CYANOPHAGES - Cyanobacteria - page 470

likely due to a change in phage receptor sites, (ii) selection for resistance to one phage simultaneously
conferred resistance to other phages, and (iii) there is no signifi cant genetic similarity of the phages
as revealed by the analysis of g20 gene sequences. Thus the evolution of multiple phage resistance
seems to infl uence population dynamics of the host. Mutation from phage sensitivity to phage
resistance is known to lead to a confi gurational change in phage receptor sites (Inoue et al ., 1995;
Bohannan and Lanski, 2000). Experimental evidences suggest that the phage-resistant mutants are
less effi cient than their sensitive counterparts in phage-free environments (Buckling and Rainey,
2002) and there exists a fi tness cost for the evolution and maintenance of virus resistance (Bohannan
and Lenski, 2000). The cost of maintaining virus resistance by Synechococcus has been investigated
by Lennon et al . (2007) who conducted experiments with phylogenetically distinct strains of the
host to resist 32 cyanomyophages (S-RIM1 to S-RIM13, S-RIM15 to S-RIM21, S-RIM23 to S-RIM36)
collected from Mount Hope Bay, Rhode Island. The cost of resistance (COR) hypothesis was tested
by comparing growth data and their resistance patterns. The growth rates have been determined
by conducting co-culture experiments with a parent strain, its phage-resistant strain along with a
reference strain. The reference strain (WH8101 lacks phycoerythin but instead contains phycocyanin
and is distinguishable from others (WH7803, WH8018 and WH8012) that possess phycoerythrin.
The quantifi cation of cell densities of these strains was followed in nutrient media at a constant
nitrogen concentration but the phosphate concentration was varied. The growth rate of virus-
resistant mutant of Synechococcus (WH7803R8) was found to be inferior to that its parent strain
(WH7803) and there appeared to be on the average a 20% reduction in relative fi tness, irrespective
of phosphorus availability. Likewise, the performance of phage-resistant mutant Synechococcus WH
8108 was signifi cantly lower leading to an almost 18% reduction in relative fi tness. Further, phage
resistance among the 22 virus-resistant strains of Synechococcus (seven isolated from WH7803, four
from WH8101, three from WH8012 and eight from WH8018) has been tested to fi nd out the breadth
of resistance (BOR). The BOR profi le for each of the resistant mutant is represented by total resistance
(summed up number of viruses to which a Synechococcus strain was resistant) and compositional
resistance (that refers to the multivariate similarity of BOR profi les among different phage resistant
strains of Synechococcus ). Competition experiments revealed that Synechococcus growth rates were
not affected by total resistance but its growth rates were affected by compositional resistance. The
strains that were sensitive to S-RIM4 and S-RIM11 showed low fi tness where as strains resistant to
S-RIM7 and S-RIM20 showed relatively high fi tness. So it depends on the fi tness cost that the viruses
impose on their hosts which matters in determining community structure of hosts. If the COR is
small then resistant bacteria may be more common with concomitant lower titre of viruses and lower
viral-induced mortality. On the other hand, if COR is high then phage-resistant cyanobacteria may
be relatively rare leading to high titer of viruses and high virus-induced mortality. Stern and Sorek
(2011) summarized bacterial and phage defensive and offensive strategies eventually leading to a
phage-host arms race and the evolution of bacterial genomes.
A number of models have been presented to explain the role of virioplankton in maintaining
microbial food web. It is generally assumed that the cell debris from lysis and the released viruses
returns to feed bacteria in a closed loop where the oxidation of carbon compounds is followed by
a regeneration of N and P (Bratbak et al ., 1990; Proctor and Fuhrman, 1991; Fuhrman, 1992). The
viruses cause a general shift in the food web activity (mostly production and respiration) from larger
organisms to bacteria (Fuhrman, 1999; Wilhelm and Suttle, 1999) and reduced protist production
(Fuhrman, 1999). The fi rst mathematical model based on empirical data confi rmed that viral-
mediated mortality of Phaeocystis globosa is an essential regulating factor for the nutrient cycling
(Ruardij et al ., 2005). Other aspects that merit mention are (i) infection of primary producers (10%