CYANOPHAGES - Cyanobacteria - page 453

possible to identify the marine cyanophage assemblages quantitatively (by cPCR) and qualitatively
following phylogenetic analysis of amplifi ed products. The amplifi cation of cyanophages belonging
to Podoviridae (S-BP1, S-BP2, S-BP3 and phi12) by primers CPS1 and CPS2 was not possible. These
primers were subsequently employed in PCR-DGGE to study cyanophage diversity along a transect
of the Atlantic Ocean (Wilson et al ., 1999, 2000). Using a myoviral-specifi c PCR-sequencing approach,
with g20 as the marker gene, Jameson et al . (2011) reported a high diversity of myoviruses along
a transect of North-South Atlantic Ocean. According to them the myoviruses are distributed over
large areas of Atlantic Ocean irrespective of the abundance and diversity of the co-occurring host
genera, Prochlorococcus and Synechococcus . DGGE was combined with PCR to “fi ngerprint” and
compare algal-viral communities both spatially and temporally (Short and Suttle, 2000, 2002, 2003).
Zhong et al . (2002) studied the phylogenetic diversity of marine cyanophage isolates and those of
natural assemblages by using CPS1 and CPS8 as primers for g20 gene sequence amplifi cation. A
portion of g20 corresponding to ~592 bp was subjected to cloning and sequence analysis. These
studies revealed the presence of nine phylogenetic groups belonging to a total of 114 different g20
homologue sequences from estuarine and oligotrophic offshore environments. A great divergence
in cyanophage community structure was found in estuaries and open-ocean samples. Of the nine
groups only three of them consisted of cyanophages and the rest of the unidentifi ed groups are
suspected to represent either bacteriophages that are infective on cyanobacteria or cyanophages
infective on other unidentifi ed members of cyanobacteria.
Instead of directly assessing cyanophage diversity by the amplifi cation of g20 gene sequence and
subjecting to DGGE analysis, Marston and Sallee (2003) fi rst identifi ed 36 different cyanomyoviruses
infecting the different strains of Synechococcus over a three-year sampling survey conducted at Rhode
Island's coastal waters. Then the g20 gene sequences from natural waters have been amplifi ed by
using a different set of PCR primers such as CPS1/CPS8, CPS1/CPS2 and CPS1/CPS4 for different
cyanophages. Most of the phage diversity has been represented by cyanomyoviruses, however, at
least 50% of the samples containing cyanophage sequences could not be amplifi ed by the primers
employed. This refl ects that other morphotypes belonging to podovirus and siphovirus categories
may also be important components of the cyanophage community in the area studied by them. The
cyanophage diversity and community structure is also shown to be directly dependent on the physical
structure of water column as indicated by temperature and salinity. This was substantiated by PCR
amplifi cation of g20 gene fragments followed by DGGE which suggests that certain cyanophages
are wide spread whereas certain others are specially distributed at greater depths in three inlets
of British Columbia (Frederickson et al ., 2003). Mann (2003) highlighted the factors affecting the
diversity of cyanophages, their interactions with their hosts and selective pressures in the marine
environment.
Studies on the genetic diversity and population dynamics of cyanophage communities in the
Chesapeake Bay revealed unique g20 clonal sequences different from open-ocean waters (Wang and
Chen, 2004). Terminal-restriction fragment length polymorphism (T-RFLP) based on the g20 was
developed by which 15 g20 genotypes have been identifi ed. Of these, only one was closely related
to the sequences from open ocean waters. The T-RFLP profi les of the g20 gene demonstrated that
the myovirus populations exhibited great change seasonally (where phage titres increased from
2.2 PFU ml -1 in April to a high of 559.6 PFU ml -1 in September) rather than spatially. The seasonal
variations appeared to be dependent on the variations of the host cell densities ( Synechococcus sp. CB
0101 ranging from 2.6 ± 0.5 x 10 2 cells ml -1 in February to 8.1 ± 1.0 x 10 4 cells ml -1 in July). Dorigo et
al ., (2004) analysed cyanophage diversity of Lake Bourget (France) by the amplifi cation of g20 gene
with the help of PCR primers CSP-1 and CSP-8 followed by DGGE. Out of 47 sequences analysed,