|M.Sc Student||Dekel Naama Pnina|
|Subject||Diversity and Abundance of Marine Cyanophages|
|Department||Department of Biology||Supervisor||Professor Debbie Lindell|
|Full Thesis text|
Marine picocyanobacteria are abundant in many marine environments and can be divided into two genera: Synechococcus and Prochlorococcus. Three families of cyanophages are known to infect these cyanobacteria: T4-like myoviruses, T7-like podoviruses and lamboid siphoviruses. T7-like podoviruses are host specific. However little is known about their diversity or abundance in the oceans. In order to assess cyanopodovirus diversity and evolution, we analyzed conserved podoviral genes: The DNA polymerase gene from the replication module, the major capsid protein gene from the structural module, and the photosynthesis gene, psbA. Phylogenetic analyses of the first two genes showed that oceanic cyanopodoviruses form two discrete clusters. Podoviruses from cluster A mainly infect Synechococcus whereas those from cluster B infect both Synechococcus and Prochlorococcus and encode the psbA gene. These combined findings suggest an ancient separation between cyanopodoviruses from cluster A and B with the acquisition or loss of psbA occurring around the time of their divergence. Cluster B phages can be further separated into a number of discrete subclusters. There is a lack of correlation between host and phage phylogeny within these clusters suggesting that cyanopodoviruses have not co-evolved with their particular hosts.
Cyanobacterial populations undergo seasonal changes. In coastal waters cyanophages infecting Synechococcus have an annual cycle that correlates with their hosts, but seasonal differences in the abundance of cyanophages infecting Prochlorococcus have yet to be tested. The relative abundance of the different cyanophage families is also unknown. Here we investigated seasonal differences in cyanophage abundance using the plaque assay with 3 Prochlorococcus and 3 Synechococcus hosts during the summer and winter of 2009 and 2010 in the Gulf of Aqaba. Furthermore, cyanophage plaque composition was assessed by PCR amplification of T4-like and T7-like marker genes. There were significantly more cyanophages infecting Synechococcus than cyanophages infecting Prochlorococcus during most cruises. Differences in host ranges were also observed: cyanophages propagated on Prochlorococcus had a narrow host range, while those propagated on Synechococcus had a broader host range and could cross-infect Prochlorococcus. In the Gulf of Aqaba the genetic diversity of Synechococcus varies during the year, while that of Prochlorococcus is stable. The composition of phages that infected Synechococcus strains changed throughout the year. In contrast, the composition of phages that infected most Prochlorococcus strains did not change much. This suggests that the composition of cyanophages coincides with the genetic complexity of their hosts in the Gulf of Aqaba.