ABSTRACT
The discovery of viruses infecting bacteria independently by F.W. Twort and Felix d’Herelle in 1915 and 1917, respectively laid a foundation for a study of these viruses. These have been termed as bacteriophages or briefl y as phages. Researches on the biology of phages can be divided into three phases. In the fi rst phase (1915-1970), basic studies on phages contributed to our understanding of phage morphology, structure and multiplication cycles. The second phase (1970-1990) dealt with genetics of phages that enabled us to have a better understanding of basic biological processes. The major breakthroughs in phage research ushered in new areas of study in molecular biology that had instant ramifi cations into genetic engineering and biotechnology. The third phase of phage research, started in 1990s, pertains to the extensive studies on the diversity of marine viruses including phages determined on the basis of identifi cation of gene sequences from marine waters. Thus the study of uncultured phage communities along with uncultured microbial gene sequences has given rise to a new branch of genetics known as metagenomics (Handelsman, 2004; Schloss and Handelsman, 2005; Zinger et al., 2011). When compared to cellular metagenomic surveys, 65 to 95% of marine viral metagenomic sequences had no counterparts in previously described sequences. In this connection, the uncultured phage communities appear to be most diverse as 3000 to 7000 viral types could be analysed in a 200 litre sample of seawater (Breitbart et al., 2002). The acquisition of host genes by some of these phages and their transfer back into hosts has provided means for the understanding of not only lateral gene transfer (LGT) in the marine ecosystem but also the evolution of phage genomes along with the genomes of their hosts (Breitbart and Rohwer, 2005; Stern and Sorek, 2010). In addition, studies on viral mortality rates of marine heterotrophic bacteria and cyanobacterial picoplanktons have highlighted the role of marine viruses in biogeochemical cycling and the probable ecological effects (Fuhrman, 1999; Wommack and Colwell, 2000; Storrie-Lombardi and Pinkart, 2007; DeCorte et al., 2010; Breitbart, 2011; Danovaro et al., 2011). Rightly, this third phase also termed as ‘third age of phages’ by Mann (2005) has diverted the attention of every one from the labaoratory to the deep seas.
