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day and mature akinetes appeared with greater frequency (up to 30%) by 10th day thus hastening
the akinete formation by 10 days period. In presence of ammonium chloride neither the period nor
the frequency of akinetes seems to have been affected as the response resembled that of cultures
from nitrate-free medium.
iii) Carbon sources : Wolk (1965) reported that akinete formation in A . cylindrica was stimulated by
acetate and calcium glucoronate. Glucose stimulated akinete production in A . doliolum (Tyagi, 1974).
On the contrary, incorporation of sucrose (0.02%) in nutrient medium prolonged the exponential
phase of cultures in Nostoc PCC 7524 and akinete differentiation was delayed (Sutherland et al ., 1979).
Acetate and citrate induced high frequency of akinetes in A . torulosa (Sarma and Malhotra, 1989).
Inorganic carbon sources, i.e. carbonates and bicarbonates of sodium, potassium and ammonium
carbonate, induced early development of akinetes in A . torulosa (Sarma and Garg, 1985).
iv) Other factors : Sarma et al . (1998) identifi ed aeration as another factor that regulates akinete
differentiation in A . torulosa . The effect of nutrients and aeration on O 2 evolution, photosynthetic
pigments and the accumulation of CGP content of cells during akinete differentiation in the same
organism were subsequently reported (Sarma et al ., 2000, 2004).
Deficiency of iron induced akinete formation in A. circinalis whereas a deficiency in
other elements either decreased their frequency (as in case of Mg and Ca) or had no effect
(Mo, SO 4 ) (Sinclair and Whitton, 1977). Depletion of iron has been found to be a trigger for akinete
differentiation in Anabaena while at the same time omission of other elements such as phosphorus,
magnesium, calcium, manganese, zinc, molybdenum, copper, boron and cobalt had not induced
akinete formation (Hori et al ., 2003). Likewise, the limitation of iron and trace elements had no effect
on akinete differentiation in A . circinalis.
Pandey and Kashyap (1987) reported that metabolic inhibitors like sodium azide, sodium
fl uoride and sodium arsenate stimulated akinete differentiation either in terms of the time required
for differentiation or akinete frequency in A . doliolum (Ads-strain). Hydrogen ion concentration in the
range of 7.9 to 9.0 was noted to be favourable in unbuffered media in case of A . cylindrica and organic
buffers of the dipeptides of DL-alanyl glycine and DL-alanyl DL-alanine favoured akinte formation
(Wolk, 1965). In case of Aph . fl os-aquae HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic
acid) buffer stimulated akinete production (Rother and Fay, 1979).
Culture fi ltrates of C. licheniforme in phosphate-free standard medium stimulated the formation
of akinetes in a fresh inoculum (Fisher and Wolk, 1976). Hirosawa and Wolk (1979b) identifi ed this
substance as having two-fused fi ve membered rings (one of which is a lactam and the other has a
thio-keto group) that induced akinetes in phosphate enriched cultures as well. Other cyanobacteria
like Aph. fl os-aquae (Rother and Fay, 1979), Nostoc PCC 7524 (Sutherland et al ., 1979), A . cylindrica
(Nichols and Adams, 1982) and Nostoc sp. (Ahluwalia and Kumar, 1983) did not respond to the
addition of culture fi ltrates. Induction of proakinetes by neo-peptone within 48 h in nitrogen-free
and ammonium grown cultures of A . cylindrica was reported (Sharma, 1984). Moreover, the inductive
response of akinetes was found to be associated with some factors with a molecular weight of
10,000-20,000 and increased with the age of the culture.
v) Carbon and nitrogen assimilation-an interaction : Four nutritional categories, i.e. photoautotrophs,
phototrophs (growth in presence of reduced carbon sources in light), photoheterotrophs and
chemoheterotrophs have been generally recognized in cyanobacteria. The differentiation of akinetes
by A . torulosa in phototrophic, photoheterotrophic and heterotrophic conditions was supported
by the presence fructose (Sarma and Khattar, 1993). Fructose incorporation in nitrogen-fi xing
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