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Functional Genomics and Ecological Impact of Viral Infection in the Toxic Haptophyte Prymnesium polylepis

Final Report Summary - VINTPRYM (Functional Genomics and Ecological Impact of Viral Infection in the Toxic Haptophyte Prymnesium polylepis)

Viruses are the most abundant and diverse biological entities in the oceans. They infect all organisms from bacteria to whales making them an active and important component of the microbial loop because they regulate microbial mortality, production, community structure, and biogeochemical cycling. Each infection has the potential to introduce new genetic information into the host or into the reproduced viruses thereby driving the evolution of both host and viral assemblages and affecting the genetic diversity of microbial communities. Due to their immobility, viruses depend on passive movement to contact a suitable host. Consequently the encounter rate between a virus and a host is directly affected by their relative abundances and viral infection is more likely in a dense host population such as an algal bloom. Some algal blooms can also be harmful because they are composed of phytoplankton which produces toxins. These harmful algae blooms devastate yearly aquaculture, mollusc populations and shellfish worldwide. As the toxin accumulates along the food web some can even be harmful to human kind. Although marine viruses control blooms and shape the evolution of biodiversity in phytoplankton, little is known about the factors governing resistance and susceptibility of their hosts. Better understanding the host-virus interaction of marine phytoplankton is therefore necessary to properly address its impact on phytoplankton dynamics, socio-economic activities, and medical applications. The project VINTPRYM investigated two different aspects of haptophytes and their corresponding dsDNA viruses: 1) characterization of the abundance and diversity of Phycodnaviridae (large dsDNA viruses that infect marine or freshwater eukaryotic algae) and their potential haptophyte hosts. 2) Transcriptomic response of the haptophyte Prymnesium kappa to viral infection of HeV-RF02. The virus HeV-RF02 is one-of-a kind because it is one of the first marine viruses which can infect different phylogenetically distinct genera: P. kappa and Haptolina ericina. This research has been conducted at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI).
The first task was accomplished by taking environmental samples along the Norwegian coast while on-board the RV Heinke during the He431 cruise in 2014. Host and virus diversity was estimated by amplification of specific molecular markers (18S rDNA, major capsid protein) and subsequent 454-sequencing. Preliminary analysis of four Fjord-systems revealed that over 50% are unknown or not identified viruses and less than 10% of the sequences belong to known haptophyte viruses, pointing out the need to isolate and bring more (and new) virus-host model systems into culture.
For the second task, studying differences in the transcriptomic response (what genes are active at a specific time point) of both host and virus during an infection cycle, I set up a 48h experiment of infected and non-infected P. kappa and H. ericina in triplicates and took samples every 3-6 hours for RNA extraction. Furthermore, samples were also taken for photochemical efficiency (Fv/Fm), growth rate, and TEM pictures (Transmission Electron Miscroscopy) to monitor the fitness of the host and the state of infection. High throughput sequencing of the RNA (RNA-Seq) via Illumina NextSeq paired-end was used. In total, 72 samples were pooled during library preparation, Currently, I am analysing these very big datasets to identify the function of over- and under-represented host genes during the infection. The combination of virus genome information and gene expression profiling will provide an understanding of the mechanisms of a successful infection. This project is the base for follow-up research: we can examine if there is a different infection strategy of the virus between the two different species; we can investigate what confers virus resistance to some strains and makes others susceptible; we can measure how biogeochemical fluxes are affected by viruses; and finally, we can inspect alterations in virus-host interactions due to change in environmental factors.