Cyanobacteria And The Cyanophage Diversity

Cyanobacteria (including genus Synechococcus) contribute to a vital role in the primary productivity in the coastal ecosystems, and prevail in huge concentrations in seas and oceans (Scanlan and West, 2002; Waterbury et al., 1986). Co-existence of distinct inherent groups of Synechococcus spp. at a single location has been reported in earlier studies (Ferris and Palenik, 1998; Palenik, 1994; Toledo and Palenik, 1997), and their distribution as well as abundance also varies across locations (Ferris and Palenik, 1998; Rocap et al., 2002). Cyanophages belong to the collection of viruses that are proficient in infecting cyanobacteria, and are abundant and diverse with concentrations of up to 108 cyanophages per liter in the marine waters observed during summer (Suttle and Chan, 1994, 1993; Waterbury and Valois, 1993). The mortality due to these viruses varies between 0.005 and 8% of the Synechococcus population per day, and influences the levels of dissolved organic matter and inorganic nutrients in seawater, and may be a crucial association in global biogeochemical cycles (Fuhrman, 1999; Wilhelm and Suttle, 1999; Wommack and Colwell, 2000; Garza and Suttle, 1998; Ortmann et al., 2002; Suttle and Chan, 1994; Waterbury and Valois, 1993). This low mortality rate implies the resistance of Synechococcus spp. to viral infection (Garza and Suttle, 1998; Suttle and Chan, 1994; Waterbury and Valois, 1993). Apart from influencing host community organization, cyanophage potentially influences the genetic diversity of host communities, through recombination and lysogenic conversion (Hambly et al., 2001; McDaniel et al., 2002; Ortmann et al., 2002).

It has been reported previously that the leading group of cyanophages isolated from coastal environments have been myoviruses, while others belong to one of the other viral families (i.e., Podoviridae, or Siphoviridae) as revelaed from the morphological studies (Lu et al., 2001; Suttle and Chan, 1993; Waterbury and Valois, 1993). The characterization of the cyanophage diversity has been achieved by employing the molecular techniques, such as restriction digestion of viral genomes (Lu et al., 2001; Wilson et al., 1993), denaturing gradient gel electrophoresis (DGGE) (Wilson et al., 2000), and g20 protein gene sequencing of viral capsid (Fuller et al., 1998; Zhong et al., 2002), being used currently. Through these techniques, cyanophage community composition has been investigated in the Atlantic Ocean (Wilson et al., 2000), Georgia river estuaries (Lu et al., 2001; Zhong et al., 2002), and the Sargasso Sea (Zhong et al., 2002), that uncovered much greater cyanophage diversity than previously reported in studies that were based only on the observable characteristics. A previous study performed by Zhong and co-workers had detected approximately 29 genetically distinct cyanophages belonging to six phylogenetic clusters in a particular water sample, by employing primers particular for the g20 gene sequence of cyanomyoviruses (Zhong et al., 2002). Further, environmental conditions namely salinity, light, nutrient availability, depth, and host cyanobacterial populations have been demonstrated to affect the genetic composition of cyanophages (Lu et al., 2001; Zhong et al., 2002; Wilson et al., 2000; Suttle and Chan, 1994). Till date, the a large number of the studies have focused on ascertaining cyanophage diversity on the basis of spatial variation. However, there is little information about how the genetic composition of cyanophage communities may change seasonally.