308
l.A. Raven
The cost-benefit analysis of photon absorption for photosynthesis as a
function of the nature of the light-harvesting pigment-protein complex and
of the size of the organism (Raven 1984b, 1986a) was extended by Raven
(1991a) to screening sensitive sites (DNA; plastoquinone (PQ» from UV-B
as a function of cell size. Such analyses could profitably be extended (cf.
the photoinhibition analysis above) with those of repair after damage (cf.
Karentz et al. 1991) for organisms of different size; just as smaller organisms
make more effective use of each molecule of chromophore of light-harvesting
machinery, they have restrictions on the effectiveness of UV-B screening
(Raven 1984b, 1989, 1991a).
A final aspect of size effects on photosynthesis by aquatic plants as
a function of variations in photon flux density relates to movement of
organisms. This has been addressed previously in terms of, for example,
wave-frequency variations in shading of understorey marine macroalgae
(Greene and Gerard 1990) and the optimization of resource acquisition
from inverse gradients of photon flux density (only available in daytime)
and nutrients by diel vertical migration of flagellates in stratified water
bodies (Raven and Richardson 1984). I now wish to address rotation of
planktophytes in a vector radiation field. Pienaar (1980) shows that swimming
flagellates (including photo litho trophic flagellates) rotate at about 1 Hz;
Raven (1993a) points out that this could, in cells with a significant fractional
photon absorption and thus a significant photon flux density gradient across
their diameter, serve to expose peripheral parts of the photosynthetic apparatus to significant variations (twofold or more for a densely pigmented
5 11m radius cell: Fig. 15.2a) when the cell is swimming at right angles to
the vector radiation. This variation is within the range of frequencies at
which constructive interactions of variations in photon flux density on photosynthetic rate, at least at high photon flux densities, occur (see Greene and
Gerard 1990, for data on a benthic macrophyte in which the light field
is in a constant spatial relationship to individual parts of the photosynthetic
apparatus).
In addition to the rotation around the axis along which flagellar motion is
occurring, all planktonic cells are subject to rotation under the influence of
very small-scale turbulence (grading into thermal vibration) which occurs
even in macroscopically still water bodies (Berg 1980; Mitchell 1991). This
rotation is inversely related to cell size; the frequency as a function of cell
radius at 20°C is indicated in Fig. 15.2b. The size range in which rotation
occurs at frequencies (0.1-1.0 Hz) at which constructive interactions of
variations in photon flux density are found (Greene and Gerard 1990) is
-0.5-1.2 11m radius, i.e., picoplankton-size cells. The extent to which this
rotation can yield increased photosynthesis and growth in a vector light field
is, of course, a function of the difference in illumination between the "lit"
and "shaded" side of the cell; even with a high chromophore content
per unit volume, the difference for a 0.51lm radius cell is not greater
than 0.35 of incident photon flux density taken as 1.0, i.e., the photon flux
l.A. Raven
The cost-benefit analysis of photon absorption for photosynthesis as a
function of the nature of the light-harvesting pigment-protein complex and
of the size of the organism (Raven 1984b, 1986a) was extended by Raven
(1991a) to screening sensitive sites (DNA; plastoquinone (PQ» from UV-B
as a function of cell size. Such analyses could profitably be extended (cf.
the photoinhibition analysis above) with those of repair after damage (cf.
Karentz et al. 1991) for organisms of different size; just as smaller organisms
make more effective use of each molecule of chromophore of light-harvesting
machinery, they have restrictions on the effectiveness of UV-B screening
(Raven 1984b, 1989, 1991a).
A final aspect of size effects on photosynthesis by aquatic plants as
a function of variations in photon flux density relates to movement of
organisms. This has been addressed previously in terms of, for example,
wave-frequency variations in shading of understorey marine macroalgae
(Greene and Gerard 1990) and the optimization of resource acquisition
from inverse gradients of photon flux density (only available in daytime)
and nutrients by diel vertical migration of flagellates in stratified water
bodies (Raven and Richardson 1984). I now wish to address rotation of
planktophytes in a vector radiation field. Pienaar (1980) shows that swimming
flagellates (including photo litho trophic flagellates) rotate at about 1 Hz;
Raven (1993a) points out that this could, in cells with a significant fractional
photon absorption and thus a significant photon flux density gradient across
their diameter, serve to expose peripheral parts of the photosynthetic apparatus to significant variations (twofold or more for a densely pigmented
5 11m radius cell: Fig. 15.2a) when the cell is swimming at right angles to
the vector radiation. This variation is within the range of frequencies at
which constructive interactions of variations in photon flux density on photosynthetic rate, at least at high photon flux densities, occur (see Greene and
Gerard 1990, for data on a benthic macrophyte in which the light field
is in a constant spatial relationship to individual parts of the photosynthetic
apparatus).
In addition to the rotation around the axis along which flagellar motion is
occurring, all planktonic cells are subject to rotation under the influence of
very small-scale turbulence (grading into thermal vibration) which occurs
even in macroscopically still water bodies (Berg 1980; Mitchell 1991). This
rotation is inversely related to cell size; the frequency as a function of cell
radius at 20°C is indicated in Fig. 15.2b. The size range in which rotation
occurs at frequencies (0.1-1.0 Hz) at which constructive interactions of
variations in photon flux density are found (Greene and Gerard 1990) is
-0.5-1.2 11m radius, i.e., picoplankton-size cells. The extent to which this
rotation can yield increased photosynthesis and growth in a vector light field
is, of course, a function of the difference in illumination between the "lit"
and "shaded" side of the cell; even with a high chromophore content
per unit volume, the difference for a 0.51lm radius cell is not greater
than 0.35 of incident photon flux density taken as 1.0, i.e., the photon flux