35
constraining global primary production (Moore et al. 2013; Redfield et al. 1963;
Tyrrell 1999).
Dugdale and colleagues (Dugdale 1967; Dugdale and Goering 1967) developed
a conceptual model of N sources which support primary productivity, distinguishing
between “new” sources which were imported into the euphotic zone where photosynthesis was occurring and “recycled” forms which arose from regeneration within
the euphotic zone itself (McCarthy and Goldman 1979). Nitrate, which can be
imported into the euphotic zone by diffusion or upwelling from large reservoirs at
depth in the ocean, is a main form of new N relative to ammonia and urea which are
released during the breakdown of organic matter (within metazoans through the
urea cycle (Carpenter et al. 1972; McCarthy 1972), or by cleavage from amino acids
during microbial degradation (Palenik and Morel 1991). We also now recognize that
some nitrate may be “recycled”, formed in the euphotic zone by nitrification (Ward
2011; Ward et al. 1989).
N 2 fixation was recognized as a form of “new” N in the original formulation of
the “new production” model (Dugdale and Goering 1967) but received little attention as it was assumed to be a minor factor, particularly in coastal and upwelling
systems where major fisheries existed and where much oceanographic research at
the time was focused (Cushing 1971, 1975). Major oceanic N cycle studies in the
1970s tended to focus on combined forms of N such as nitrate (Blasco et al. 1984;
Eppley et al. 1969, 1979). Ironically, N 2 fixation was also assumed by some researchers to make up any overall deficits that might occur in the ocean N cycle (Redfield
1958). Interestingly, Trichodesmium was used as an indicator of the outer edges of
upwelling regions by some phycologists (Peired et al. 1985).
An infusion of new technologies and approaches to study marine N 2 fixation
began in the early 1960s and included the use of enriched isotopic tracers of dinitrogen (
15
N 2 ), natural abundance studies of the N 2 isotopes, and introduction of a simple enzyme based assay amenable to field studies, the acetylene reduction method
(see Chap. 5 for details). The discovery that Trichodesmium was indeed diazotrophic was a key turning point, although it was met with some skepticism as the only
known cyanobacterial diazotrophs at that time were heterocyst-forming forms
(Fogg 1978). Some suggested that the measured N 2 fixation might be due to associated bacteria, not Trichodesmium itself (Bunt et al. 1970; Taylor et al. 1973). Kaori
Ohki (Ohki and Fujita 1982; Ohki et al. 1986) first brought Trichodesmium into
unialgal culture (NIBB 1067) which led to a suite of studies confirming its role in
N 2 fixation (Ohki and Fujita 1988), its regulation (Ohki et al. 1991; Ohki et al. 1992;
Zehr et al. 1991) and physiology (Mulholland et al. 1999, 2001). A second culture
(IMS 101) which has been widely disseminated was obtained by Prufert-Bebout
et al. (1993).
With advances in methods, accelerating field studies in the 1970s measured substantial N 2 fixation rates in the water column of some tropical and sub-tropical areas
and coastal seas (e.g. the Caribbean, South China Sea) largely, but not exclusively,
focused on Trichodesmium (Carpenter and McCarthy 1975; Carpenter and Price
1976; Dugdale et al. 1961, 1964; Goering et al. 1966; Mague et al. 1974, 1977;
Martinez et al. 1983; Saino and Hattori 1978, 1980; Villareal 1990, 1991). Ironically,
3.2 Process Oriented Studies: Water Column
constraining global primary production (Moore et al. 2013; Redfield et al. 1963;
Tyrrell 1999).
Dugdale and colleagues (Dugdale 1967; Dugdale and Goering 1967) developed
a conceptual model of N sources which support primary productivity, distinguishing
between “new” sources which were imported into the euphotic zone where photosynthesis was occurring and “recycled” forms which arose from regeneration within
the euphotic zone itself (McCarthy and Goldman 1979). Nitrate, which can be
imported into the euphotic zone by diffusion or upwelling from large reservoirs at
depth in the ocean, is a main form of new N relative to ammonia and urea which are
released during the breakdown of organic matter (within metazoans through the
urea cycle (Carpenter et al. 1972; McCarthy 1972), or by cleavage from amino acids
during microbial degradation (Palenik and Morel 1991). We also now recognize that
some nitrate may be “recycled”, formed in the euphotic zone by nitrification (Ward
2011; Ward et al. 1989).
N 2 fixation was recognized as a form of “new” N in the original formulation of
the “new production” model (Dugdale and Goering 1967) but received little attention as it was assumed to be a minor factor, particularly in coastal and upwelling
systems where major fisheries existed and where much oceanographic research at
the time was focused (Cushing 1971, 1975). Major oceanic N cycle studies in the
1970s tended to focus on combined forms of N such as nitrate (Blasco et al. 1984;
Eppley et al. 1969, 1979). Ironically, N 2 fixation was also assumed by some researchers to make up any overall deficits that might occur in the ocean N cycle (Redfield
1958). Interestingly, Trichodesmium was used as an indicator of the outer edges of
upwelling regions by some phycologists (Peired et al. 1985).
An infusion of new technologies and approaches to study marine N 2 fixation
began in the early 1960s and included the use of enriched isotopic tracers of dinitrogen (
15
N 2 ), natural abundance studies of the N 2 isotopes, and introduction of a simple enzyme based assay amenable to field studies, the acetylene reduction method
(see Chap. 5 for details). The discovery that Trichodesmium was indeed diazotrophic was a key turning point, although it was met with some skepticism as the only
known cyanobacterial diazotrophs at that time were heterocyst-forming forms
(Fogg 1978). Some suggested that the measured N 2 fixation might be due to associated bacteria, not Trichodesmium itself (Bunt et al. 1970; Taylor et al. 1973). Kaori
Ohki (Ohki and Fujita 1982; Ohki et al. 1986) first brought Trichodesmium into
unialgal culture (NIBB 1067) which led to a suite of studies confirming its role in
N 2 fixation (Ohki and Fujita 1988), its regulation (Ohki et al. 1991; Ohki et al. 1992;
Zehr et al. 1991) and physiology (Mulholland et al. 1999, 2001). A second culture
(IMS 101) which has been widely disseminated was obtained by Prufert-Bebout
et al. (1993).
With advances in methods, accelerating field studies in the 1970s measured substantial N 2 fixation rates in the water column of some tropical and sub-tropical areas
and coastal seas (e.g. the Caribbean, South China Sea) largely, but not exclusively,
focused on Trichodesmium (Carpenter and McCarthy 1975; Carpenter and Price
1976; Dugdale et al. 1961, 1964; Goering et al. 1966; Mague et al. 1974, 1977;
Martinez et al. 1983; Saino and Hattori 1978, 1980; Villareal 1990, 1991). Ironically,
3.2 Process Oriented Studies: Water Column