Suttle, C.A., M.T. Cottrell, A.M. Chan and D.R. Garza. 1995. The effect of viruses on the mortality of natural communities of phytoplankton (invited). International Workshop on Aquatic Microbial Ecology, Konstanz, Germany, April 1995.

Results based on decay rates of viral infectivity, contact theory and the effect of concentrated natural virus communities on primary productivity, indicate that viruses can be responsible for significant mortality in natural populations of phytoplankton from coastal waters. Mortality in populations of Synechococcus spp. was inferred from the cyanophage production rates required to balance removal rates of isolates and natural communities of viruses infecting Synechococcus sp. (strain DC2). For offshore waters the proportion of the Synechococcus community that was lysed was estimated to range from 6.6% for a burst size of 250 to 32.8% for a burst size of 50. Based on transport theory, the proportion of the Synechococcus community that could be contacted by infective cyanophages on a daily basis would be ca 5%, thus implying that most cyanobacteria in offshore populations are susceptible to infection by viruses. In nearshore waters about 80% of the Synechococcus would be contacted by infective cyanophages, suggesting that most cells are resistant to infection or the efficiency of infection is very low. For the photosynthetic flagellate Micromonas pusilla, estimates based on decay rates and virus adsorption kinetics ranged from 9 to 51% of the population lysed per day assuming that all contacts result in infection, or from 2 to 10% per day if measured adsorption rates are employed. These data are in agreement with those determined by adding concentrated natural communities of viruses to seawater and measuring the effect on rates of photosynthesis. The results suggest that rates of primary production would be about 1 to 4% higher in the absence of viruses. The results from these studies provide strong evidence that viruses are significant agents of mortality for phytoplankton populations in nature.