Photosynthesis in Aquatic Plants
311
equilibrium value suggests that the fitness of aquatic plants is significantly
enhanced by such mechanisms. Does this mean that productivity of aquatic
plants would routinely be limited by diffusive CO2 supply from air-equilibrium solution under natural conditions of growth? The answer to this question
is undoubtedly "yes" for larger organisms in otherwise resource-rich environments; rapid growth by such organisms requires the operation of a
COz-concentrating mechanism (entries 3-5 in Table 15.3) which is, indeed,
present in these organisms (Raven 1991b). The occurrence of such mechanisms in smaller aquatic photolithotrophs, and especially picoplankton cells,
is less obviously essential if CO2 diffusion from an air-equilibrium solution
to Rubisco is considered if the kinetics of Rubisco are taken as those of
terrestrial C 3 plants. However, most aquatic plants have Rubisco kinetics
which are markedly less favorable to net CO2 fixation in air-equilibrium
solutions than those of the terrestrial Cs plant Rubisco (Raven 1984a; Raven
et al. 1990a), thus requiring a COrconcentrating mechanism if CO2 fixation
per unit Rubisco is to be more than a small fraction of the potential rate.
Furthermore, the possibility, (proved in some cases), that the use of CO2
concentrating mechanisms economizes in the use of other resources could
evolutionarily favor their occurrence in many natural environments, Including HCO-3-rich seawater (Raven 1991b,c).
A final, vital, question on the ecophysiology of inorganic carbon acquisition by aquatic plants is the extent to which carbon supply limits net
productivity in aquatic environments. My tentative answer is "relatively
infrequently". COz-concentrating mechanisms, and/or CO2 enrichment from
terrestrial inputs to freshwater, mean that aquatic plants are more frequently
limited by other nutrients, by light or by temperature (MaberJy 1985a,b;
Raven and Richardson 1986; Raven and Johnston 1991b; Raven 1993b).
This is not to say that the inorganic carbon supply might not have a
significant influence on the relative performance of different species, e.g.,
organisms with CO2 concentrating mechanisms (e.g., diatoms) relative to
those with diffusive CO 2 entry (e.g., coccolithophorids): Raven (1991c,
1993b). Limitation of the growth of aquatic plants by factors other than
inorganic carbon supply is exemplified by many flowing bodies of freshwater,
supplied with CO2 (at above air-equilibrium concentrations) and other
nutrients as "leaks" from terrestrial communities in groundwater. Here
there is typically a net CO2 loss from the water body to the atmosphere;
growth of the aquatic plants is limited by some factor other than CO 2 supply
to Rubisco (e.g., light if riparian vegetation shades the stream, or a nutrient
other than CO2), so these organisms merely serve to decrease the net
evasion of CO2 to the atmosphere which could occur from a similar but
sterile stream (Raven 1992b).
311
equilibrium value suggests that the fitness of aquatic plants is significantly
enhanced by such mechanisms. Does this mean that productivity of aquatic
plants would routinely be limited by diffusive CO2 supply from air-equilibrium solution under natural conditions of growth? The answer to this question
is undoubtedly "yes" for larger organisms in otherwise resource-rich environments; rapid growth by such organisms requires the operation of a
COz-concentrating mechanism (entries 3-5 in Table 15.3) which is, indeed,
present in these organisms (Raven 1991b). The occurrence of such mechanisms in smaller aquatic photolithotrophs, and especially picoplankton cells,
is less obviously essential if CO2 diffusion from an air-equilibrium solution
to Rubisco is considered if the kinetics of Rubisco are taken as those of
terrestrial C 3 plants. However, most aquatic plants have Rubisco kinetics
which are markedly less favorable to net CO2 fixation in air-equilibrium
solutions than those of the terrestrial Cs plant Rubisco (Raven 1984a; Raven
et al. 1990a), thus requiring a COrconcentrating mechanism if CO2 fixation
per unit Rubisco is to be more than a small fraction of the potential rate.
Furthermore, the possibility, (proved in some cases), that the use of CO2
concentrating mechanisms economizes in the use of other resources could
evolutionarily favor their occurrence in many natural environments, Including HCO-3-rich seawater (Raven 1991b,c).
A final, vital, question on the ecophysiology of inorganic carbon acquisition by aquatic plants is the extent to which carbon supply limits net
productivity in aquatic environments. My tentative answer is "relatively
infrequently". COz-concentrating mechanisms, and/or CO2 enrichment from
terrestrial inputs to freshwater, mean that aquatic plants are more frequently
limited by other nutrients, by light or by temperature (MaberJy 1985a,b;
Raven and Richardson 1986; Raven and Johnston 1991b; Raven 1993b).
This is not to say that the inorganic carbon supply might not have a
significant influence on the relative performance of different species, e.g.,
organisms with CO2 concentrating mechanisms (e.g., diatoms) relative to
those with diffusive CO 2 entry (e.g., coccolithophorids): Raven (1991c,
1993b). Limitation of the growth of aquatic plants by factors other than
inorganic carbon supply is exemplified by many flowing bodies of freshwater,
supplied with CO2 (at above air-equilibrium concentrations) and other
nutrients as "leaks" from terrestrial communities in groundwater. Here
there is typically a net CO2 loss from the water body to the atmosphere;
growth of the aquatic plants is limited by some factor other than CO 2 supply
to Rubisco (e.g., light if riparian vegetation shades the stream, or a nutrient
other than CO2), so these organisms merely serve to decrease the net
evasion of CO2 to the atmosphere which could occur from a similar but
sterile stream (Raven 1992b).