Biology Reference
In-Depth Information
XI. MARINE CYANOPHAGES
Viruses constitute the most abundant biological entities in marine waters exceeding the numbers
of their hosts and as per present estimates there are nearly 10
27
to 10
31
viruses in the world's Oceans
(Wilhelm and Suttle, 1999; Suttle, 2005; Breitbart and Rohwer, 2005) and these amount to ~270 Mt of C.
If these viruses are laid end to end, they would span between Earth and the Sun 10
13
times (Hendrix,
2003). A number of workers have demonstrated the existence of viruses in high concentrations
(ranging from 10
5
to 10
8
particles ml
-1
seawater) with transmission electron microscopy, after
concentrating either by fi ltration or through centrifugation (Bergh
et al
., 1989; Børsheim
et al
., 1990;
Bratbak
et al
., 1990, 1992; Hara
et al
., 1991; Heldal and Bratbak, 1991; Proctor and Fuhrman, 1990,
1991, 1992; Suttle
et al
., 1990, 1991). The titres of viruses or virus-like particles (VLPs) represented
in different environmental samples are presented in Table 5. Attempts to isolate cyanophages from
seawaters were very few and sporadic (Moisa
et al
., 1981; Suttle
et al
., 1990) but it was only in 1993
that the marine cyanophages have been well characterized (Suttle and Chan, 1993; Waterbury and
Valois, 1993; Wilson
et al
., 1993).
Table 5:
Virus-like particles in different environmental samples
Sr. No.
Type of water body
Titre ml
-1
water/
g
-1
soil
Authors
1
Surface seawater
10
7
Fuhrman, 1999; Wommack and Colwell, 2000
2
Marine sediment
10
8
Danovaro
et al.
, 2001
3
Soil
~10
8
Williamson
et al
., 2003
4
Freshwater
10
5
to 10
9
Kepner
et al
., 1998; Chen
et al
., 2001; Vrede
et al
., 2003
5
Hotsprings
10
5
to 10
9
Breitbart
et al
., 2004
6
Hypersaline
waters
10
6
to 10
9
Oren
et al.
, 1997; Dyall-Smith
et al
., 2003
7
Solar salterns
~10
10
Guixa-Boixareu
et al
., 1996; Pedros-Alio
et al
., 2000a,b
Studies on marine cyanophages have dealt upon the isolation and morphological characterization
of cyanophages infecting
Synechococcus
and
Prochlorococcus
from marine waters of different
geographical locations. Phage titres have been assayed on the basis of culture-based plaque assay
and the phage diversity has been determined by a number of techniques developed for the purpose.
Culture-independent assays of phage diversity included the assay of phage-specifi c genes (encoding
different proteins of capsid, sheath or tail) such as
g20
,
g23
or
g91
or some of the host genes identifi ed
in the marine cyanophages such as
psbA
and
psbD
gene sequences or other allied photosynthesis
genes. Another important characteristic of viruses that can be readily determined is the genome
size. The genome sizes range from few kb to several hundred thousand kb. This chraracteristic of
phages has been utilized for determining the frequency distribution of genome sizes. As conventional
electrophoresis does not provide necessary resolution, pulsed-fi eld gel electrophoresis (PFGE) or
denaturing gradient gel electrophoresis (DGGE) have been employed. This has revolutionized
marine viral enumeration studies. The role of marine cyanophages in the control of prokaryotic
marine phytoplanktons has received much attention. All these aspects are discussed in detail in the
following sections.
i)
Isolation and morphological characterization
:
Waterbury and Valois (1993) isolated marine
cyanophages infecting marine
Synechococcus
strains belonging to two clusters. Cluster A is
characterized by strains (WH8012, WH8103, WH8108, WH8018, WH8109 and WH8017) that occur
in open ocean and coastal waters, possess phycoerythrin as the primary light-harvesting pigment
