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

Abstract

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 cyanobacteria 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 interactions: at low light (20 µmol quanta m −2 s −1) the latent period was 1.3 times longer compared with at mid light (90 µmol 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 µmol 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°C) was reduced to just 11% (compared with a mid temperature of 22°C), but still the burst size increased to 541 CrV per lysed host cell; at low temperature (15°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.

Cylindrospermopsis raciborskii Virus and host: genomic characterization and ecological relevance

“…NCBI Genbank NC_020851, unpublished). The assembled CrV genome size is consistent with an estimate of size using pulse‐field gel electrophoresis (PFGE) (Steenhauer et al ., ). Its GC content is 39%.…”

“…Absence of CrV‐like genes in sequenced strains of C. raciborskii and absence of key lysogeny‐associated genes within CrV are consistent with the idea that CrV gained the ability to infect C. raciborskii by means of lytic‐only infection in recent evolutionary times. Along these lines, Steenhauer and colleagues () tested the host range of CrV against 10 species/strains of filamentous cyanobacteria and found CrV to have a restricted range as only C. raciborskii Cr2010 supported lytic infections. It is important to note that the method of isolation of CrV (Steenhauer et al ., ) and continued culturing technique have favoured or selected for the lytic lifestyle.…”

“…Along these lines, Steenhauer and colleagues () tested the host range of CrV against 10 species/strains of filamentous cyanobacteria and found CrV to have a restricted range as only C. raciborskii Cr2010 supported lytic infections. It is important to note that the method of isolation of CrV (Steenhauer et al ., ) and continued culturing technique have favoured or selected for the lytic lifestyle. That closely related CrV‐like viruses in nature have lysogenic ability is a possibility.…”

“…This difference in threshold cycles was significant for all tested conditions (Supporting Information Table S2). Finally, the size of the CrV genome was estimated at ~ 110 kb via PFGE by Steenhauer and colleagues () in their initial characterization work. Segment B existing as inverted repeats results in a genome size consistent with the Steenhauer estimate.…”

“…Yet, despite availability of a few isolates, we have no sequences of cyanophages infecting C. raciborskii . To close this knowledge gap, we sequenced and analysed the genome of the cyanosiphovirus CrV (Steenhauer et al ., ) and its host C. raciborskii strain Cr2010. Along with annotating the genome, we have used this sequence information to demonstrate the presence of CrV‐like viral genes within other filamentous cyanobacteria as well as the presence of CrV‐like viruses within large blooms of C. raciborskii .…”