Wherever there is life, there are viruses, even in the expanses of the open ocean. In the photic zone, viruses are known to infect phytoplankton, and busting an otherwise booming phytoplankton bloom can have important consequences for how nutrients and energy cycles. Just like viral infections in higher-order species, viruses infecting eukaryotic algae can be highly virulent, reproducing rapidly and killing their hosts without much concern for the conditions around them.
A recent paper published in Nature Communications suggests not all viral infections are the same, however, and viral infections in eukaryotic algae may depend more on the host environment than previously thought. Using the globally important Emiliania huxleyi as their model organism, Knowles and colleagues did not observe any virus-initiated deaths at algal densities similar to natural bloom conditions. Viral lysis was only observed in more crowded cultures. Further experiments also showed the virus population scaled with E. huxleyi carrying-capacity and remained dormant until the host was physiologically stressed, suggesting temperate infection. This means that infection isn't just dependent on host density per se, but rather viral lysis is a "Coup de grâce" once the host population is already on the decline.
A recent paper published in Nature Communications suggests not all viral infections are the same, however, and viral infections in eukaryotic algae may depend more on the host environment than previously thought. Using the globally important Emiliania huxleyi as their model organism, Knowles and colleagues did not observe any virus-initiated deaths at algal densities similar to natural bloom conditions. Viral lysis was only observed in more crowded cultures. Further experiments also showed the virus population scaled with E. huxleyi carrying-capacity and remained dormant until the host was physiologically stressed, suggesting temperate infection. This means that infection isn't just dependent on host density per se, but rather viral lysis is a "Coup de grâce" once the host population is already on the decline.
What does this mean for algae in the ocean? The authors acknowledge this is a difficult question to answer since there are many unknowns about the physiological state of a bloom. It's hard to tell what the bloom carrying capacity may be, just as it is hard to tell whether the bloom has even been infected in the first place. But they also suggest the "Coup de grâce" framework may be a more sustainable relationship at the ecosystem level if it allows for infections at different times. Temperate infection in heterogeneous bloom populations would be an interesting next line of inquiry, particularly if the if this model holds true for subsequent infections. Ultimately what this study means for biogeochemistry and ecology in areas where blooms form is still an open question as well. A previous study showed viral lysis of E. huxleyi can even influence cloud formation, so influence of viruses on of these microscopic coccolithophores certainly has major consequences.