Photosynthesis in Aquatic Plants
313
emersion. This finding is of interest in the context of the extent of limitation
of productivity by inorganic carbon supply considered under Section 15.6
above. If light is limiting productivity, then emersed CO2 acquisition in the
light might be advantageous for plants growing in the turbid coastal waters.
However, if nutrients other than carbon are limiting productivity, carbon
acquisition during emersion (when the supply of combined nitrogen or
phosphorus is negligible) serves to further exacerbate the potential imbalance
in supply of carbon relative to the more rate-limiting nutrient. Thomas et al
(1987) have shown a substantial post-emersion stimulation of uptake of
(noncarbon) nutrients when submersed which could occur if nitrogen or
phosphorus supply were limited by diffusion through boundary layers unless
very large changes in the capacity for transport across the plasmalemma at
lower concentrations at the transporter site. Further work is clearly needed.
Two further aspects are of current interest. One relates to the relationship between carbon gain during dehydration during emersion and the
mechanism of inorganic carbon assimilation. Surif and Raven (1990) have
suggested that intertidal macroalgae with COr concentrating mechanisms
could, by maintaining a larger CO2 concentration gradient through gaseous
diffusion boundary layers than could plants with a purely diffusive CO2
supply to Rubisco, fix more CO2 per unit water evaporated. This means
that, with a fixed supply of thallus water (the dowry left by the retreating
tide), more inorganic carbon could be fixed before the extent of water loss
curtailed photosynthesis in each desiccation event. This suggestion should be
tested. Analogous work on terrestrial lichens (Cowan et al. 1992) is more
complex since the sites of CO2 uptake and water vapor loss are more likely
to be different in these lichens than in the intertidal macroalgae.
The other aspect of water relations of intertidal macroalgae which can
be related to those of certain terrestrial lichens and related free-living
terrestrial algae is that of the possibility of uptake of water vapor from an
unsaturated atmosphere by desiccated algae. Terrestrial lichens with green
photobionts, and related free-living terrestrial green algae, have this capacity
to rehydrate without contact with liquid water (Bertsch 1966; Lange 1989).
Such a water uptake mechanism has not been reported for intertidal algae,
and may be unlikely (although it should be tested for in such possibly
secondarily aquatic littoral fringe algae as Prasiola, which furthermore forms
lichen-like associations with fungi: Raven and Johnston 1991a). However,
results of Cooper and De Niro (1989) on the 2H/IH and lS0/160 of water in
intertidal macroalgae and seagrasses may give a pointer here. Net loss of
water from emersed plants by evaporation preferentially removes water
depleted in the heavy isotopes 2H and ISO; however, the water in emersed
specimens of the intertidal plants is itself depleted in 2H and ISO, instead of
being enriched in the heavy isotopes as would be expected from net loss of
2H_ and ISO-depleted water vapour (Cooper and De Niro 1989). The suggested explant at ion is exchange of water in the plant with 2H_ and lSO_
depleted water vapor overcoming the effect of net evaporative loss (Cooper
313
emersion. This finding is of interest in the context of the extent of limitation
of productivity by inorganic carbon supply considered under Section 15.6
above. If light is limiting productivity, then emersed CO2 acquisition in the
light might be advantageous for plants growing in the turbid coastal waters.
However, if nutrients other than carbon are limiting productivity, carbon
acquisition during emersion (when the supply of combined nitrogen or
phosphorus is negligible) serves to further exacerbate the potential imbalance
in supply of carbon relative to the more rate-limiting nutrient. Thomas et al
(1987) have shown a substantial post-emersion stimulation of uptake of
(noncarbon) nutrients when submersed which could occur if nitrogen or
phosphorus supply were limited by diffusion through boundary layers unless
very large changes in the capacity for transport across the plasmalemma at
lower concentrations at the transporter site. Further work is clearly needed.
Two further aspects are of current interest. One relates to the relationship between carbon gain during dehydration during emersion and the
mechanism of inorganic carbon assimilation. Surif and Raven (1990) have
suggested that intertidal macroalgae with COr concentrating mechanisms
could, by maintaining a larger CO2 concentration gradient through gaseous
diffusion boundary layers than could plants with a purely diffusive CO2
supply to Rubisco, fix more CO2 per unit water evaporated. This means
that, with a fixed supply of thallus water (the dowry left by the retreating
tide), more inorganic carbon could be fixed before the extent of water loss
curtailed photosynthesis in each desiccation event. This suggestion should be
tested. Analogous work on terrestrial lichens (Cowan et al. 1992) is more
complex since the sites of CO2 uptake and water vapor loss are more likely
to be different in these lichens than in the intertidal macroalgae.
The other aspect of water relations of intertidal macroalgae which can
be related to those of certain terrestrial lichens and related free-living
terrestrial algae is that of the possibility of uptake of water vapor from an
unsaturated atmosphere by desiccated algae. Terrestrial lichens with green
photobionts, and related free-living terrestrial green algae, have this capacity
to rehydrate without contact with liquid water (Bertsch 1966; Lange 1989).
Such a water uptake mechanism has not been reported for intertidal algae,
and may be unlikely (although it should be tested for in such possibly
secondarily aquatic littoral fringe algae as Prasiola, which furthermore forms
lichen-like associations with fungi: Raven and Johnston 1991a). However,
results of Cooper and De Niro (1989) on the 2H/IH and lS0/160 of water in
intertidal macroalgae and seagrasses may give a pointer here. Net loss of
water from emersed plants by evaporation preferentially removes water
depleted in the heavy isotopes 2H and ISO; however, the water in emersed
specimens of the intertidal plants is itself depleted in 2H and ISO, instead of
being enriched in the heavy isotopes as would be expected from net loss of
2H_ and ISO-depleted water vapour (Cooper and De Niro 1989). The suggested explant at ion is exchange of water in the plant with 2H_ and lSO_
depleted water vapor overcoming the effect of net evaporative loss (Cooper