Lysogenic cyanophages; molecular mechanisms of lysogeny and their role in marine ecosystems

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Recent studies have revealed that cyanophages are found not only in polluted waters near the coastal areas but also in clean waters such as the open ocean. These cyanophages are thought to have an important role in the regulation of cyanophyta (cyanobacterial) populations. Most of the isolated cyanophages are of the virulent type, and cells are rapidly lysed shortly after infection. The contribution of lysogenic phages to the regulation of cyanobacterial populations is unknown. The basic knowledge such as distribution of lysogenic cyanophages among marine cyanobacteria and molecular mechanisms for lysogeny are required to estimate the possible role of lysogenic cynophages in marine ecosystems. In our previous work, we found that a cyanophage had established the stable lysogeny in marine cyanobacteria, Phormidium persicinum Provasoli strain. The lytic cycle of lysogenized phage was induced by the addition of mitomicyn C or a brief UV-illumination. (1) Using the culture of P. persicinum … More cultures, I found that considerably large part of the lysogenized cyanophage were detected to be in lytic cycle in the Phormidium percicinum cultures without any artificial induction. (2) Considarably large numbers of marine Phormidium strains isolated from different areas harbored lysogenic cyanophages. Morphological characters, DNA-DNA hybridization and Western analysis of phage proteins suggested that different family of cyanophages were established the stable lysogeny in marine cyanobacteria. These results indicated that lysogenic cyanophages are ecologically important, because not only they regulated the host algal populations but also they become the source for continuous supply of free phage particles to the ambient sea waters. (3) A possible role of lysogenic cyanophage(s) in regulation of marine, aerobic diazotrophic cyanobacteria, Trichodesmium was studied using both isolated strains (NIBB 1067) and field samples collected from Caribbean Sea off Carrie Bow Cay, Belize. Trichodesmium spp. Are the most important primary producer in oligotrophic, tropical and sub-tropical oceans. The rapid cell lysis of Trichodesmium caused by the addition of mitomycin C was observed in both cultured and natural samples. Cyanophage particles were isolated from Trichodesmium NIBB1067. These results suggested that the lysogenic cyanophage(s) must play an important role in the food web of oligotrophic oceans. Less