VIRALP: Virus-host relationships in an alpine lake

Viruses are key players in regulating aquatic microbial communities. Through large-scale infection and lysis of their microbial hosts, they are known drivers of biogeochemical cycles, microbial diversity and evolution. Considerable research efforts are beginning to uncover virus diversity and activity in marine systems, however, freshwaters have received comparatively little attention despite the numerical dominance of viruses. In extreme, alpine lake ecosystems, exemplified by Gossenköllesee in the Austrian Alps, the microbial community is subjected to high summer UV radiation levels, seasonal ice cover and oligotrophic conditions. These factors may all theoretically restrict virus propagation, yet viruses are the most abundant biological entity and account for up to 28% of bacterial mortality, a value which is comparable to marine systems. Most unusually however, the virus community is dominated by previously uncharacterized large filamentous forms, in contrast to the mainly icosahedral forms observed in other aquatic environments. We expect that this lake contains a highly novel virus community which may use alternative life strategies, such as lysogeny, pseudolysogeny or chronic infection strategies to maintain a long-term association with their hosts. This study aims to produce the first genetic assessment of virus diversity in an alpine lake, reconstruct complete novel virus genomes from the unusual virus forms and identity hosts and the metabolic processes the virus community may influence. Hence uncover previously overlooked controls on the microbial community in an Alpine lake.

VIRALP - Results Summary The VIRALP project investigated aquatic viruses present in the water column of a high altitude alpine lake. Unusually large virus forms have previously been observed in these habitats and we predicted that many of these would be new to science. In this study we sequenced all the collective virus DNA from a large water sample to decode the genomes of thousands of lake viruses and predict which hosts they may infect. The overall aim was to characterise novel viruses and increase research into the microbiology of alpine lakes. Our analysis discovered thousands of new viruses, most of which infected bacteria and had little similarity to known aquatic viruses. By sequencing the DNA from these new viruses we have significantly expanded known freshwater virus diversity and will provide a large, publicly available data-set for microbiologists to use. The most significant result was the discovery of several major new groups of viruses called Polinton-like viruses, which infect a wide-range of microscopic plants, animals and fungi. We showed for the first time that Polinton-like viruses are among the most abundant viruses present in a lake, hence are important and previously unrecognised members of aquatic communities. By analysing their genomes in detail and searching global DNA sequence databases, we further showed that similar viruses are found in lakes around the world, hence we uncovered a large network of globally distributed viruses. The discovery of abundant Polinton-like viruses in lake water is particularly important to the fields of microbiology and virology as these viruses are very similar to short DNA regions found in the genomes of hundreds of diverse organisms, from microscopic plants to large animals, suggesting that they infect an even wider range of organisms than currently known. The VIRALP project has been successful in highlighting alpine lakes as important locations for discovering novel microbial entities and in opening up a new research area in the field of virology. The hundreds of new Polinton-like virus genomes we have discovered are based on the in-depth analysis of a single alpine lake. We therefore anticipate that as further environments are investigated with the methodology we have developed, many more genomes will be discovered, hence our findings represent only the tip of the iceberg for their true global diversity. Many new research questions have been generated by the discovery of these new Polinton-like viruses. We know little about the full range of organisms they infect and nothing about their life cycles or impact on their hosts. We predict that these viruses significantly affect the growth of algae and microscopic animals in aquatic ecosystems. Further research must now address these questions by isolating and characterising these new viruses in a laboratory setting.