Biology Reference
In-Depth Information
occurred in both the wide (basal or prostrate) fi laments and narrow branching fi laments with a
tendency of almost all the cells in the former type having a potential to develop into heterocysts.
Mostly in these, continuous rows of four or more heterocysts are generally found. The fi ne structure
details agreed with the heterocysts of
A
.
cylindrica
except that the thylakoid membranes are densely
packed with occasional lamellar stacks embedded in granular cytoplasm (Nierzwicki-Bauer
et
al
., 1984a).
3) BIOCHEMICAL COMPOSITION
The differentiation of heterocysts in
A
.
cylindrica
was accompanied by a gradual loss of phycocyanin
and the ability to fi x CO
2
. The presence of photosystem I (PSI) with a gradual increase in the content
of β-carotene is a characteristic feature of heterocysts with a simultaneous decrease in the content of
xanthophyll (Fay, 1969; Wolk and Simon, 1969). A relationship between the age of culture and the
contents of the major phycobiliproteins in the heterocysts of
Anabaena
sp. L-31 has been established
where heterocysts from 2-day old cultures exhibited very less amounts of these pigments whereas
heterocysts from 5-day old cultures acquired more of the biliproteins very much similar to their
contents in vegetative cells (Thomas, 1972). Reconstitution experiments with thylakoid membranes
from heterocysts revealed that PSI activity was maximal when cytochrome
c
-
553
or plastocyanin
were present. Hydrogen, NADH/NADPH or ascorbate or dichlorophenolindophenol couple served
as electron donors (Hawkesford
et al
., 1983). The presence of cytochromes
c-553
,
b-563
and
f-557
(corresponding to cytochrome
f
of higher plants and algae) and absence of cytochrome
b
-
559
in
the isolated heterocysts of
N.
muscorum
suggested the degradation of photosystem II (PSII; Almon
and Böhme, 1980). However, the presence of phycobiliproteins in the heterocysts of
A
.
variabilis
(Peterson
et al
., 1981; Ke
et al
., 1983), an
Anabaena
sp. (Yamanaka and Glazer, 1983) and the endophyte
of
Azolla
,
A
.
azollae
(Ke
et al
., 1983; Tyagi
et al
., 1981) has been unequivocally demonstrated based
on microspectrophotometric and fl uorescence action spectra. Further, the phycobiliproteins in the
heterocysts of
A
.
variabilis
appeared to be organized into phycobilisomes (Ke
et al
., 1983). During the
thylakoid rearrangement, a 20 kDa polypeptide component of the heterocyst phycobiliprotein particle
helps in attachment of the phycobiliproteins to the photosynthetic lamellae (Yamanaka and Glazer,
1983). The endosymbionts of liverworts and
Gunnera
lack the phycobiliproteins (Rodgers and Stewart,
1977; Silvester, 1976) but the heterocysts from the endophytes of
Azolla caroliniana
and
A
.
pinnata
(Kaplan and Peters 1981; Tyagi
et al
., 1981; Kaplan
et al
., 1986) not only possess the phycobiliproteins
but they are also effective in harvesting light energy for production of O
2
evolution (Ray
et
al
., 1979) and
PSI-linked acetylene reduction (Tyagi
et al
., 1981). Peterson
et al
. (1981) observed fl uorescence emission
bands specifi c for phycocyanin and allophycocyanin in the heterocysts of
A
.
variabilis
and this has also
been confi rmed by the fl uorescence microscopy of the individual heterocysts. The presence of PSI and
absence of PSII was confi rmed and the phycobiliproteins in heterocysts were effective in supporting
light-dependent acetylene reduction. Kaplan
et al
. (1986) found that there is no appreciable difference
in the composition of phycobiliproteins of vegetative cells and heterocysts of the endophyte of
A
.
caroliniana
when a comparison was made from the young apices to the mature portions of leaves.
In contrast, fi laments of
Nostoc
sp. and a free-living
Nostoc
sp. 7422 from
Cycas revoluta
revealed the
localization of phycoerythrins in the vegetative cells of both strains but absent in the heterocysts
(Lindblad and Bergman, 1989). Fluorescence emission and absorption spectra of single
Anabaena
sp. strain PCC 7120 cells at excitation and detection areas of less than 1.0 µm showed the existence
of low PSII activity (Ying
et al
., 2002).