HETEROCYST - Cyanobacteria - page 175

N 2 grown cultures, the expression of cph2 gene cluster was higher in the absence of ammonium in
the medium. The contribution of cph1 gene cluster for CGP accumulation in vegetative cells and
heterocysts was more than that of cph2 gene cluster. Mutational analysis revealed that the breakdown
of CGP appeared to have more signifi cant bearing on the diazotrophic growth potential rather than
the synthesis of CGP. Though CGP synthesis in the heterocysts is a normal phenomenon it is not
required for heterocyst function (Picossi et al . 2004).
7) DEVELOPMENT
Around the same time when Fritsch (1951) was discussing the enigmatic nature of heterocyst,
Fogg (1942, 1944, 1949 and 1951) made important observations on the inhibition of heterocyst
development in A . cylindrica due to the presence of combined nitrogen sources and an inverse
relationship was noted by him between the frequency of heterocysts and concentration of nitrogen
source in the medium. Certain sources like nitrate nitrogen, glycine and asparagine caused a decrease
in the frequency of heterocysts and ammonium nitrogen totally suppressed differentiation. The
induction of heterocysts in the nitrogen-defi cient media and their repression in nitrate-, nitrite- or
ammonium-containing media correlated their role in conferring the ability to grow in the absence
of nitrogen and so their role in nitrogen fi xation. This aspect received considerable attention
by employing different heterocystous cyanobacteria. Species of Anabaena , Nostoc and Nodularia
differentiate heterocysts in a regularly spaced pattern with almost constant number of vegetative
cells (8-10) intervening two heterocysts. The frequency of heterocysts is usually represented as
percentage, i.e. their number per 100 vegetative cells and thus 8-10% of cells in nitrogen-defi cient
cultures represent heterocysts. Species of other heterocystous cyanobacteria Aulosira , Calothrix ,
Cylindrospermum , Gloeotrichia, Hapalosiphon , Mastigocladus , Scytonema , Stigonema and Tolypothrix
have been studied for the frequency of heterocysts in relation to the presence of nitrogen sources.
Exceptionally, Chlorogloea fritschii initially described as a unicellular heterocystless nitrogen-fi xing
genus (Mitra, 1950; Fay and Fogg, 1962) later turned out to be a fi lamentous form producing
heterocysts (Fay et al ., 1964; Stanier et al ., 1971). Thus heterocyst formation has been found to be
inhibited in the presence of combined nitrogen (nitrate, nitrite and ammonium ions) sources in the
medium in batch cultures of Camptylonema lahorense (Pandey and Mitra, 1962), C. fritschii (Fay et
al ., 1964; Peat and Whitton, 1967), Anabaena ambigua Rao (Talpasayi and Kale, 1967), A . fl os-aquae
A-37 (Mickelson et al ., 1967), A . variabilis (Ogawa and Carr, 1969); Anabaenopsis raciborskii Wolosz,
C . majus Kuetz, Anabaena sp. (Singh et al ., 1972) and Anabaena doliolum (Singh and Srivastava, 1968;
Tyagi, 1973a). Of the three nitrogen sources ammonium repressed the formation of heterocysts and
nitrogen fi xation at even very low concentrations (Mickelson et al ., 1967). Thomas and David (1971)
observed that the presence of nitrate nitrogen completely inhibited the induction of heterocysts in
Anabaena sp. L-31 in batch cultures but in continuous cultures no inhibition was observed at high
dilution rates with a consequent increase in heterocyst frequency. Kulasooriya et al . (1972) conducted
a detailed study on the frequency of heterocysts and the rate of nitrogen fi xation. Since growth of
A . cylindrica in ammonium nitrogen caused complete repression of both heterocyst differentiation
and nitrogen fi xation, transfer of cultures into nitrogen-defi cient medium resulted in a resumption of
the two processes. They correlated the ability to differentiate heterocysts with an attainment of a C:N
ratio from 4.5:1 to 6:1. Photoautotrophically grown cultures of A . cylindrica when starved of nitrogen
(bubbled with argon +CO 2 ) formed higher frequency of heterocysts than when aerated with N 2 +CO 2
in continuous cultures (Kulasooriya et al ., 1972). Heterocystless cultures derived from combined
nitrogen sources when shifted to a medium defi cient in nitrogen (nutrient shift or nitrogen step-