ocean:
149
6.3
Extracellular Hydrolysis of Peptides in Sea Water and Sediments
Peptides and proteins are important components of dissolved organic matter, accounting for 5-20% of dissolved organic nitrogen and 3-4% of dissolved organic carbon
(DOC) in seawater (Sharp 1983; Thurman 1985). In surface sediments, the protein pool
accounts for ca. 10% of organic carbon (Degens 1970; Burdige and Martens 1988) and
30-40% of total nitrogen (Rosenfeld 1979; Henrichs et al.1984-; Burdige and Martens 1988).
In the ocean, photosynthetic organic matter produced in the photic zone involves
the cellular synthesis of macromolecules. On average, more than 50% of the dry weight
of phytoplankton is in the form of proteins, and more than 30% in polysaccharides
(Parsons et al.1961). In summary, most organic matter (about 80%) in phytoplankton
is in the form of macromolecules. 2 As discussed before, most of this planktonic material is degraded in the water column or in surface sediments.
The current model of macromolecular degradation in the ocean (Fig. 6.9) assumes
that extracellular hydrolysis of macromolecules yields smaller molecules (e.g. Billen
1984; Chr6st 1991; Hoppe 1991). Thus, protein degradation will produce peptides and
free amino acids via free enzymes or enzymes associated with microorganisms or other
particles. Although the presence of peptides (amino acid sequences of MW < 6 000 D)
in marine environments has not been clearly documented, they are believed to be
important intermediates in the degradation of organic matter, and most of the carbon and nitrogen in protein may be cycled through this pathway.
Although larger organisms can consume protein and hydrolyze it internally, bacteria must initially hydrolyze proteins and peptides to smaller substrates outside the cell
(Payne 1980). When they are small enough (oligopeptides and amino acids), they are
incorporated across the cell membrane and mineralized. Molecules larger than approximately 600 Daltons cannot be transported across microbial cell membranes
(Nikaido and Vaara 1985).
6.3.1
Experimental Approaches to Studying Degradation of Proteinaceous Material
in the Ocean
6.3.1.1
Radiolabelled Substrates
Several radiolabelled substrates have been used to measure rates of degradation of
proteins in seawater: methyl-e 4 C]methemoglobin and [125I]BSA 3 (Hollibaugh and
Azam 1983), [14C]-methylated protein mixture (Billen 1991), and [14C]-uniformly-Iabelled Rubisc0 4 and [14C]-glucosylated Rubisco (Kiel and Kirchman 1993). In sediments,
2 Macromolecules are defined here as compounds of molecular weight larger than about 1000 Daltons.
To give a sense of the units: molecular weight of water = 18, average amino acid = 141, a large
protein = 600 000 Daltons.]
3 Bovine serum albumin.
4 Ribulose-l,5-bisphosphate carboxylase/oxygenase.
149
6.3
Extracellular Hydrolysis of Peptides in Sea Water and Sediments
Peptides and proteins are important components of dissolved organic matter, accounting for 5-20% of dissolved organic nitrogen and 3-4% of dissolved organic carbon
(DOC) in seawater (Sharp 1983; Thurman 1985). In surface sediments, the protein pool
accounts for ca. 10% of organic carbon (Degens 1970; Burdige and Martens 1988) and
30-40% of total nitrogen (Rosenfeld 1979; Henrichs et al.1984-; Burdige and Martens 1988).
In the ocean, photosynthetic organic matter produced in the photic zone involves
the cellular synthesis of macromolecules. On average, more than 50% of the dry weight
of phytoplankton is in the form of proteins, and more than 30% in polysaccharides
(Parsons et al.1961). In summary, most organic matter (about 80%) in phytoplankton
is in the form of macromolecules. 2 As discussed before, most of this planktonic material is degraded in the water column or in surface sediments.
The current model of macromolecular degradation in the ocean (Fig. 6.9) assumes
that extracellular hydrolysis of macromolecules yields smaller molecules (e.g. Billen
1984; Chr6st 1991; Hoppe 1991). Thus, protein degradation will produce peptides and
free amino acids via free enzymes or enzymes associated with microorganisms or other
particles. Although the presence of peptides (amino acid sequences of MW < 6 000 D)
in marine environments has not been clearly documented, they are believed to be
important intermediates in the degradation of organic matter, and most of the carbon and nitrogen in protein may be cycled through this pathway.
Although larger organisms can consume protein and hydrolyze it internally, bacteria must initially hydrolyze proteins and peptides to smaller substrates outside the cell
(Payne 1980). When they are small enough (oligopeptides and amino acids), they are
incorporated across the cell membrane and mineralized. Molecules larger than approximately 600 Daltons cannot be transported across microbial cell membranes
(Nikaido and Vaara 1985).
6.3.1
Experimental Approaches to Studying Degradation of Proteinaceous Material
in the Ocean
6.3.1.1
Radiolabelled Substrates
Several radiolabelled substrates have been used to measure rates of degradation of
proteins in seawater: methyl-e 4 C]methemoglobin and [125I]BSA 3 (Hollibaugh and
Azam 1983), [14C]-methylated protein mixture (Billen 1991), and [14C]-uniformly-Iabelled Rubisc0 4 and [14C]-glucosylated Rubisco (Kiel and Kirchman 1993). In sediments,
2 Macromolecules are defined here as compounds of molecular weight larger than about 1000 Daltons.
To give a sense of the units: molecular weight of water = 18, average amino acid = 141, a large
protein = 600 000 Daltons.]
3 Bovine serum albumin.
4 Ribulose-l,5-bisphosphate carboxylase/oxygenase.