Recent investigations have indicated the presence of high concentrations of viruses in the marine environment. It has been clearly demonstrated that infective cyanophages are extremely abundant in surface seawater, and it is likely they will mediate some form of control to cyanobacterial community structure but very little data has been reported on their characterisation. This study presents data on the isolation and molecular characterisation of cyanophages propagated on the marine phycoerythrin-containing SYNECHOCOCCUS sp. WH7803. The subsequent drive towards developing a cyanophage- specific probe is also described. Marine cyanophages propagated on SYNECHOCOCCUS sp. WH7803 were isolated from six different oceanographic provinces; coastal water from the Sargasso Sea, Bermuda; Woods Hole harbour, Massachusetts, USA; Red Sea, Eilat; Raunefjorden, Bergen, Norway; coastal water from the Gulf of Mexico, Miami and coastal water from the English Channel, off Plymouth Sound, U.K. Cyanophage isolates were found to belong to three families, Myoviridae, Styloviridae and Podoviridae on the basis of their morphology observed in the transmission electron microscope. SDS-PAGE analysis revealed that a major, presumably structural, polypeptide (approximately 55 - 57 kDa) was present in each cyanophage isolate. DNA purified from each of the cyanophage isolates was restricted with a selection of restriction endonucleases and distinguishably different patterns were observed. Southern blotting analysis revealed that there was a limited degree of homology between all the cyanophage DNA probed. The homologous region was cloned and partially sequenced from two cyanophage isolates and was found to have significant homology to the structural head protein of bacteriophage T4. Development of a PCR-based assay to interrogate natural cyanophage populations, based on these sequences, was discussed. Antibodies were raised against one of the cyanophage strains and a putative immuno-assay was developed to enumerate cyanophages in the marine environment.

The effect of nutrient limitation on cyanophage infectivity was investigated using adsorption and one step growth kinetics. Differences were observed in the one step growth kinetics of cyanophage strain S-PS1 propagated on phosphate deplete SYNECHOCOCCUS sp. WH7803 compared to S-PS1 propagated on normal grown host cells. These differences suggested that cyanophage strain S-PS1 is temperate and host cells revert to lysogeny when infected during phosphate depletion. Host SYNECHOCOCCUS sp. WH7803 reverted back to the lytic cycle when phosphate was added back to the media. Phosphate depletion of host cells has no effect on the uptake kinetics of cyanophage strain S-PS1 compared to host cells grown in normal (nutrient replete) media.

Virus particles were observed in association with lysis of the marine coccolithophorid EMILIANIA HUXLEYI and the marine diatom THALASSIOSIRA WEISSFLOGII during an investigation into other marine viruses. E. HUXLEYI viruses were found to be inherently temperate (cf. lytic) and eventually reverted back to their natural state as temperate viruses after several generations as a lytic virus. Viral lysis of T. WEISSFLOGII only occurred in association with a member of the CYTOPHAGA spp., which are marine Gram negative heterotrophs capable of producing degradative exoenzymes.