1. This study addressed the prevalence of lysogeny within 19 strains of freshwater Synechococcus sp. rich in phycocyanin (PC) that were isolated from a temperate lake or obtained, in four cases, from a culture collection. 2. Lysis of 16 Synechococcus strains (84%) was inducible with mitomycin C, but the required concentration (20 lg mL )1 ) was higher than in other studies. Mitomycin C induction yielded burst sizes of 3.5-23.7 phages per cell. 3. Transmission Electron Microscopy of the resultant temperate phages showed that all belonged to the family Siphoviridae with capsid diameters ranging from 40.3 to 53.5 nm and tail lengths from 129 to 186 nm. None of the cyanophages could be maintained in the lytic cycle using the three non-lysogenic strains, or any of the other 16 lysogenic strains of Synechococcus. 4.These results suggest a potentially high level of lysogeny within PC-rich Synechococcus species in natural freshwaters, but that the presence of these temperate cyanosiphoviruses may go undetected with the lower mitomycin C concentrations commonly used.

Fifteen strains of naked amoebae were presented with 19 strains of Synechococcus on an agar surface. After 14 days of incubation, each of the 285 combinations yielded one of three responses. 42.1% of combinations showed clearing (digestion) of the Synechococcus (C), 56.5% of combinations showed no clearing of the Synechococcus (N) while 1.4% of combinations showed partial clearing of the Synechococcus (P). In general, the Synechococcus strains showed variability in their susceptibility to digestion by the amoebae and the amoebae showed variability in their ability to digest the Synechococcus strains. There was no evidence for amoebae actively selecting profitable prey and equivalent-sized Synechococcus strains were ingested at the same rate, irrespective of their fate. There was some evidence of 'size-selective' grazing in that amoebae ingested the smaller Synechococcus strains at higher rates than the larger strains. However, there was no correlation between prey size and their ultimate fate. These data suggest that amoebae are not selective with regard to the ingestion of synechococci, but that 'selection' occurs at the digestion stage, i.e. whether the synechococci are digested or not.

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