Both phosphorus- and nitrogen limitation constrain viral proliferation in marine phytoplankton

Through cell lysis, viruses shape phytoplankton community composition and stimulate biogeochemical cycling in the oceans. Earlier studies indicate that reduced phosphorus (P) availability can affect phytoplankton virus proliferation. The effects of nitrogen (N) availability are claimed to be weaker than those for P, but this has not been thoroughly studied. Here, we investigated how N-limiting growth conditions, resulting in altered algal elemental stoichiometry and physiology, affected virus proliferation in the phytoplankters Micromonas pusilla and Phaeocystis globosa. Algal cultures were adapted to balanced nutrient-limited growth, i.e. N-, P- and NP-controlled growth, before infection with their respective viruses MpV-08T and PgV-07T. The viral infection experiments were then performed in batch cultures to allow optimal 1-step virus growth cycles. Compared to the nutrient-replete cultures, infection of nutrient-controlled hosts resulted in elongated latent periods (time until first virus release) and reduced viral burst sizes (viruses lysed host cell^-1) for both MpV and PgV. For MpV, the viral burst size was reduced by 70%, independent of the type of nutrient. The burst size of PgV was most reduced under N-limitation, by as much as 92%, compared to 70% under P-limitation. Overall, our results demonstrate that algal virus production can be strongly impaired by N-limitation and that the effects are of a similar magnitude to or even larger than for P. Our study indicates that viral control of natural phytoplankton populations might be strongly driven by both P- and N-availability.