Aquatic photosynthetic microorganisms were surveyed (algae and cyanobacteria) for novel lysogenic/latent viruses and new methodology was established, using a variety of techniques such as AFC, electron microscopy, and molecular tools. The first study assessed Symbiodinium sp. cultures as a model system to investigate the induction of potential latent vi ruses. The study of Symbiodinium sp. showed that ca. 37% of the strains tested had a group of filamentous VLPs that is inducible by UV-C treatment. Extrapolation of this virus-host interaction and its effects on zooxanthellae viability provides a novel link to the impact of latent infection on symbiotic dinoflagellates of cnidarians and the subsequent disruption of the reef ecosystems. The second study examined the interaction of a freshwater cyanobacterium and its inducible VLPs. The work carried out on Pseudanabaena, strain PPt10905 suggests that this freshwater cyanobacterium harbours a prophage. An unusual interaction was observed in this freshwater cyanobacterium, where the abundance of carboxysome-like particles increased 10 times in heat-treated cultures. The cyanobacterium PPt10905, its inducible VLPs and the co-occurring increase in carboxysomes could be a new mechanism in which lysogeny benefits freshwater cyanobacteria, possibly increasing the host's photosynthetic efficiency. Finally, the third study investigated the presence of latent viruses in the Plymouth culture collection of marine algae. The characterisation and isolation of inducible viruses from this algal culture collection has revealed much novel information on the prevalence of latent viruses in algae. From the 30 algal species examined in this study, over 35% appear to contain an inducible infectious agent. AFC and TEM images have confirmed the presence of VLPs, and thin sections of UV-induced cultures further supported the presence of VLPs in the UV-induced cultures. This work's contribution increases the knowledge of latent and temperate viruses of aquatic microbes, which are underrepresented in previous studies. Additionally, this research established novel techniques for the study of unique biological interactions between aquatic viruses and their hosts that will facilitate and improve subsequent investigation of similar systems.

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