267
hypothesized that nitrogen fixation was instrumental in the development of the blooms. More
recently, Heinbokel (1986), using epi-fluorescence microscopy5, found that Richelia were
much more frequent than previously thought (i.e. present in about 80% of the hosts' cells) in
two species of the diatom genus Hemiaulus from the Pacific Ocean, and concluded that the
significance of this kind of symbiosis in the nitrogen dynamics of the ocean may have been
largely underestimated in previous studies. Indeed, Carpenter (1983), in evaluating the global
contribution of N2 fixation effected by marine pelagic cyanobacteria (which he put at about
5 10 12 g a-I), pointed out the unfortunate lack of information on the contribution of fixed
nitrogen by Richelia; he emphasized that taking into account this contribution could result in
a major upward revision of the estimates.
Coral reefs are also thought to account for a significant share of global N2 fixation (Capone,
1983) and to play an important role in carbon sequestration. They develop amid an
environment that is particularly poor in nitrogen compounds, and have nevertheless be found
to be net exporters of N-nutrients, at a rate that could exhaust the reserves in the local
biomass pool within months (Webb et al., 1975). Interactions between the members of coral
reef communities are extremely complex (see, for instance, Mann, 1982); one of the major
links is the association between corals and zooxanthellae (endosymbiotic dinoflagellates),
which allows the corals to globally behave as functional photoautotrophs. In the same way as
phytoplankton, however, the algal symbionts require a conventional source of N-nutrients,
which cannot be supplied by the surrounding oligotrophic waters. Nitrogen fixation in coral
reefs is effected by the cyanobacterium Calothrix crustacea, that occurs in the form of a thin
film or a feltlike mat over different portions of the reefs (Wiebe et al., 1975). The process
has been estimated to take place at as high a rate as has been found in any other system,
marine or terrestrial (see Capone, 1983), and to be the most important nitrogen source for the
coral community, through various steps, which are still incompletely elucidated and may
include grazing by fish, excretion, nitrification by bacteria, etc. Alternatively, it has been
hypothesized (Rougerie and Wauthy, 1986) that the major source is an upward flow of
nutrient-rich deep water driven, within the porous structure of the reefs, by the heat flux from
the subsiding volcanoes on which atolls generally grow. Whatever their nitrogen source,
SLarge cells containing chloroplasts and endophytic cyanobacteria would be ideal tagets for flow-cytometric
studies.
hypothesized that nitrogen fixation was instrumental in the development of the blooms. More
recently, Heinbokel (1986), using epi-fluorescence microscopy5, found that Richelia were
much more frequent than previously thought (i.e. present in about 80% of the hosts' cells) in
two species of the diatom genus Hemiaulus from the Pacific Ocean, and concluded that the
significance of this kind of symbiosis in the nitrogen dynamics of the ocean may have been
largely underestimated in previous studies. Indeed, Carpenter (1983), in evaluating the global
contribution of N2 fixation effected by marine pelagic cyanobacteria (which he put at about
5 10 12 g a-I), pointed out the unfortunate lack of information on the contribution of fixed
nitrogen by Richelia; he emphasized that taking into account this contribution could result in
a major upward revision of the estimates.
Coral reefs are also thought to account for a significant share of global N2 fixation (Capone,
1983) and to play an important role in carbon sequestration. They develop amid an
environment that is particularly poor in nitrogen compounds, and have nevertheless be found
to be net exporters of N-nutrients, at a rate that could exhaust the reserves in the local
biomass pool within months (Webb et al., 1975). Interactions between the members of coral
reef communities are extremely complex (see, for instance, Mann, 1982); one of the major
links is the association between corals and zooxanthellae (endosymbiotic dinoflagellates),
which allows the corals to globally behave as functional photoautotrophs. In the same way as
phytoplankton, however, the algal symbionts require a conventional source of N-nutrients,
which cannot be supplied by the surrounding oligotrophic waters. Nitrogen fixation in coral
reefs is effected by the cyanobacterium Calothrix crustacea, that occurs in the form of a thin
film or a feltlike mat over different portions of the reefs (Wiebe et al., 1975). The process
has been estimated to take place at as high a rate as has been found in any other system,
marine or terrestrial (see Capone, 1983), and to be the most important nitrogen source for the
coral community, through various steps, which are still incompletely elucidated and may
include grazing by fish, excretion, nitrification by bacteria, etc. Alternatively, it has been
hypothesized (Rougerie and Wauthy, 1986) that the major source is an upward flow of
nutrient-rich deep water driven, within the porous structure of the reefs, by the heat flux from
the subsiding volcanoes on which atolls generally grow. Whatever their nitrogen source,
SLarge cells containing chloroplasts and endophytic cyanobacteria would be ideal tagets for flow-cytometric
studies.