Photosynthesis in Aquatic Plants
309
density at the "shaded" side is 0.65 that at the "lit" side (Fig. IS.2a). A
larger difference (up to 0.5) could occur for 111m radius cells, again with
chromophore content per unit volume at the upper end of the observed
range (Fig. IS.2a). Such differences could be significant for photosynthetic
rates in vector radiation fields close to light saturation for photosynthesis.
We note that an analogous analysis for rather larger cells by Babin et al.
(1992) concludes that there is unlikely to be a constructive effect of rotation
on photosynthetic rate. While concurring with this conclusion for these
larger, slowly rotating cells (cf. Greene and Gerard 1990), Babin et al.
(1992) used the specific reaction rate of redox reactions of photosynthesis
( - 20- 200 mol electron transferred [mol photoreaction 2r 1 s -1) to estimate
the rotation frequency (20-200 Hz) at which constructive effects on photosynthesis could be expected, rather than the observed lower frequency
at which such stimulations do occur; the extension of stimulatory effects
to lower frequencies relates to the finite pool sizes of intermediates of
photosynthetic reactions. Since the rotation rate is a linear function of
the viscosity of the medium, the occurrence of intermittency effects on
photosynthesis as a function of rotation rate could be examined by altering
the viscosity of the medium with solutes (e.g., polymers) which do not
impose an osmotic (or other) metabolic challenge to the cells.
15.6 Inorganic Carbon Acquisition by Aquatic Plants:
When Does It Limit Net Productivity?
Much work has been devoted over the past two decades to the study of
inorganic carbon acquisition by aquatic plants (see Raven 1991b). It has
been established that some aquatic plants acquire their inorganic carbon
in situ in a manner analogous to that of terrestrial C3 plants, i.e., by
CO 2 diffusion from the bulk phase to ribulose bisphosphate carboxylaseoxygenase (Rubisco), although in the great majority of aquatics all of the
diffusion pathway is in the aqueous phase. However, this mechanism may
yield a relatively low potential rate of photosynthesis if CO2 concentration in
solution is at or below the air-equilibrium concentration, especially at high
temperatures, in view of the low diffusion coefficient of CO2 in water (about
10- 4 of the value in air) which is not offset by the thinner diffusion boundary
layers in aquatic environments except in the case of the smallest (picoplanktonic) cells (Raven 1991b,c). Accordingly, a majority of aquatic plants rely
on more complex mechanisms of inorganic carbon acquisition. Some of
these are listed in Table 15.3. A common feature of all of these mechanisms
is that they give a steady-state CO2 concentration at the site of Rubisco
activity which exceeds that in air-equilibrium solution. This has theoretical
implications (in some cases tested and verified) for the rate of CO2 fixation
309
density at the "shaded" side is 0.65 that at the "lit" side (Fig. IS.2a). A
larger difference (up to 0.5) could occur for 111m radius cells, again with
chromophore content per unit volume at the upper end of the observed
range (Fig. IS.2a). Such differences could be significant for photosynthetic
rates in vector radiation fields close to light saturation for photosynthesis.
We note that an analogous analysis for rather larger cells by Babin et al.
(1992) concludes that there is unlikely to be a constructive effect of rotation
on photosynthetic rate. While concurring with this conclusion for these
larger, slowly rotating cells (cf. Greene and Gerard 1990), Babin et al.
(1992) used the specific reaction rate of redox reactions of photosynthesis
( - 20- 200 mol electron transferred [mol photoreaction 2r 1 s -1) to estimate
the rotation frequency (20-200 Hz) at which constructive effects on photosynthesis could be expected, rather than the observed lower frequency
at which such stimulations do occur; the extension of stimulatory effects
to lower frequencies relates to the finite pool sizes of intermediates of
photosynthetic reactions. Since the rotation rate is a linear function of
the viscosity of the medium, the occurrence of intermittency effects on
photosynthesis as a function of rotation rate could be examined by altering
the viscosity of the medium with solutes (e.g., polymers) which do not
impose an osmotic (or other) metabolic challenge to the cells.
15.6 Inorganic Carbon Acquisition by Aquatic Plants:
When Does It Limit Net Productivity?
Much work has been devoted over the past two decades to the study of
inorganic carbon acquisition by aquatic plants (see Raven 1991b). It has
been established that some aquatic plants acquire their inorganic carbon
in situ in a manner analogous to that of terrestrial C3 plants, i.e., by
CO 2 diffusion from the bulk phase to ribulose bisphosphate carboxylaseoxygenase (Rubisco), although in the great majority of aquatics all of the
diffusion pathway is in the aqueous phase. However, this mechanism may
yield a relatively low potential rate of photosynthesis if CO2 concentration in
solution is at or below the air-equilibrium concentration, especially at high
temperatures, in view of the low diffusion coefficient of CO2 in water (about
10- 4 of the value in air) which is not offset by the thinner diffusion boundary
layers in aquatic environments except in the case of the smallest (picoplanktonic) cells (Raven 1991b,c). Accordingly, a majority of aquatic plants rely
on more complex mechanisms of inorganic carbon acquisition. Some of
these are listed in Table 15.3. A common feature of all of these mechanisms
is that they give a steady-state CO2 concentration at the site of Rubisco
activity which exceeds that in air-equilibrium solution. This has theoretical
implications (in some cases tested and verified) for the rate of CO2 fixation