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1.A. Raven
absorption sepctrum as a result of a greater package effect. The end-point of
this process is a black organism with no features in its absorption spectrum:
absorptance is 1.0 at all wavelengths between 400-700nm. Accordingly, the
nature of the pigments has a much greater influence on the effectiveness of
absorption of particular wavelengths in small cells with minimal package
effects. Thus, while fully concurring with the conclusions of Dring (1981) as
to the minimal contribution of "chromatic adaptation" to photosynthetic
performance of marine benthic macroalgae in situ, the situation could well
be very different for smaller cells with much lower absorptances (Raven
1986a). This tendency is, of course, most marked for the very smallest Or
evolving photolithotrophic cells, the picoplankton (Raven 1986a) where,
even with the highest plausible chromophore concentration per unit volume,
a 0.5 ~m radius chlorophyte cell only absorbs 0.35 of incident vector radiation
at 435 nm (the blue absorption peak): Fig. 15.2a. This argument, based on
photophysical first principles, is consistent with the finding that the greatest
diversity of tetrapyrrol-based chromophores within a single organism, and
of novel carotenoids, occurs in phytoplankton organisms. Of course, the
phylogenetic influence must not be ignored; the organisms in which this
great diversity of chromophores occurs are found only in the plankton.
However, I would argue that this is in itself significant; the selective advantage of diverce pigmentation is greatest in small organisms with a minimal
package effect. Coming down to specifics, the eukaryotic organisms which
have the greatest diversity of tetrapyrrols and carotenoids are in the Cryptophyceae and the Micromonadophyceae. The Cryptophyceae are the
organisms which have chlorophylls a and C2 as well as phycoerythrobilin and
phycocyanobilin, and thus combine the attributes of the chlorophyll a plus
chlorophyll c organisms and the chlorophyll a plus phycobilin organisms; all
of them are planktonic, and a few approach the picoplankton size range
(Thomsen 1986). The Micromonadophyceae include many picoplankters
(Thomsen 1986; Guillard et al. 1991); all of them have the chlorophyll clike MgDVP as well as light-harvesting carotenoids (e.g., prasinoxanthin;
siphonein/siphonoxanthin) with significant absorption in vivo out to 540 nm.
It is also worthy of note that the greatest diversity of c-type chlorophylls (C3
and other "new" molecular species as well as Cl and cz) occurs in the smaller
chromophytes (see Wilhelm and Wiedemann 1991, and references therein).
Among the prokaryotic Oz-evolvers the picoplanktonic Prochlorococcus
marinus is distinguished by having not only divinyl analogs of chlorophylls a
and b, but also probably the chlorophyll c-like Mg 2,8 divinyl, 2,8 desethyl,
pheoporphyrin as (Goericke and Repeta 1992).
I would not, of course, attempt to contest data showing that modification
of the basal chromophore composition of members of the three main pigment groups such as to extend the range of wavelengths at which a high
absorptance is attained is also found in organisms with a very substantial
package effect (high chromophore content per m Z surface, and/or very large
internal scattering). Examples are some macroscopic marine Ulvophyceae,
1.A. Raven
absorption sepctrum as a result of a greater package effect. The end-point of
this process is a black organism with no features in its absorption spectrum:
absorptance is 1.0 at all wavelengths between 400-700nm. Accordingly, the
nature of the pigments has a much greater influence on the effectiveness of
absorption of particular wavelengths in small cells with minimal package
effects. Thus, while fully concurring with the conclusions of Dring (1981) as
to the minimal contribution of "chromatic adaptation" to photosynthetic
performance of marine benthic macroalgae in situ, the situation could well
be very different for smaller cells with much lower absorptances (Raven
1986a). This tendency is, of course, most marked for the very smallest Or
evolving photolithotrophic cells, the picoplankton (Raven 1986a) where,
even with the highest plausible chromophore concentration per unit volume,
a 0.5 ~m radius chlorophyte cell only absorbs 0.35 of incident vector radiation
at 435 nm (the blue absorption peak): Fig. 15.2a. This argument, based on
photophysical first principles, is consistent with the finding that the greatest
diversity of tetrapyrrol-based chromophores within a single organism, and
of novel carotenoids, occurs in phytoplankton organisms. Of course, the
phylogenetic influence must not be ignored; the organisms in which this
great diversity of chromophores occurs are found only in the plankton.
However, I would argue that this is in itself significant; the selective advantage of diverce pigmentation is greatest in small organisms with a minimal
package effect. Coming down to specifics, the eukaryotic organisms which
have the greatest diversity of tetrapyrrols and carotenoids are in the Cryptophyceae and the Micromonadophyceae. The Cryptophyceae are the
organisms which have chlorophylls a and C2 as well as phycoerythrobilin and
phycocyanobilin, and thus combine the attributes of the chlorophyll a plus
chlorophyll c organisms and the chlorophyll a plus phycobilin organisms; all
of them are planktonic, and a few approach the picoplankton size range
(Thomsen 1986). The Micromonadophyceae include many picoplankters
(Thomsen 1986; Guillard et al. 1991); all of them have the chlorophyll clike MgDVP as well as light-harvesting carotenoids (e.g., prasinoxanthin;
siphonein/siphonoxanthin) with significant absorption in vivo out to 540 nm.
It is also worthy of note that the greatest diversity of c-type chlorophylls (C3
and other "new" molecular species as well as Cl and cz) occurs in the smaller
chromophytes (see Wilhelm and Wiedemann 1991, and references therein).
Among the prokaryotic Oz-evolvers the picoplanktonic Prochlorococcus
marinus is distinguished by having not only divinyl analogs of chlorophylls a
and b, but also probably the chlorophyll c-like Mg 2,8 divinyl, 2,8 desethyl,
pheoporphyrin as (Goericke and Repeta 1992).
I would not, of course, attempt to contest data showing that modification
of the basal chromophore composition of members of the three main pigment groups such as to extend the range of wavelengths at which a high
absorptance is attained is also found in organisms with a very substantial
package effect (high chromophore content per m Z surface, and/or very large
internal scattering). Examples are some macroscopic marine Ulvophyceae,