CYANOPHAGES - Cyanobacteria - page 437

and require high salt concentration for growth while Cluster B is composed of strains (WH7803 and
WH8101) that occur in coastal waters, possess phycocyanin as the primary light-harvesting pigment
and do not require elevated salt concentration but at the same time are halophilic. Among the phages
isolated by them (75 in total) representatives from each of the three families of tailed phages are
included. Phage 9 infected as many as 10 of 13 strains tested from Marine Cluster (MC)-A. Phages 5
and 7 infected only the host strain used for isolation. Phage 20 not only infected strains of MC-A but
also infected strain WH8101 from MC-B. Phage 1 isolated on WH8101 from MC-B also infected 5 of
13 strains belonging to MC-A. All the cyanophages isolated are virulent phages. Attempts to induce
lysogenic cells to produce phage by using temperature shock, light shifts, UV or X-ray irradiation and
treatment with mitomycin C have not been successful. According to them, these lytic cyanophages
have negligible effect in regulating the populations of Synechococcus in marine waters. They based
their conclusions on the loss of infectivity by cyanophages due to solar radiation and viral decay
rates (Heldal and Bratbak, 1991; Suttle and Chan, 1992). Further, they suggested that evolution of
phage-resistant strains and their coexistence with phages is the likely fi nal outcome. The emergence
of phage-sensitive cells from the phage-resistant populations enables the viruses to maintain the
titres up to 10 3 to 10 4 cyanophages ml -1 in both inshore and offshore waters. Garza and Suttle (1998)
studied the rates of mortality of Synechococcus and the evolution of UV-resistant viral communities
by incubating two cyanophage isolates S-PWM1 and S-PWM3 either with or without host cells of
Synechococcus sp in UV-transparent bags at the surface and at different depths of seawater at Port
Aransas, Texas. Likewise, natural populations of Synechococcus sp. and cyanophages were (collected,
fi ltered to remove zooplankton grazers, bacteria and host cells) put in UV-transprent bags and were
held at the surface and various depths of the experimental site. The mortality rates of the host in two
offshore locations have been noted to be 1 and 8%. In contrast, nearshore incubations of cyanophages
were responsible for removing only ≤1% of Synechococcus cells on a daily basis during winter and
spring. Natural cyanophage communities tolerate damage by solar radiation better in summer than
in winter. Net decay rates of cyanophage infectivity in sunlight were similar whether host cells were
present or not indicating that detectable cyanophage production did not occur during day time in
situ incubations. Natural viral communities appeared to be more resistant to DNA damage than
cultured cyanophage Syn-M3 infecting Synechococcus sp. DC2 (WH7803) and the marine Vibrio phage
PWH3a-P1 when incubated from dawn to dusk in offshore and coastal waters of western Gulf of
Mexico. The DNA damage was measured by estimating the number of sun-light induced formation
of cyclobutane pyrimidine dimmers (CPDs) and pyrimidine pyrimidone photoproducts (PPs) by a
highly specifi c radioimmunoassay (Weinbauer et al ., 1999).
Wilson et al . (1993) isolated cyanomyoviruses (S-BM1, S-PM1, S-PM2 and S-WHM1) and
cyanosiphoviruses (S-BS1 and S-PS1) infecting marine Synechococcus . The diameter of the head of
S-PM2 is 90 nm with a tail of 165 nm long and 20 nm wide (Fig. 4 A and B). Cyanophages S-BM1
and S-PM1 had a similar morphology in having isometric heads with a diameter ~88 nm, extended
tails ~57 nm long and 31 nm wide (at the base plate) which are connected to the head with a collar,
8nm long and 14 nm wide and tail plates ~45 nm wide. Cyanophage S-WHM1 had an isometric
head of approximately the same size as those of S-BM1 and S-PM1 and a longer tail ~108 nm long
and 23 nm wide. Cyanosiphoviruses (S-PS1 and S-BS1) have isometric heads with a diameter of 90
nm and had rigid tails ~165 nm long and 20 nm wide. A base plate of 43 nm wide was connected
to the tail. Protruding from each plate corner was a spike ~45 nm long and tail fi bres that are 1.5
times the length of the tail (Table 6). The genome size of cyanomyoviruses isolated were ca. 80-85
kb and those of cyanosiphoviruses were ~90-100 kb. The restriction patterns of cyanomyoviruses
from Woods Hole appeared to be unique from others. The restriction patterns of cyanomyoviruses