Photosynthesis in Aquatic Plants
307
especially deep water representatives (see Raven 1987), which have siphonein
and/or siphonoxanthin, extending absorptance into the green region from
the blue peak. The high overall absorptance of at least some of these algae
makes this extension of limited value (see Ramus 1978; Dring 1981). In
any case, the occurrence of these light-harvesting carotenoids is limited
to primarily aquatic plants; secondarily aquatic plants with chlorophyll a
plus chlorophyll b appear to lack them. Although deep water (secondarily)
aquatic bryophytes have a high content, relative to chlorophyll a, of a
number of the usual higher plant carotenoids, these do not seem to have any
additional light-harvesting role (Boston et al. 1991).
The role of "anomalous" light-harvesting chromophores in large and
small photolithotrophs clearly needs further investigation to test the suggestion that such additional chromophore species are of greater selective
advantage in small cells, granted the radiation climate in which the organisms
normally occur. Such studies should be integrated with energetic cost-benefit
analyses of photon harvesting as a function of cell size and the nature and
cost of synthesis of chromophores and the polypeptides which bind them
(Raven 1984b, 1986a,b; Alberte 1989; Andrews 1991).
A further aspect of light effects on aquatic plants is that of photoinhibition. The homologous nature of the photoreaction 2 reaction center in
all OTevolvers is consistent with the potential for photoinhibition in all OT
evolvers (Raven 1984a,b; Kyle et al. 1987). The only point I wish to make
here is that the differences in pigmentation and thylakoid organization
among the three main pigment groups of aquatic plants means that some of
the means of avoiding photoinhibition damage are different in, for example,
the chromophytes as compared to the green plants in the strict sense. Thus,
the option of detaching light-harvesting complexes specifically from photoreaction 2 reaction centers ("state transition") does not seem to be of
general occurrence in chromophytes (Raven et al. 1989; Lichtle et al.
1992; Pyszniak and Gibbs 1992; Raven 1993a). Furthermore, the option
of a "carotenoid cycle" as a means of quenching excess excitation of the
photoreaction 2 centers is present in chromophytes as well as green plants,
but with different carotenoids (Willemoes and Monas 1991). As with the
nature of the light-harvesting complexes discussed above, cost-benefit
analyses of the various approaches which photolithotrophs have to photoinhibition are possible (Raven and Samuelsson 1986; Raven 1989). It
is possible (Raven 1989) to compare the energetic costs and benefits of
avoidance of photoinhibition (taking into account any decrease in the
potential for light-saturated or light-limited photosynthesis as well as the
costs of avoidance of damage per se) with those of permitting photo inhibition
to occur, with subsequent repair (again taking into account the decrease in
photosynthetic rate in high or low light as well as the direct energy costs of
repair): Raven (1989). The rather specialized avoidance mechanism modeled
by Raven (1989) should be extended to a wider range of avoidance mechanisms, such as the state transitions and carotenoid cycles discussed above.
307
especially deep water representatives (see Raven 1987), which have siphonein
and/or siphonoxanthin, extending absorptance into the green region from
the blue peak. The high overall absorptance of at least some of these algae
makes this extension of limited value (see Ramus 1978; Dring 1981). In
any case, the occurrence of these light-harvesting carotenoids is limited
to primarily aquatic plants; secondarily aquatic plants with chlorophyll a
plus chlorophyll b appear to lack them. Although deep water (secondarily)
aquatic bryophytes have a high content, relative to chlorophyll a, of a
number of the usual higher plant carotenoids, these do not seem to have any
additional light-harvesting role (Boston et al. 1991).
The role of "anomalous" light-harvesting chromophores in large and
small photolithotrophs clearly needs further investigation to test the suggestion that such additional chromophore species are of greater selective
advantage in small cells, granted the radiation climate in which the organisms
normally occur. Such studies should be integrated with energetic cost-benefit
analyses of photon harvesting as a function of cell size and the nature and
cost of synthesis of chromophores and the polypeptides which bind them
(Raven 1984b, 1986a,b; Alberte 1989; Andrews 1991).
A further aspect of light effects on aquatic plants is that of photoinhibition. The homologous nature of the photoreaction 2 reaction center in
all OTevolvers is consistent with the potential for photoinhibition in all OT
evolvers (Raven 1984a,b; Kyle et al. 1987). The only point I wish to make
here is that the differences in pigmentation and thylakoid organization
among the three main pigment groups of aquatic plants means that some of
the means of avoiding photoinhibition damage are different in, for example,
the chromophytes as compared to the green plants in the strict sense. Thus,
the option of detaching light-harvesting complexes specifically from photoreaction 2 reaction centers ("state transition") does not seem to be of
general occurrence in chromophytes (Raven et al. 1989; Lichtle et al.
1992; Pyszniak and Gibbs 1992; Raven 1993a). Furthermore, the option
of a "carotenoid cycle" as a means of quenching excess excitation of the
photoreaction 2 centers is present in chromophytes as well as green plants,
but with different carotenoids (Willemoes and Monas 1991). As with the
nature of the light-harvesting complexes discussed above, cost-benefit
analyses of the various approaches which photolithotrophs have to photoinhibition are possible (Raven and Samuelsson 1986; Raven 1989). It
is possible (Raven 1989) to compare the energetic costs and benefits of
avoidance of photoinhibition (taking into account any decrease in the
potential for light-saturated or light-limited photosynthesis as well as the
costs of avoidance of damage per se) with those of permitting photo inhibition
to occur, with subsequent repair (again taking into account the decrease in
photosynthetic rate in high or low light as well as the direct energy costs of
repair): Raven (1989). The rather specialized avoidance mechanism modeled
by Raven (1989) should be extended to a wider range of avoidance mechanisms, such as the state transitions and carotenoid cycles discussed above.