Seasonal variations in virus-host populations in Norwegian coastal waters: focusing on the cyanophage community infecting marine Synechococcus spp

Abstract

Viruses are ubiquitous components of the marine ecosystem. In the current study we investigated seasonal variations in the viral community in Norwegian coastal waters by pulsed-field gel electrophoresis (PFGE). The results demonstrated that the viral community was diverse, displaying dynamic seasonal variation, and that viral populations of 29 different sizes in the range from 26 to 500 kb were present. Virus populations from 260 to 500 kb and dominating autotrophic pico- and nanoeukaryotes showed similar dynamic variations. Using flow cytometry and real-time PCR, we focused in particular on one host-virus system: Synechococcus spp. and cyanophages. The two groups covaried throughout the year and were found in the highest amounts in fall with concentrations of 7.3 x 10(4) Synechococcus cells ml(-1) and 7.2 x 10(3) cyanophage ml(-1). By using primers targeting the g20 gene in PCRs on DNA extracted from PFGE bands, we demonstrated that cyanophages were found in a genomic size range of 26 to 380 kb. The genetic richness of the cyanophage community, determined by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified g20 gene fragments, revealed seasonal shifts in the populations, with one community dominating in spring and summer and a different one dominating in fall. Phylogenetic analysis of the sequences originating from PFGE and DGGE bands grouped the sequences into three groups, all with homology to cyanomyoviruses present in cultures. Our results show that the cyanophage community in Norwegian coastal waters is dynamic and genetically diverse and has a surprisingly wide genomic size range.

FIG. 1.

Development of bacteria and viruses (A) and algae belonging to the nano- and picoeukaryote groups (B) as measured by FCM. (C) Synechococcus (measured by FCM) and cyanophages (measured in real time using primers against the g20 gene). Panel C also shows water temperature measured at the sampling site in Raunefjorden, Norway, from March to November 2004.

FIG. 2.

(A) Schematic outline of the relative abundance (indicated by the size of the dots) of viral populations determined by PFGE. Viral populations are defined by genome size, and the outline is based on three different electrophoresis runs for each viral concentrate. (B) Cluster analysis of the total viral populations from individual samples. The dendrogram is constructed from a binary matrix of similarity values, using a distance calculation algorithm based on the absence or presence of bands. Clustering is based on the simple matching algorithm, while the dendrogram was drawn using the complete link method.

FIG. 3.

Cluster analysis of the DGGE profiles of amplified g20 gene fragments from individual samples. The dendrogram is constructed from a binary matrix of similarity values, using a distance calculation algorithm based on the absence or presence of bands. Clustering is based on the simple matching algorithm, while the dendrogram was drawn using the complete link method.

FIG. 4.

Phylogenetic analysis using neighbor-joining analysis (PAUP*) of sequences from DGGE and PFGE bands and representative g20 sequences retrieved from the GenBank (NCBI) database. Groups I, II, and III refer to the cyanophage classification by Zhong et al. (59). Boldface indicates g20 sequences from GenBank. Bootstrap values were generated with 1,000 replicates; values of <50 are not shown. The scale bar represents 0.1 substitution per site.