Wilson, W.H.
Department of Biological Sciences, University of Warwick, Coventry, CV4 7AL, U.K. Email: wl@dna.bio.warwick.ac.uk (NB from July 1st 1998 my address will be: Marine Biological Association of the UK., The laboratory, Citadel Hill, Plymouth, PL1 2PB, UK.)
Cyanophages (viruses that infect cyanobacteria) are abundant in the marine environment and are thought to be a significant factor in determining the population dynamics of members of the unicellular phycoerythrin-containing cyanobacteria of the genus Synechococcus. In an effort to use molecular techniques to characterise cyanophage populations, a conserved region from the cyanophage genome was identified in 3 genetically distinct marine cyanomyoviruses and sequence analysis revealed that they exhibited significant similarity to a gene encoding a capsid assembly protein (gp20) from the enteric coliphage T4. Comparison of these sequences permitted the design of PCR primers which specifically amplified a region of 165 bp from cyanomyovirus isolates tested. Denaturing gradient gel electrophoresis (DGGE)was then used to separate 165 bp DNA fragments from a range of different cyanomyovirus isolates which had been PCR-amplified together. DGGE was subsequently used to investigate the population structure of cyanophages during the course of a seawater mesocosm study. A large Synechococcus spp. bloom developed in one mesocosm enclosure which was maintained at a high N:P ratio, simulating phosphate-deplete growth conditions. Following phosphate addition to this enclosure, there was a large increase in estimated total virus numbers shortly before an apparent collapse of the Synechococcus bloom. DGGE analysis of the > 0.3 µm fraction revealed that one cyanophage genotype disappeared following the addition of phosphate to the phosphate-deplete enclosure. It is tentatively suggested that lysogenic viruses were induced following phosphate addition to the phosphate-limited enclosures. Such observations further indicate that nutrient availability is responsible for the switch between lysogeny and lytic production.