The unicellular prokaryote Prochlorococcus is the smallest but most abundant photosynthetic microorganism on Earth. Prochlorococcus thrives in oligotrophic oceans where phosphorus is depleted. Cyanophages, viruses that infect Prochlorococcus are tenfold abundant than their hosts. Due to the large amount, around 20% of cyanobacteria are infected each day and competitions among two cyanophages infecting the same host is common. Besides, cyanobacteria exhibit diurnal rhythms in response to the natural light-dark cycle. Meanwhile light is essential for cyanophage infection. In this thesis, studies were performed on how cyanophages maintain their replications under adverse conditions from three distinct aspects, phosphorus, superinfection, and infection initiating at night.

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The unicellular prokaryote Prochlorococcus is the smallest but most abundant photosynthetic microorganism on Earth. Prochlorococcus thrives in oligotrophic oceans where phosphorus is depleted. Cyanophages, viruses that infect Prochlorococcus are tenfold abundant than their hosts. Due to the large amount, around 20% of cyanobacteria are infected each day and competitions among two cyanophages infecting the same host is common. Besides, cyanobacteria exhibit diurnal rhythms in response to the natural light-dark cycle. Meanwhile light is essential for cyanophage infection. In this thesis, studies were performed on how cyanophages maintain their replications under adverse conditions from three distinct aspects, phosphorus, superinfection, and infection initiating at night.

The first project focused on how cyanophages overcome phosphate limitation. Prochlorococcus uses PstS to harvest phosphate from environments in response to phosphate limitation. The pstS gene has also been identified in the genomes of cyanophages, but it is unknown whether the cyanophage PstS can be expressed under phosphate limitation conditions. Comparison of phage and host PstS expression levels demonstrated that under phosphate starvation condition, phage-encoded PstS protein is expressed and total PstS amount is substantially increased in infected cyanobacteria. Then, attention is turned from nutrient stress to peer pressure, which is the superinfection study. Cyanophages were inoculated one after the other and their infection dynamics were monitored. It turned out that the order of inoculation is crucial and the latter one has much smaller burst size, which could be explained by superinfection exclusion. At last, how cyanophages adapt to the darkness period in diel cycle was studied. Previous study showed that cyanophage P-SSM2 adsorbs to its host but does not replicate at night. Here, it was found that infection of cyanophage P-SSM2 initiating at night had significantly larger burst size than that in the day, suggesting that adsorption at night provides P-SSM2 with fitness advantage.