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15.7 Water Relations ofIntertidal Aquatic Plants
in Relation to Photosynthesis
I.A. Raven
Primarily aquatic plants are all poikilohydric. Those which live in habitats in
which exposure to air is predictable (the marine intertidal, seasonal freshwater pools, streams subject to seasonal drawdown) and is sufficiently
prolonged to substantially desiccate the populations, have a desiccationtolerant stage in the life cycle. For the marine algae the desiccation tolerance
involves the vegetative phases(s) (Rugg and Norton 1987). For freshwater
algae desiccation tolerance may involve the vegetative body (e.g., the diploid
perennial "Chantransia" phase, but not the annual semi-erect haploid
gametophyte phase, of the batrachospermalean red algae) or phases associated with sexual reproduction and/or dispersal (Raven 1992b).
Among secondarily aquatic plants, the bryophytes are (like their terrestrial ancestors) poikilohydric; the vascular plants which are most
thoroughly aquatic have lost the homoiohydric capacities of their terrestrial
ancestors. Perennial haptophytes of predictably annually desiccated environments are desiccation-tolerant (Raven 1986b).
Before considering the implications of these attributes for photosynthesis,
it is worth pointing out that at least one primarily aquatic marine macroalga,
the high intertidal brown Pelvetia canaliculata, requires periodic emersion
for its survival (Rugg and Norton 1987). The habitat of such (secondarily)
aquatic marine lichens as Lichina pygmaea, i.e., high intertidal of exposed
shores, is consistent with a requirement for periodic emersion (Raven et al.
1990a,b) such as might be expected from the known properties of other
lichen symbioses.
The acquisition of inorganic carbon by periodically emersed aquatics
as a function of wetting-drying cycles is analogous to the terrestrial lichen
situation which Professor Dr. Lange has so successfully addressed (Lange et
al. 1970; Lange 1989). Much of the work on intertidal algae has suffered
from the use of O2 evolution as a measure of submersed photosynthesis and
CO2 fixation to measure emersed photosynthesis (e.g., Surif and Raven
1990). Recent work has overcome this by measuring inorganic C uptake
(Maberly and Madsen 1990) or O2 evolution (Britting 1992) for both emersed
and submersed thalli, in each case using natural inorganic carbon levels
in both air and seawater. A further problem concerns putting short-term
measurement into the context of environmental variability. Integration of
the observed dependence of emersed CO2 fixation on water content during a
drying cycle, and the delay in resumption of submersed photosynthesis upon
re-immersion while any damage incurred during desiccation is repaired, with
the in situ variables (tides, light-dark cycles, weather) is complex. The bestinvestigated example is that of the mid-high intertidal brown macro alga
Fucus spiralis; Maberly and Madsen (1990) and Madsen and Maberly (1990)
showed that this alga in situ fixes about a quarter of its total carbon during
15.7 Water Relations ofIntertidal Aquatic Plants
in Relation to Photosynthesis
I.A. Raven
Primarily aquatic plants are all poikilohydric. Those which live in habitats in
which exposure to air is predictable (the marine intertidal, seasonal freshwater pools, streams subject to seasonal drawdown) and is sufficiently
prolonged to substantially desiccate the populations, have a desiccationtolerant stage in the life cycle. For the marine algae the desiccation tolerance
involves the vegetative phases(s) (Rugg and Norton 1987). For freshwater
algae desiccation tolerance may involve the vegetative body (e.g., the diploid
perennial "Chantransia" phase, but not the annual semi-erect haploid
gametophyte phase, of the batrachospermalean red algae) or phases associated with sexual reproduction and/or dispersal (Raven 1992b).
Among secondarily aquatic plants, the bryophytes are (like their terrestrial ancestors) poikilohydric; the vascular plants which are most
thoroughly aquatic have lost the homoiohydric capacities of their terrestrial
ancestors. Perennial haptophytes of predictably annually desiccated environments are desiccation-tolerant (Raven 1986b).
Before considering the implications of these attributes for photosynthesis,
it is worth pointing out that at least one primarily aquatic marine macroalga,
the high intertidal brown Pelvetia canaliculata, requires periodic emersion
for its survival (Rugg and Norton 1987). The habitat of such (secondarily)
aquatic marine lichens as Lichina pygmaea, i.e., high intertidal of exposed
shores, is consistent with a requirement for periodic emersion (Raven et al.
1990a,b) such as might be expected from the known properties of other
lichen symbioses.
The acquisition of inorganic carbon by periodically emersed aquatics
as a function of wetting-drying cycles is analogous to the terrestrial lichen
situation which Professor Dr. Lange has so successfully addressed (Lange et
al. 1970; Lange 1989). Much of the work on intertidal algae has suffered
from the use of O2 evolution as a measure of submersed photosynthesis and
CO2 fixation to measure emersed photosynthesis (e.g., Surif and Raven
1990). Recent work has overcome this by measuring inorganic C uptake
(Maberly and Madsen 1990) or O2 evolution (Britting 1992) for both emersed
and submersed thalli, in each case using natural inorganic carbon levels
in both air and seawater. A further problem concerns putting short-term
measurement into the context of environmental variability. Integration of
the observed dependence of emersed CO2 fixation on water content during a
drying cycle, and the delay in resumption of submersed photosynthesis upon
re-immersion while any damage incurred during desiccation is repaired, with
the in situ variables (tides, light-dark cycles, weather) is complex. The bestinvestigated example is that of the mid-high intertidal brown macro alga
Fucus spiralis; Maberly and Madsen (1990) and Madsen and Maberly (1990)
showed that this alga in situ fixes about a quarter of its total carbon during