CYANOPHAGES - Cyanobacteria - page 469

by a number of workers, these observations, however, have not been correlated with the existence
of marine phages that were specifi c for the respective marine planktons (Fuhrman and Suttle, 1993).
Mayer and Taylor (1979) fi rst isolated a large polyhedral virus measuring 130-135 nm in diameter that
was specifi c for Micromonas pusilla , a marine Chrysophyte. In the Gulf of Mexico, the concentrations
of cyanophages infecting a single strain of Synechococcus ranged in between 10 3 L -1 to 10 8 L -1 (Suttle
et al ., 1993; Suttle and Chan, 1993). The concentrations of bacteriophages infecting various bacterial
hosts were found to be of the order of 10 4 L -1 to 3.7 x 10 7 phage particles L -1 . However, great variability
was noted in the titres of viruses that infect eukaryotic photosynthetic fl agellates like M . pusilla
(Cottrell and Suttle, 1991). Primary productivity was reduced by as much as 78% due to infection of
phytoplankton by the respective viruses of diatoms, Cryptophytes, Prasinophytes and chroococcoid
cyanobacteria in marine waters. So in addition to grazing and nutrient limitation, phage infection
seems to play a regulating factor in the development of the phytoplankton communities (Suttle et
al ., 1990). Proctor and Fuhrman (1990) suggested that up to 7% of the heterotrophic bacteria and
5% of the cyanobacteria from diverse marine locations contained mature phage and this indicates
that up to 70% of the prokaryotes could be infected. This led to the understanding that a new
pathway of carbon and nitrogen cycling in marine food webs with probable infl uence on LGT in
marine microorganisms. Viral-mediated release of dissolved organic matter (DOM) can be a good
source of major nutrients (C, N, and P) and trace elements (e.g. Fe) for other photosynthetic and
heterotrophic microorganisms (Middelboe et al ., 1996; Göbler et al ., 1997; Middelboe et al ., 2003). This
was shown to infl uence bacteria and phytoplankton species composition and succession (Göbler
et al ., 1997; Brussaard et al ., 2004). Another area of global signifi cance infl uenced by viruses is the
augmentation in production of dimethyl sulfi de (DMS) that helps in cloud formation (Charlson et
al ., 1987). The production of dimethylsulfoniopropionate (DMSP) by many phytoplanktons and its
subsequent cleavage by algal lyases and/or other lyases of other organisms into DMS and acrylic
acid has been demonstrated. The build up of DMSP in nutrient media as a result of viral lysis of M .
pusilla , P . pouchetii and Emiliana huxleyi has been confi rmed (Hill et al ., 1998; Malin et al ., 1998; Wilson
et al ., 1998). It seems, therefore, viral lysis of phytoplankton may constitute an important source for
production of DMSP in the marine environment.
Suttle and Chan (1993) fi rst re-emphasized the role of marine cyanophages in controlling
cyanobacterial community structure in marine waters. The dynamics and distribution of marine
cyanophages infecting Synechococcus DC2 and SYN48 along the transect of Western Gulf of Mexico
in Aransas Pass, Texas revealed that the cyanophage concentrations ranged from few hundred ml -1
at a depth of 97 m to 4 x 10 5 ml -1 at the surface waters (83 km offshore). The highest concentrations
of cyanophages at the surface waters coincided with Synechococcus density. As the density of the
host increased beyond 10 3 ml -1 , then the titers of cyanophages increased suggesting that a minimum
concentration of host is required for infection to occur. They estimated that on daily basis about
5 to 7% of Synechococcus cells are lysed. Cyanophages thus seem to constitute a dynamic marine
planktonic communitiy and are responsible for causing signifi cant losses of Synechococcus population
on a daily basis. A comparison of viral production rates and the contact rates between cyanophage
particles and the host cells at the farthest offshore waters and nearshore waters brought to light that
most of the Synechococcus cells in the former locality are susceptible to infection while in the latter
only 80% host cells are liable to be infected. This was explained on the basis that most of the cells
are probably resistant as reported earlier by Waterbury and Valois (1993). This aspect of study has
received attention during recent years. Stoddard et al . (2007) isolated 23 phage-resistant mutants by
employing four strains of Synechococcus sp. (WH7803, WH8018, WH8012 and WH8101) challenged
by 32 previously isolated cyanomyophages. The important fi ndings are that (i) resistance is more