Author:
Gunning B. E. S.,Schwartz O. M.
Abstract
Confocal microscopy was used to examine heterogeneity of chlorophyll
fluorescence in chloroplasts of selected green algae, in the light of evidence
that the technique reveals the distribution of photosystem II (PSII). Three
levels of complexity were seen: (1) uniform fluorescence
(Codium) or intergrading zones of bright and less bright
fluorescence in genera known from electron microscopy to have irregular areas
of thylakoid appression (e.g. Chlamydomonas — in
which Bertos and Gibbs (J. Phycol.,
34, 1009, 1998) have found absence of segregation of
photosystem I (PSI) and PSII, Ulothrix,
Stigeoclonium, Draparnaldia); (2)
a pattern of 1–2 µm patches of fluorescence on a less bright
uniform background, in taxa where more organized thylakoid stacking (but not
grana sensu higher plants) is seen by electron
microscopy (Ulva, Oedogonium); and
(3) Discrete 0.5–2 µm spots of fluorescence in a relatively
fluorescence-free background, closely resembling higher plant grana
(Cladophorophyceae, Zygnematales, Coleochaete,
Nitella). Further investigation of these states may
illuminate the evolution of higher plant thylakoid systems, where PSII is
segregated into grana, and may provide clues concerning the adaptive
advantages of grana. Association of putative PSII fluorescence with pyrenoids
was found in most taxa, although fluorescence of trans-pyrenoid thylakoids was
seen in one case only. The association prevails whether or not there is a
starch sheath around the pyrenoids, and is persistent in pyrenoids isolated
from Spirogyra. We speculate that this fluorescence may
represent a subset of PSII that functions with thylakoid carbonic anhydrase to
provide locally high concentrations of CO2 to the
Rubisco in the pyrenoid core, as predicted by Raven
(Plant, Cell and Environment,
20, 147, 1997).
Subject
Plant Science,Agronomy and Crop Science