CYANOPHAGES - Cyanobacteria - page 433

myoviruses and podoviruses. Because of the fact Prochlorococcus and Synechococcus phages' MazG
genes do not cluster with their host's genes it seems that they have not acquired it from their hosts
but acquired them by LGT from other sources. It seems S-PM2 might have acquired this gene from
an ancestor like Chlorofl exus aurantiacus rather than from its host Synechococcus .
Other metabolic changes relate to the activity of enzymes of carbon and nitrogen metabolism.
The infection of phage AS-1 was shown to be responsible for transformation of a low activity form
of glucose-6-phosphate dehydrogenase to a hyperactive form (Cseke et al ., 1981). Blashka et al . (1982)
studied the incorporation of 22 radioactive metabolites and identifi ed three classes of substances,
i.e. (i) substances exclusively incorporated in the infected state, (ii) compounds incorporated at a
2- to 3-fold rate in infected cells and (iii) compounds incorporated relatively equally by infected
and non-infected cells. Studies on the metabolic changes associated with cyanophage N-1 infection
of N . muscorum cells revealed an increase in the level of glucose-6-phosphate dehydrogenase, and a
lowering of glutamine synthetase activity. However, nitrogenase activity remained high throughout
eclipse period which decreased rapidly soon after 5 h (Amla et al ., 1987). Nitrate reductase activity of
phage N-1 infected cells of N . muscorum was elevated (Bisen et al ., 1986a) and this was suggested to
be at the level of synthesis of new enzyme by phage N-1 mediated molybdenum co-factor synthesis
(Bagchi et al ., 1987). Phage N-1 was shown to require products of glutamine synthetase activity, i.e.
glutamine and its further metabolic products for its replication while the inhibition of glutamate-
oxaloacetate transaminase and signifi cant increase of glutamate-pyruvate transaminase has been
suggested to involve a shift in nitrogen metabolism in favour of N-1 multiplication (Kashyap et
al ., 1988). Mendzhul et al . (1988) observed changes in the specifi c activity of glucose-6-phosphate
dehydrogenase in cyanophage LPP-3 infected cells of P . boryanum that were shown to be dependent on
the illumination. Low illumination (400 Lux) that supported optimum virus development enhanced
the activity of the enzyme by 75% while high illumination (2000 Lux) lowered the specifi c activity
of the enzyme by 62%.
IX. ROLE OF FRESHWATER CYANOPHAGES IN THE CONTROL OF
CYANOBACTERIAL BLOOMS
The rapid disappearance of cyanobacterial blooms under natural conditions suggested the presence
of a factor that is generally associated with degeneration. Krauss (1961) fi rst suspected that the
sudden disappearance of cyanobacterial blooms is due to the presence of viral agents. Safferman and
Morris (1964b) demonstrated that the control of cyanobacteria with virus by artifi cially producing
algal blooms in stainless-steel tanks containing 112 litres of tap water supplemented with inorganic
nutrients. The temperature of this pond was maintained at 32°C. The viral agent was added after
considerable growth of P . boryanum and degeneration of the organism was assessed by counting
the fragments of fi laments. Complete lysis or disappearance of algal cells occurred about 7 days
after viral treatment.
Upon a close examination of three hosts sensitive to LPP-1, it was suggested that these three
genera Lyngbya , Phormidium and P . boryanum have close morphological characteristics (Safferman
and Morris, 1964b). These observations were further confi rmed and in a re-classifi cation of the
members of Oscillatoriaceae, Drouet (1963) showed that all the three members belonged to a single
species of the genus Schizothrix, i.e. S . calcicola . This has been considered as one of the most widely
distributed and most frequently encountered cyanobacterium on earth. Safferman and Morris (1967)
thus speculated that LPP-1 had a lot of ecological signifi cance in naturally controlling the populations
of Plectonema and related organisms since none of the cyanobacterial strains were found dominantly