AKINETES - Cyanobacteria - page 153

photoautotrophic cultures enhanced nitrogenase activity by nearly 30 times and led to akinete
differentiation within 10 days as it occurred in the presence of nitrate. However, when fructose
was supplemented in nitrate medium there was no synergistic effect of these two nutrients. In
cyanobacteria, the breakdown of fi xed carbon sources is known to occur through oxidative pentose
phosphate (OPP) pathway (Cheung and Gibbs, 1965; Pelroy and Bassham, 1972; Doolittle and
Singer, 1974) with glucose-6-phosphate dehydrogenase (G6PDH) as the fi rst enzyme involved in
this route (Stewart, 1980; Smith, 1982). The levels of G6PDH in relation to nitrogen source and phase
of akinete differentiaition revealed that nitrogen-fi xing cultures exhibited a very high activity than
in presence of nitrate in all the four nutritional conditions (Sarma and Khattar, 1993). The fact that
nitrate is lowering the rate of fructose dissimilation through OPP-pathway suggests that fructose
might be channeled to accumulate in the form of glycogen. Under all the four nutritional conditions,
the presence of nitrate favoured the accumulation of more levels of glycogen when compared to
nitrogen fi xing conditions. These observations point out that (i) in nitrogen-fi xing cultures fructose
favoured nitrogen fi xation due to generation of reducing power through the OPP-pathway (Neur et
al. , 1983); (ii) in presence of nitrate a lowering of G6PDH suggests a slowing down of OPP-pathway
and the extra carbon is channelized to form glycogen; (iii) nitrate uptake and reduction proceed
under photoheterotrophic and chemoheterotrophic conditions leading to akinete differentiation
and (iv) the cells of A . torulosa are committed to undergo akinete differentiation when a balance
between carbon and nitrogen nutrition is maintained, suggesting that C:N ratios govern akinete
differentiation in this organism (Sarma and Khattar, 1993). Rao et al. (1987) opined that a continuation
of photosynthesis with a simultaneous cessation in nitrogen fi xation led to an increase in C:N
ratios during akinete differentiation in A . doliolum . But the fi ndings of Sarma and Khattar (1993) on
A . torulosa and those of Sarma and Ghai (1998) on S . fritschii revealed that in all nutrient conditions
favouring akinete differentiation the C:N ratios decreased due to a decrease in photosynthesis and
concomitant accumulation of nitrogen reserves. Evidences for the existence of an interplay between
carbon and nitrogen metabolism during akinete differentiation have been presented by Ahuja et al.
(2008) who compared enzymes of carbon metabolism (glycolytic cycle-pyruvate kinase; glyoxylate
cycle -NAD + linked malate dehydrogenase and C 4 carbon fi xation pathway-NADP + linked malate
dehydrogenase) in nutrient-enriched and -depleted cultures under aerated and unaerated conditions.
Alongside a comparison of respiratory O 2 uptake, nitrate uptake and nitrate reductase levels under
the same conditions prompted them to conclude that the cells of A . torulosa are able to sense the
nitrogen and carbon levels and thus are able to modulate the changes required to trigger either
akinete development or continued vegetative growth (Fig. 9).
vi) Mutational studies : Mutants defective in akinete differentiation arose spontaneously in C . majus
(Singh and Sinha, 1965) and in A . doliolum following UV (Singh, 1967) or MNNG-treatment (Dhar,
1979) and in G . ghosei (Singh and Tiwari, 1970). The simultaneous mutational loss of the ability to
produce both heterocysts and akinetes in Cylindrospermum sp. (Singh et al. , 1972b) and A . variabilis
(Singh, 1978) suggests a common component of wall polysaccharides. The altered structure of the
akinete envelope in a Het - mutant of Nostoc PCC 7524 (Sutherland et al ., 1980) supports the existence
of such a common subunit component in both the cell types. This is further substantiated by the
molecular genetic analysis that has been presented in the following section. However, isolation of
oligosporogenic and non-sporulating mutants in A . doliolum by acridine dyes indicated the probable
involvement of an extrachromosomal determinant in this cyanobacterium (Dikshit et al ., 1981).
The mutants with the loss of akinete phenotype can be recognized into two categories. The
fi rst type does not differentiate akinetes at all (non-sporulating) as observed in A . doliolum (Singh,
1967; Singh and Dikshit, 1976; Dhar, 1979), A . variabilis (Singh, 1978), N . linckia (Singh et al ., 1972b)