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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).
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