Ecological Physiology of Photosynthesis in Corals
313
host tissues in a quite different way. The above mechanism of dissolved
glycerine and amino acid excretion is of secondary importance in this case.
The translocation process from zooxanthellae to their hosts' tissues proceeds
as the transfer of tiny droplets of lipids via a mechanism resembling
pinocytosis (Crossland 1980; Patton and Burris 1983). This process has been
observed directly under the microscope. The droplet of fat, extruded from
the cell, draws off the external cell membrane. Then it separates from the
cell, which is surrounded by this membrane (Fig. 8.7). The droplets of fat
covered with membrane might circulate via channels connecting the polyps
and inside the polyps, the same as they have been observed to do in the
body of the actin ian Condylactis (Kellog and Patton 1983). They also form
the lipid fraction of mucus, upon whose excretion corals lose up to 20-40%
of their total photosynthetic production (Crossland 1980; Sorokin et al.
1983). The fat, which corals get mainly from their algal symbionts, serves as
a main energy source in their metabolism, being also a basic stock material
in polyps, which accumulates in them under favorable feeding conditions
(Szamant-Froelich and Pilson 1980). The dry tissues of hermatypic corals
contain 25-30% of fat and other lipoid products (Bergmann et al. 1956).
Only about 25% of this fat is represented by the structural lipids
(phospholipids, galactolipids). The rest is mostly the share of reserve fat and
wax (Patton et al. 1977). The lipogenesis itself is localized mainly in cells of
the zooxanthellae, from which lipoids thus formed are instantly transferred
to the cells of the polyps and are absorbed into the total stock of fat and
other lipoid materials of the coral. Besides fat, the translocation of other
organic compounds, like amino acids and carbohydrates, by zooxanthellae
was also observed (Mascatine et al. 1972; Burris 1983; Wafar et al. 1985).
These products may be then transported through the colony to the points of
its growth and calcification (Pearse and Muscatine 1971).
The translocation has been usually estimated as a percentage of the net or
of the gross photosynthetic production. Estimated by a most reliable method
of calculation from the values of photosynthetic production minus the
metabolic and growth expenditures of the zooxanthellae themselves, it
appears to be very high - within 95-99% of the net, and 90-93% of the
gross autotrophic production in a coral colony (Muscatine et al. 1984, 1985).
8.4 Ecological Physiology of Photosynthesis in Corals
The course of the coral's photosynthesis rate during the day is characterized
by curves with one maximum at midday (Porter 1980; Porter et al. 1984;
Sorokin 1984a; cf. Figs. 8.8-8.10). In full sunlight, the midday depression
due to excess light was observed only in some weakly branching (Porites) or
foliose (Merulina) corals (Fig. 8.10). In most of the ramose, cribose,
313
host tissues in a quite different way. The above mechanism of dissolved
glycerine and amino acid excretion is of secondary importance in this case.
The translocation process from zooxanthellae to their hosts' tissues proceeds
as the transfer of tiny droplets of lipids via a mechanism resembling
pinocytosis (Crossland 1980; Patton and Burris 1983). This process has been
observed directly under the microscope. The droplet of fat, extruded from
the cell, draws off the external cell membrane. Then it separates from the
cell, which is surrounded by this membrane (Fig. 8.7). The droplets of fat
covered with membrane might circulate via channels connecting the polyps
and inside the polyps, the same as they have been observed to do in the
body of the actin ian Condylactis (Kellog and Patton 1983). They also form
the lipid fraction of mucus, upon whose excretion corals lose up to 20-40%
of their total photosynthetic production (Crossland 1980; Sorokin et al.
1983). The fat, which corals get mainly from their algal symbionts, serves as
a main energy source in their metabolism, being also a basic stock material
in polyps, which accumulates in them under favorable feeding conditions
(Szamant-Froelich and Pilson 1980). The dry tissues of hermatypic corals
contain 25-30% of fat and other lipoid products (Bergmann et al. 1956).
Only about 25% of this fat is represented by the structural lipids
(phospholipids, galactolipids). The rest is mostly the share of reserve fat and
wax (Patton et al. 1977). The lipogenesis itself is localized mainly in cells of
the zooxanthellae, from which lipoids thus formed are instantly transferred
to the cells of the polyps and are absorbed into the total stock of fat and
other lipoid materials of the coral. Besides fat, the translocation of other
organic compounds, like amino acids and carbohydrates, by zooxanthellae
was also observed (Mascatine et al. 1972; Burris 1983; Wafar et al. 1985).
These products may be then transported through the colony to the points of
its growth and calcification (Pearse and Muscatine 1971).
The translocation has been usually estimated as a percentage of the net or
of the gross photosynthetic production. Estimated by a most reliable method
of calculation from the values of photosynthetic production minus the
metabolic and growth expenditures of the zooxanthellae themselves, it
appears to be very high - within 95-99% of the net, and 90-93% of the
gross autotrophic production in a coral colony (Muscatine et al. 1984, 1985).
8.4 Ecological Physiology of Photosynthesis in Corals
The course of the coral's photosynthesis rate during the day is characterized
by curves with one maximum at midday (Porter 1980; Porter et al. 1984;
Sorokin 1984a; cf. Figs. 8.8-8.10). In full sunlight, the midday depression
due to excess light was observed only in some weakly branching (Porites) or
foliose (Merulina) corals (Fig. 8.10). In most of the ramose, cribose,