Marine and freshwater cyanophages in a Laurentian Great Lake: evidence from infectivity assays and molecular analyses of g20 genes

Abstract

While it is well established that viruses play an important role in the structure of marine microbial food webs, few studies have directly addressed their role in large lake systems. As part of an ongoing study of the microbial ecology of Lake Erie, we have examined the distribution and diversity of viruses in this system. One surprising result has been the pervasive distribution of cyanophages that infect the marine cyanobacterial isolate Synechococcus sp. strain WH7803. Viruses that lytically infect this cyanobacterium were identified throughout the western basin of Lake Erie, as well as in locations within the central and eastern basins. Analyses of the gene encoding the g20 viral capsid assembly protein (a conservative phylogenetic marker for the cyanophage) indicate that these viruses, as well as amplicons from natural populations and the ballast of commercial ships, are related to marine cyanophages but in some cases form a unique clade, leaving questions concerning the native hosts of these viruses. The results suggest that cyanophages may be as important in freshwater systems as they are known to be in marine systems.

FIG. 1.

Map of Lake Erie indicating stations sampled for this study. Black circles represent stations where no lytic activity was observed for Synechococcus sp. strain WH7803; white circles represent stations where lytic activity was detected. Stations discussed in more detail are labeled numerically (Environment Canada station coding) or with acronyms: GR, station located within the mouth of the Grand River; LPB, station located within Long Point Bay.

FIG. 2.

Phylogenetic affiliation (neighbor-joining method) of inferred g20 amino acid sequences from Lake Erie phage isolates as well as direct amplifications from virus concentrates, whole water, ship's ballast water, and particulate samples collected during this study. Samples are color-coded to represent different source materials, and accession numbers are given to clarify sequences from other studies: black, natural samples (9, 18, 21, 31, 39, 49); black with underline, isolates (14, 34, 49); red, isolates (this study); blue, natural samples (this study); green, ballast water samples (this study). Bootstrap values of <50 are not shown. The scale bar equals 0.1 substitutions per site.

FIG. 3.

Transmission electron micrograph of cyanophage MC15-d2 after passage through a culture of the marine cyanobacterium Synechococcus sp. strain WH7803. The image shows the dominant morphotype of intact, tailed bacteriophage, as well as the remnants of phage (primarily tails) damaged during the process of ultracentrifugation.