CYANOPHAGES - Cyanobacteria - page 467

(30 µg ml -1 ) was followed by electron microscopy (Ohki and Fujita, 1996). A temperate phage-specifi c
to Trichodesmium sp. has been isolated by Ohki (1999) and it is speculated that the phage had a specifi c
role in the regulation of Trichodesmium biomass.
ii) Lysogeny in marine cyanophages : Phosphate availability has been shown to affect kinetics of
cyanophage infection and also is probably responsible for induction of lysogenic Synechococcus
cells (Wilson et al ., 1996, 1998). A study conducted over a period of one year (during October,
1999 to October, 2000) at Tampa Bay, Florida revealed prophage induction by Synechococcus cells
after treatment with mitomycin C. Due to this, there was appreciable increase in the titre of virus
particles that was inversely correlated with cyanobacterial abundance. Induction of prophage was
predominantly noted during summer months. The limited lysogeny noted during summer months
has been explained on the basis that exposure to UV or high temperature, lysogenic Synechococcus
naturally had undergone induction thereby decreasing the population density. It was concluded that
lysogenization is not a random event but a seasonal pattern that is consistent with the occurrence
of lysogeny when host population was poorly represented in nature (McDaniel et al ., 2002). During
a Synechococcus bloom in a pristine fjord in British Columbia, Canada, it was noted that 0.6% of
the cells are lysogens as they could be induced by the addition of mitomycin C (Ortmann et al .,
2002). McDaniel and Paul (2005) demonstrated induction of lysogenic cells of marine Synechococcus
collected from ten locations of Gulf of Mexico when amended with either nutrients (such as nitrate,
ammonium, urea or phosphate) or mitomycin C. In fi ve of the nine experiments induction occurred
only in the latter. Phosphate enrichment led to induction in only one Tampa Bay sample. Nutrient
enrichment, on the contrary, resulted in a decrease of the lytic phage production.
Put together, the number of Synechococcus cells occurring at any given time and the cyanophage
titers, lysogeny in Synechococcus is a most likely event taking place during low ambient cyanophage
abundance in marine waters. Similar results have been reported from the Gulf of Mexico and
Mississippi River plume by Long et al . (2008). The extent to which the strains of Synechococcus enter
into lysogeny or liable to be infected by lytic phages has been investigated by McDaniel et al . (2006).
Of the 25 strains of Synechococcus that occur in the Gulf of Mexico, 11 strains showed induction after
treatment with mitomycin C and the released particles have been stained with SBYR Gold and viewed
through epifl uorescence microscopy. One of the lysogenic strains appeared to be unique because of
its induction not only by continuous light but also by the release of particles that contained single-
stranded DNA as genome.
XIII. ROLE OF CYANOPHAGES IN MARINE ECOSYSTEM
The virtual abundance of viruses in marine waters superceding the numbers of marine phytoplanktons
has led to the concept of marine viruses being treated as virioplankton. Viral abundance in relation
to environmental parameters, the probable impact of viral-mediated lysis of marine phytoplankton,
the concept of microbial loop and the role of viruses in maintaining host community diversity have
received attention (Fig. 15; Wommack and Colwell, 2000). Two genera of cyanobacteria that are
important components of marine phytoplankton are Prochlorococcus and Synechococcus . Of these two,
Prochlorococcus is numerically the dominant prokaryote inhabiting the temperate and tropical oceans
(Partensky et al ., 1999). It is less than one micron in diameter and contributes signifi cantly to global
photosynthesis (Liu et al ., 1997, 1998). In marine waters the organism is represented to a density of
10 5 cells ml -1 . This cyanobacterium is adapted to live in oligotrophic waters and is represented by
two ecotypes adapted to grow under HL- and LL-conditions and are physiologically and genetically