AKINETES - Cyanobacteria - page 156

germination (Huber, 1985). During the fi rst 24 h of the germination process the energy required
has been suggested to be derived from the aerobic oxidation of endogenous carbon reserves (Rai et
al ., 1988). The energetic and metabolic requirements for the germination of akinetes of Nostoc PCC
7524 during the fi rst 9 to 10 h could not be met either from cyclic photophosphorylation of PSI or
respiration but for complete germination it required resumption of full photosynthetic activity with
both photosystems functional (Chauvat et al ., 1982). Similarly, germination potential has been found
to be highest when both photosystems are functional and if PSII inhibition is caused by DCMU,
cyclic photophosphorylation needed the support of respiration to provide the required energy for
the germination process (Herdman, 1987, 1988).
The nascent germlings of A . doliolum acquire fi rst the capabilities of oxygenic photosynthesis
and expression of nitrate reductase and GS activities prior to the establishment of a regular pattern
of heterocyst differentiation which generally takes 24-72 h. During this period the presence of
signifi cant activities of GOT and GPT indicates a rapid turnover of the required amino acids for
growth of the germlings (Rai et al ., 1988). Another alternative that has been suggested to meet the
requirements of nitrogen before the establishment of nitrogen fi xation capability is the dissolution of
the thick peptidoglycan layer that facilitates the release of a lipopolysaccharide-like laminated layer
in Nostoc PCC 7524 (Sutherland et al ., 1979) and Cyanospira sp. (Sili et al ., 1994). Neely-Fisher et al .
(1989), who reported akinete germination of the symbionts of Azolla pinnata in presence of fructose
in darkness, observed that the addition of nitrogen sources in presence of fructose did not cause any
additional advantage. On the contrary, the addition of ammonium chloride (5 mM) in presence of
fructose reduced the frequency of akinete germination in dark. Sutherland et al . (1985a,b) observed
synchronized akinete germination in Nostoc PCC 7524 by dilution of akinetes into fresh medium
with or without nitrate in light and the germlings in these media differentiated fi rst heterocyst from
the terminal cell of a germling around 19 h followed by the second heterocyst that appeared at the
other terminus after few cell divisions. During the synchronized akinete germination, the synthesis
of RNA and proteins began immediately and continued during the germination process but DNA
synthesis started only after 90 h by which time the germling growth with ten vegetative cells and
two terminal heterocysts would have taken place. The continued cell division in the absence of DNA
synthesis has been explained by Adams and Duggan (1999) on the basis that the akinetes possess
10 genome equivalents of DNA so that these are redistributed to the ten vegetative cells during
germling growth (Sutherland et al ., 1979).
Relatively well synchronized germination of akinetes (up to 90%) was noted after 43 h of transfer
to fresh medium (Stulp and Stam, 1982). In certain members like Nostoc PCC 7524, Sutherland et al .
(1985b) obtained nearly 100% synchronous germination in fresh medium within 24 h. In A . torulosa
induction of akinete formation occurred in nitrogen (nitrate and nitrite) and carbon sources (acetate
and citrate) and their combinations on the 10th day (Sarma and Malhotra, 1989). Akinetes in these
nutrients had undergone germination too readily in the same medium with maximum percentage
in presence of citrate with nitrite followed by acetate. The percentage of germination was found
to be highest in nitrite followed by nitrate. The resulting 2-3 celled-germlings have immediately
undergone a second round of akinete formation circumventing the process of vegetative growth.
This has been designated as microcycle akinete formation. Maximum percentage of microcycle
akinetes was observed in nitrite, citrate with nitrate and lowest in case of acetate in presence of the
two nitrogen sources (Fig. 10; Sarma and Malhotra, 1989). Ammonium chloride inhibited the process
of germination in case of No . spumigena (Huber, 1985), A . circinalis (van Dok and Hart, 1997) and
A . torulosa (Sarma and Malhotra, 1989).