Marine phytoplankton contributes to about one half of global primary production, and a significant proportion of their photosynthetically fixed organic carbon is released after viral infection as dissolved organic matter (DOM). This DOM pool is known to be consumed by heterotrophic microorganisms; however, its impact on the uninfected co-occurring phytoplankton remains largely unknown. Here, we conducted transcriptomic analyses to study the effects of viral lysis products on the unicellular

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... obacterium Prochlorococcus, which is the most abundant photosynthetic organism on Earth. While Prochlorococcus growth was not affected by viral lysis products, many tRNAs increased in abundance, which was also seen after amino acid addition, suggesting that amino acids are one of the compounds in viral lysis products that affected the expression of tRNA genes. The decreased transcript abundances of N metabolism genes also suggested that Prochlorococcus responded to organic N compounds, consistent with abundant amino acids in viral lysis products. The addition of viral lysis products to Prochlorococcus reduced the maximum photochemical efficiency of photosystem II and CO2 fixation while increased its respiration rate, consistent with differentially expressed genes related to photosynthesis and respiration. One of the highest positive fold-changes was observed for the 6S RNA, a non-coding RNA functioning as a global transcriptional regulator in bacteria. The high level of 6S RNA might be responsible for some of the observed transcriptional responses. Taken together, our results revealed the transcriptional regulation of Prochlorococcus in response to viral lysis products and suggested its metabolic potential to utilize organic N compounds.