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1.A. Raven
land. Thus, instead of shade habitats depleted mainly in the blue and red
regions of the spectrum due to attenuation by chlorophylls and carotenoids
as occurs on land, aquatic shade habitats can be predominantly depleted in
the red region (low plant biomass; low content of blue-absorbing organic
solutes) or even in the blue (low plant biomass; very high content of blueabsorbing solutes).
This diversity of spectral quality of shade light in aquatic habitats is
paralleled by a diversity of photosynthetic pigments in at least the algal
component of the aquatic flora relative to that of most terrestrial vegetation
(Rowan 1989). It must be emphasized that this diversity of pigments increases as one moves outward (counter to the direction of movement of the
excitation energy derived from absorbed photons) from the reaction centers
of the two photosystems. It is clear that the two reaction centers are
homologous throughout the Orevolvers; some pigment substitutions occur
in the dedicated antenna pigment-protein complexes of the two photosysterns, while major differences occur in both chromophores and polypeptides
among the analogous light-harvesting pigment-protein complexes (Raven
1984a,b; Rowan 1989). The three major pigment groups are chlorophyll a
plus phycobilins, chlorophyll a plus chlorophylls c and chlorophyll a plus
chlorophyll b. In addition to their photoprotective role, the carotenoids of at
least the two latter groups can have a major photon-harvesting role, serving
to extend the blue peak of chlorophyll(s) absorption into the green, although
not to the extent which is possible with phycobilins (Rowan 1989). Before
considering the ecological role of these pigments, it is worth noting that
recent evidence suggests a polyphyletic origin of chlorophyll b- but not
chlorophyll c-based light-harvesting systems (Palenik and Haselkorn 1992;
Urbach et al. 1992; Fujiwara et al. 1993). Furthermore, several chlorophyll
b-containing primarily aquatic Orevolvers have a chlorophyll c-like pigment
(Mg 2,4 divinyl, 2,4 desethyl, pheoporphyrin as mono methyl ester or
MgDVP) which functions in light harvesting (Rowan 1989). Spanning of two
of the three pigment groups in the Cryptophyceae relates to monophylly
of the chlorophyll a plus c and phycobilin-containing plastids. Finally, the
carotenoids of primarily aquatic photolithotrophs show a great diversity
which is difficult to accommodate in phyletic schemes without assuming
multiple origins of some carotenoids (Rowan 1989; Fawley 1991).
Consideration of the package effect, i.e., the decreased achieved specific
absorption coefficient of pigments when they are present at high areal
densities (mol m- 2 ) in terms of the projection of individual cells or extended
thalli at right angles to a vector photon field, or (via a more complicated
analysis) to a scalar photon field, is necessary if the potential ecological
implications of the variations in pigmentation are to be explored (e.g.,
Ramus 1978; Dring 1981). The decrease in the achieved in vivo specific
absorption coefficient of the pigment at high areal chromophore densities
means that, in addition to the decreased catalytic utility of individual pigment
molecules, there is also a much smaller ratio of "peaks" to "troughs" in the
1.A. Raven
land. Thus, instead of shade habitats depleted mainly in the blue and red
regions of the spectrum due to attenuation by chlorophylls and carotenoids
as occurs on land, aquatic shade habitats can be predominantly depleted in
the red region (low plant biomass; low content of blue-absorbing organic
solutes) or even in the blue (low plant biomass; very high content of blueabsorbing solutes).
This diversity of spectral quality of shade light in aquatic habitats is
paralleled by a diversity of photosynthetic pigments in at least the algal
component of the aquatic flora relative to that of most terrestrial vegetation
(Rowan 1989). It must be emphasized that this diversity of pigments increases as one moves outward (counter to the direction of movement of the
excitation energy derived from absorbed photons) from the reaction centers
of the two photosystems. It is clear that the two reaction centers are
homologous throughout the Orevolvers; some pigment substitutions occur
in the dedicated antenna pigment-protein complexes of the two photosysterns, while major differences occur in both chromophores and polypeptides
among the analogous light-harvesting pigment-protein complexes (Raven
1984a,b; Rowan 1989). The three major pigment groups are chlorophyll a
plus phycobilins, chlorophyll a plus chlorophylls c and chlorophyll a plus
chlorophyll b. In addition to their photoprotective role, the carotenoids of at
least the two latter groups can have a major photon-harvesting role, serving
to extend the blue peak of chlorophyll(s) absorption into the green, although
not to the extent which is possible with phycobilins (Rowan 1989). Before
considering the ecological role of these pigments, it is worth noting that
recent evidence suggests a polyphyletic origin of chlorophyll b- but not
chlorophyll c-based light-harvesting systems (Palenik and Haselkorn 1992;
Urbach et al. 1992; Fujiwara et al. 1993). Furthermore, several chlorophyll
b-containing primarily aquatic Orevolvers have a chlorophyll c-like pigment
(Mg 2,4 divinyl, 2,4 desethyl, pheoporphyrin as mono methyl ester or
MgDVP) which functions in light harvesting (Rowan 1989). Spanning of two
of the three pigment groups in the Cryptophyceae relates to monophylly
of the chlorophyll a plus c and phycobilin-containing plastids. Finally, the
carotenoids of primarily aquatic photolithotrophs show a great diversity
which is difficult to accommodate in phyletic schemes without assuming
multiple origins of some carotenoids (Rowan 1989; Fawley 1991).
Consideration of the package effect, i.e., the decreased achieved specific
absorption coefficient of pigments when they are present at high areal
densities (mol m- 2 ) in terms of the projection of individual cells or extended
thalli at right angles to a vector photon field, or (via a more complicated
analysis) to a scalar photon field, is necessary if the potential ecological
implications of the variations in pigmentation are to be explored (e.g.,
Ramus 1978; Dring 1981). The decrease in the achieved in vivo specific
absorption coefficient of the pigment at high areal chromophore densities
means that, in addition to the decreased catalytic utility of individual pigment
molecules, there is also a much smaller ratio of "peaks" to "troughs" in the