CYANOPHAGES - Cyanobacteria - page 418

XII. LYSOGENY 466

i) Lysogeny in freshwater cyanophages
ii) Lysogeny in marine cyanophages
XIII. ROLE OF CYANOPHAGES IN MARINE ECOSYSTEM 467

XIV. METAGENOMICS 471

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.
It was almost fi fty years after the discovery of bacteriophages that the isolation of fi rst virus,
LPP-1 active against three genera of cyanobacteria, i.e. Lyngbya , Phormidium and Plectonema boryanum
was made (Safferman and Morris 1963, 1964a). This generated a lot of enthusiasm for two reasons.
Firstly, it was thought that the discovery of these viruses would help in the study of genetics of
cyanobacteria and secondly in the control of algal blooms. Thus a number of workers reported the
isolation of cyanophages from all over the world. But the euphoria generated by the discovery of
these viruses died down without meeting the two expectations. Freshwater cyanophages infective
against various unicellular and fi lamentous cyanobacteria are summarized along with their sensitive
hosts and country of origin (Tables 1 and 2).