Autotrophic Feeding of Corals
329
coefficients could be estimated by using the curves of diurnal course of
photosynthesis and respiration. The value of Pt per day (Ptd) is equal to: Ptd
= Kp' Pth (d. Table 8.5). Most authors accepted that Mth was
approximately constant during the 24-h cycle. Thus: Mt = Mth x 24. The
Mth values in this case were taken as an average of those measured in
daytime and at night (Wethey and Porter 1976; Porter 1980; Sorokin 1984c).
There is also an opinion that the daytime respiration of corals could be
1.5-2 times less than the nocturnal one (Porter et al. 1984; Titlyanov et al.
1988a). But the corresponding data could also be an artifice, because the
daytime experiments were conducted in waters oversaturated with oxygen,
which might lose part of it during the bottle experiments as bubbles.
Also, the first estimations of photosynthesis and respiration in corals
made in the early 1930s by M. Yonge and S. Kawaguti showed that, as in
green plants, photosynthesis outweighs respiration in diurnal balance (see
Kawaguti 1937). To date we have a wide range of data on the rates of these
processes, which were obtained by various methods and techniques. But the
data available were presented by authors in various dimensions: per hour or
per day, per cm 2 of surface, per g of weight of colony, per mg of chlorophyll
or per 10 6 cells of zooxanthellae, expressed in units of O2 or carbon.
Therefore, it is quite obvious that the fluctuation of data, given by different
authors, could sometimes reflect mostly differences in skeletal composition,
colony forms or in the biomass of individual corals used in experiments
rather than the efficient functioning of their photosynthesis apparatus. It
seems reasonable therefore to express the metabolic rates measured, either
of photosynthesis or respiration, in carbon units per biomass of active tissues
of the corals colony. Davils (1980) and Lewis and Post (1982) have
calculated them per biomass of colony decalcinated matter. But a significant
part of it - more than half - is comprised of inert organic substances of
coenosarc and skeleton. Therefore, it seems even more reasonable to
calculate their rates per biomass of the polyps, isolated from coenosarc
(Sorokin 1984b).
The data on the photosynthesis rates per cm 2 of coral colony surface, or
per projective area, permit one to calculate also the metabolic rates of corals
in their natural communities (Wethey and Porter 1976). An important value
in this respect is the ratio Pt/Mt per day. The same ratios calculated per hour
and measured at midday are much less informative for the evaluation of the
importance of photosynthesis in the energy balance of corals. Nevertheless,
the latter sometimes have been also used to evaluate the relative
"autotrophy" of various corals and to estimate the depth of the
compensation point, where Pt = Mt, which is obviously wrong. But even if
the depth of the compensation point could be estimated correctly, e.g., with
a 24-h balance of Pt/Mt, it remains largely conventional, because corals
spend energy not only for respiration, but also for their multiplication,
growth and mucus production. The respiration comprises only a part (6070%) of the total energy demand. So the coral would need additional
329
coefficients could be estimated by using the curves of diurnal course of
photosynthesis and respiration. The value of Pt per day (Ptd) is equal to: Ptd
= Kp' Pth (d. Table 8.5). Most authors accepted that Mth was
approximately constant during the 24-h cycle. Thus: Mt = Mth x 24. The
Mth values in this case were taken as an average of those measured in
daytime and at night (Wethey and Porter 1976; Porter 1980; Sorokin 1984c).
There is also an opinion that the daytime respiration of corals could be
1.5-2 times less than the nocturnal one (Porter et al. 1984; Titlyanov et al.
1988a). But the corresponding data could also be an artifice, because the
daytime experiments were conducted in waters oversaturated with oxygen,
which might lose part of it during the bottle experiments as bubbles.
Also, the first estimations of photosynthesis and respiration in corals
made in the early 1930s by M. Yonge and S. Kawaguti showed that, as in
green plants, photosynthesis outweighs respiration in diurnal balance (see
Kawaguti 1937). To date we have a wide range of data on the rates of these
processes, which were obtained by various methods and techniques. But the
data available were presented by authors in various dimensions: per hour or
per day, per cm 2 of surface, per g of weight of colony, per mg of chlorophyll
or per 10 6 cells of zooxanthellae, expressed in units of O2 or carbon.
Therefore, it is quite obvious that the fluctuation of data, given by different
authors, could sometimes reflect mostly differences in skeletal composition,
colony forms or in the biomass of individual corals used in experiments
rather than the efficient functioning of their photosynthesis apparatus. It
seems reasonable therefore to express the metabolic rates measured, either
of photosynthesis or respiration, in carbon units per biomass of active tissues
of the corals colony. Davils (1980) and Lewis and Post (1982) have
calculated them per biomass of colony decalcinated matter. But a significant
part of it - more than half - is comprised of inert organic substances of
coenosarc and skeleton. Therefore, it seems even more reasonable to
calculate their rates per biomass of the polyps, isolated from coenosarc
(Sorokin 1984b).
The data on the photosynthesis rates per cm 2 of coral colony surface, or
per projective area, permit one to calculate also the metabolic rates of corals
in their natural communities (Wethey and Porter 1976). An important value
in this respect is the ratio Pt/Mt per day. The same ratios calculated per hour
and measured at midday are much less informative for the evaluation of the
importance of photosynthesis in the energy balance of corals. Nevertheless,
the latter sometimes have been also used to evaluate the relative
"autotrophy" of various corals and to estimate the depth of the
compensation point, where Pt = Mt, which is obviously wrong. But even if
the depth of the compensation point could be estimated correctly, e.g., with
a 24-h balance of Pt/Mt, it remains largely conventional, because corals
spend energy not only for respiration, but also for their multiplication,
growth and mucus production. The respiration comprises only a part (6070%) of the total energy demand. So the coral would need additional