Viruses are extremely abundant in the marine environment and are thought to profoundly influence ecosystem functioning. As host-specific agents of mortality they influence the population dynamics, diversity and evolution of their hosts. However, we are presently unable to assess the abundance or dynamics of discrete viral populations.Furthermore, estimates of viral-induced mortality are rudimentary at best, due to the lack of appropriate tools to quantify infected host cells. I propose to investigate the diversity of viruses that infect the ecologically important marine cyanobacteria, Synechococcus and Prochlorococcus, as well as assess their impact on host mortality. I will focus on the ubiquitous podoviruses for which little information is presently available. The diversity of cyanobacterial podoviruses will be determined by sequencing the DNA polymerase gene from podovirus isolates.We will use these findings to develop primers for the specific identification of cyanobacterial podoviruses and will employ a novel molecular approach, solid-phase polymerase chain reaction, for their quantification. The abundance and population dynamics of these podoviruses will then be determined over spatial and temporal scales in the Red Sea and will be compared to host population and total virus dynamics.Furthermore, this method will be used to amplify viral DNA from single cyanobacterial cells to investigate the extent of podovirus infection. The results from this research will significantly enhance our understanding of the influence of podoviruses on cyanobacterial mortality, population dynamics and community composition, and thus their impact on primary production and recycling of matter in the oceans.I bring to this project and Europe, unique expertise acquired in the US on physiological, genomic and evolutionary aspects of cyanobacteria-virus interactions. EC funding of this project will advance my progress towards a permanent position at the Technion.