Changing phosphorus concentration and subsequent prophage induction alter composition of a freshwater viral assemblage

Abstract

Summary1. The effects of nutrients on the temporal variation in viral assemblage composition, and in particular the occurrence of temperate phages, were assessed in mesotrophic Lake Erken over 5 months of the ice‐free period. The percentage of the bacterial community that contained inducible prophages (lysogenic bacteria, LB) changed over the season, being lowest in late spring and highest in early autumn. The most important variables for predicting LB were concentrations of total nitrogen (TN), total phosphorus (TP) and temperature.2. The viral assemblage composition, as determined by pulsed‐field gel electrophoresis (PFGE), also changed over the season. Prophages were induced by incubations with mitomycin C and we show, for the first time for natural communities, that the resulting temperate phages could be detected using PFGE.3. A substantial fraction (19%) of the number of detected operational taxonomic units (OTUs: defined as unique genome sizes) appeared unique to temperate phages and 41% of OTUs increased in relative abundance after treatment with mitomycin C.4. Different viral OTUs were induced at different times during the season. The most important environmental factor covarying with viral assemblage composition over the period of study, as determined by multivariate analysis, was concentration of TP. In re‐growth cultures with natural bacteria and lowered viral abundance (VA) (decreased virus to bacteria ratio), addition of PO4‐P induced prophages and resulted in subsequent production of temperate phages, as indicated by a decreased percentage of LB and increased VA. Incubations of natural bacterial communities with mitomycin C (field data) or PO4‐P (experiment) changed the viral assemblage composition at a similar rate as the observed monthly changes in the lake.