Through cell lysis, viruses shape phytoplankton community composition and stimulatebiogeochemical cycling in the oceans. Earlier studies indicate that reduced phosphorus (P)availability can affect phytoplankton virus proliferation. The effects of nitrogen (N) availability areclaimed to be weaker than those for P, but this has not been thoroughly studied. Here, we investigatedhow N-limiting growth conditions, resulting in altered algal elemental stoichiometry andphysiology, affected virus proliferation in the phytoplankters Micromonas pusilla and Phaeocystisglobosa. Algal cultures were adapted to balanced nutrient-limited growth, i.e. N-, P- and NPcontrolledgrowth, before infection with their respective viruses MpV-08T and PgV-07T. The viralinfection experiments were then performed in batch cultures to allow optimal 1-step virus growthcycles. Compared to the nutrient-replete cultures, infection of nutrient-controlled hosts resulted inelongated latent periods (time until first virus release) and reduced viral burst sizes (viruses lysedhost cell−1) for both MpV and PgV. For MpV, the viral burst size was reduced by 70%, independentof the type of nutrient. The burst size of PgV was most reduced under N-limitation, by as much as92%, compared to 70% under P-limitation. Overall, our results demonstrate that algal virusproduction can be strongly impaired by N-limitation and that the effects are of a similar magnitudeto or even larger than for P. Our study indicates that viral control of natural phytoplanktonpopulations might be strongly driven by both P- and N-availability.
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