Isolation of cyanophage CrV infecting Cylindrospermopsis raciborskii and the influence of temperature and irradiance on CrV proliferation

A lytic virus that infects a European strain of the freshwater filamentous cyano-bacterium Cylindrospermopsis raciborskii was isolated from a lake in the Netherlands and partially characterised. With a genome size of 110 +/- 15 kb, an icosahedral capsid of 65 +/- 1 nm (n = 22) and a long non-contractile tail of 612 +/- 31 nm (n = 15), this dsDNA cyanophage CrV appears to belong to the Siphoviridae family. CrV was highly host specific, not infecting other filamentous cyano bacteria species isolated from the same lake, nor 4 Australian strains of C. raciborskii. The latent period of this cyanophage was 20-24 h. Varying the irradiance affected cyanophage-host inter actions: at low light (20 mu mol quanta m(-2) s(-1)) the latent period was 1.3 times longer compared with at mid light (90 mu mol quanta m-2 s(-1)); burst size at mid light was 332 CrV per lysed host cell, at low light it was halved (48%) and at high light (250 mu mol quanta m(-2) s(-1)) the burst size was further reduced to only 14% of that of mid light. Temperature also affected the virus growth characteristics: the CrV latent period at high temperature (30 degrees C) was reduced to just 11% (compared with a mid temperature of 22 degrees C), but still the burst size increased to 541 CrV per lysed host cell; at low temperature (15 degrees C) the latent period was prolonged 1.3-fold and the burst size was reduced to 43%. Our findings indicate that ecologically relevant environmental factors can affect the extent of viral lysis of C. raciborskii, advancing our understanding of the spread of this invasive cyanobacterium across Europe.