266
NITROGEN FIXATION (AND DENITRIFICATION)
Direct fixation of N2 gas by phytoplankton is one mechanism by which marine ecosystems
may influence the export of carbon to depth. Until recently, it had been assumed that direct
fixation of N2 by cyanobacteria was negligible in surface waters, because this process does
not normally take place in the presence of oxygen. However, recent evidence (reviewed by
Legendre and Gosselin, 1989) suggests that the upper layer of the ocean may offer a variety
of reducing environments, potentially favourable for N2 fixation, which include
oxygen-depleted microzones associated with surfaces of organic and inorganic aggregates and
internal microzones within aggregates (bundles) of the filamentous N-fixing cyanobacterium
Oscillatoria spp. In addition, Ohki and Fujita (1988) have shown that N2 fixation could
sometimes occur in the presence of relatively high oxygen concentrations. Under steady state
conditions, phytoplankton production derived from N2 fixation should lead to carbon export.
Indeed, Lewis et al. (1986) pointed out that estimates of nitrogen supply by vertical turbulent
transport, in the oligotrophic ocean, are too low to meet the demand implied by production
estimates derived from indirect methods such as annual changes in oxygen concentration, and
that part of the discrepancy could be accounted for if the amount of N2 fixation were higher
than previously thought. It must actually be stressed that, because N2 is the most abundant gas
dissolved in seawater, the export of biogenic carbon fuelled by nitrogen fixation is
independent of any hydrodynamically or atmospherically driven influx of N0 3 · or NHiNH4 +.
It follows that conditions enhancing N2 fixation may be of major significance for the export
of biogenic carbon from the surface waters, especially in oligotrophic oceanic waters where
nitrate-driven new production is usually thought to be low (Eppley and Peterson, 1979),
although the most recent estimates tend to be somewhat higher than the previous ones (see
Eppley, 1989, and Platt et al., 1989b).
Eukaryotic cells provide an ideal reducing environment for N2 fixation by cyanobacteria. In
the N-depleted oligotrophic waters of the central North Pacific Ocean, for example, the
endophytic cyanobacterium Richelia intracellularis, which has been shown to be N 2 -fixing
(Mague et al., 1974), is frequently associated with several species of the diatom genus
Rhizosolenia. Venrick (1974) found it in more than 90% of the cells in three Rhizosolenia
species that were responsible for localized blooms in the North Pacific Central Gyre, and she
NITROGEN FIXATION (AND DENITRIFICATION)
Direct fixation of N2 gas by phytoplankton is one mechanism by which marine ecosystems
may influence the export of carbon to depth. Until recently, it had been assumed that direct
fixation of N2 by cyanobacteria was negligible in surface waters, because this process does
not normally take place in the presence of oxygen. However, recent evidence (reviewed by
Legendre and Gosselin, 1989) suggests that the upper layer of the ocean may offer a variety
of reducing environments, potentially favourable for N2 fixation, which include
oxygen-depleted microzones associated with surfaces of organic and inorganic aggregates and
internal microzones within aggregates (bundles) of the filamentous N-fixing cyanobacterium
Oscillatoria spp. In addition, Ohki and Fujita (1988) have shown that N2 fixation could
sometimes occur in the presence of relatively high oxygen concentrations. Under steady state
conditions, phytoplankton production derived from N2 fixation should lead to carbon export.
Indeed, Lewis et al. (1986) pointed out that estimates of nitrogen supply by vertical turbulent
transport, in the oligotrophic ocean, are too low to meet the demand implied by production
estimates derived from indirect methods such as annual changes in oxygen concentration, and
that part of the discrepancy could be accounted for if the amount of N2 fixation were higher
than previously thought. It must actually be stressed that, because N2 is the most abundant gas
dissolved in seawater, the export of biogenic carbon fuelled by nitrogen fixation is
independent of any hydrodynamically or atmospherically driven influx of N0 3 · or NHiNH4 +.
It follows that conditions enhancing N2 fixation may be of major significance for the export
of biogenic carbon from the surface waters, especially in oligotrophic oceanic waters where
nitrate-driven new production is usually thought to be low (Eppley and Peterson, 1979),
although the most recent estimates tend to be somewhat higher than the previous ones (see
Eppley, 1989, and Platt et al., 1989b).
Eukaryotic cells provide an ideal reducing environment for N2 fixation by cyanobacteria. In
the N-depleted oligotrophic waters of the central North Pacific Ocean, for example, the
endophytic cyanobacterium Richelia intracellularis, which has been shown to be N 2 -fixing
(Mague et al., 1974), is frequently associated with several species of the diatom genus
Rhizosolenia. Venrick (1974) found it in more than 90% of the cells in three Rhizosolenia
species that were responsible for localized blooms in the North Pacific Central Gyre, and she