Genetic diversity and temporal variation in the cyanophage community infecting marine Synechococcus species in Rhode Island's coastal waters.

Viruses in the marine environment are abundant and diverse. Surface seawater samples typically contain 1010 viral particles per liter, and the viral genotypes identified within these samples can be extremely diverse (2, 5, 37). Virus-caused mortality of bacteria and phytoplankton affects levels of dissolved organic matter and inorganic nutrients in seawater and may be a key link in global biogeochemical cycles (5, 32, 37). In addition, viruses have been shown to influence the structure and diversity of marine microbial communities (9). Cyanobacteria, including the genus Synechococcus, are important contributors to primary productivity in marine ecosystems and are found in high concentrations in coastal waters (20, 31). Recent studies have shown that genetically distinct groups of Synechococcus spp. can coexist in a single location (4, 17, 27) and that the distribution and abundance of Synechococcus spp. vary across locations (4, 19). Cyanophages are viruses that can infect Synechococcus spp. and other cyanobacteria. They are ubiquitous in marine environments and are found in great abundance in coastal waters with concentrations of up to 108 cyanophages per liter observed in summer months (22, 23, 30). It has been estimated that in coastal waters, 80% of Synechococcus cells may encounter infectious cyanophage particles per day (22). Nevertheless, the mortality due to cyanophage infection ranges from only 0.005 to 8% of the Synechococcus community per day (7, 15, 22, 30). This low mortality rate suggests that resistance to viral infection is common in Synechococcus spp. (7, 22, 30). Indeed, studies indicate that many Synechococcus clones isolated from coastal waters are resistant to cyanophages (22, 30). Even with low mortality rates, cyanophages can still influence the clonal composition of Synechococcus communities (30) and could account for some of the cyanobacterial diversity observed in natural communities (25, 27, 37). Morphological studies show that cyanophages belong to one of three viral families (i.e., Myoviridae, Podoviridae, or Siphoviridae), although the majority of cyanophages isolated from coastal environments have been myoviruses (12, 23, 30). Recently, molecular techniques, including restriction digestion of viral genomes (12, 34), denaturing gradient gel electrophoresis (DGGE) (36), and viral capsid assembly protein (g20) gene sequencing (6, 40), have been used to characterize cyanophage diversity. By the use of these techniques, cyanophage community composition has been examined along a south-north Atlantic Ocean transect (36), in Georgia river estuaries (12, 40), and from stations in the Sargasso Sea (40). These studies have revealed much greater cyanophage diversity than previously reported in studies that were based solely on phenotypic characteristics. For example, using primers specific for the g20 gene sequence of cyanomyoviruses, Zhong et al. (40) identified up to 29 different cyanophage genotypes falling into six phylogenetic clusters in a single water sample. The genetic composition of cyanophage communities appears to be influenced by salinity, light, nutrient availability, depth, and presumably host cyanobacterial populations (12, 22, 36, 40). To date, most studies examining cyanophage diversity have focused on spatial variation. There is little information about how the genetic composition of cyanophage communities may change over time. In this study we characterized the diversity of cyanophages in Mt. Hope Bay, R.I., over a 3-year period of time. The overall abundance, g20 genotypes, phylogenetic relationships, and host ranges of cyanophages capable of infecting marine Synechococcus spp. were determined. We also examined temporal and spatial variations in the relative abundances of different cyanophage genotypes in Mt. Hope Bay and Narragansett Bay, R.I.