Ecological aspects of viral infection and lysis in the harmful brown tide alga Aureococcus anophagefferens

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Although many bloom-forming phytoplankton are susceptible to viral lysis, the per- sistence of blooms of algal species that are susceptible to viral infection suggests that mechanisms prohibiting or minimizing viral infection and lysis are common. We describe the isolation of viruses capable of lysing the harmful brown tide pelagophyte Aureococcus anophagefferens and an investi- gation of factors which influence the susceptibility of A. anophagefferens cells to viral lysis. Nine strains of A. anophagefferens-specific viruses (AaV) were isolated from 2 New York estuaries which displayed 3-order of magnitude differences in ambient A. anophagefferens cell densities. The host range of AaV was species-specific and AaV was able to lyse 9 of 19 clonal A. anophagefferens cul- tures, suggesting that resistance to viral infection is common among these algal clones. Viral lysis of A. anophagefferens cultures was delayed at reduced light intensities, indicating that the low light conditions which prevail during blooms may reduce virally induced mortality of this alga. Some A. anophagefferens clones which were resistant to viral infection at 22°C were lysed by AaV at lower temperatures, suggesting that the induction of viral resistance at higher temperatures could allow the proliferation of blooms during summer months. The addition of laboratory propagated viruses to bottle-incubated bloom waters (>10 5 cells ml -1 ) from New York and Maryland estuaries resulted in a significant reduction in (p < 0.05), but not complete loss of, A. anophagefferens densities, suggesting that a resistant sub-population survived during experiments. In summary, the results demonstrate that clonal resistance, combined with the lower light levels and higher temperatures which are found during brown tides, may allow A. anophagefferens to form blooms during summer months.