1. We used the freshwater alga Chlorella NC64A (Division Chlorophyta) and its virus Paramecium bursaria Chlorella virus-1 (PBCV-1) as a model system to test for potential stoichiometric constraints on a virus–host interaction.
2. Media phosphorus concentrations were manipulated to create Chlorella NC64A host cells with low (91 ± 23) or high (453 ± 246) C : P ratio. In contrast, the C : P ratio of PBCV-1, calculated from its biochemical composition, was 17 : 1.
3. Stoichiometric theory predicts that infection success and postinfection viral production should be depressed in high C : P cultures due to insufficient intracellular P for production of P-rich viral particles.
4. Consistent with this hypothesis, viral production was strongly affected by host C : P ratio. While host C : P ratio did not affect viral attachment or the percentage of new viral particles that were infectious, in the low C : P Chlorella NC64A treatment, nine times more viruses were produced per infected cell than in the high C : P treatment (158 ± 138 versus 18 ± 18), indicating that the low C : P cells were higher quality for PBCV-1 proliferation.
5. This result implies that the stoichiometric quality of algal cells can have a major effect on host–virus population dynamics.