Suttle, C.A., A.M. Chan, K.M. Rodda, S.M. Short, M.G. Weinbauer, D.R. Garza and S.W. Wilhelm. 1996. The effect of cyanophages on Synechococcus spp. during a bloom in the western Gulf of Mexico. Eos 76(3 suppl):OS207-OS208.

A monospecific bloom of Synechococcus spp. (10⁵ cells ml^-1) in the western Gulf of Mexico was associated with high concentrations of cyanophages (up to 10⁶ ml^-1) which infected Synechococcus strain DC2. During a two day diel study cyanophages remained relatively constant in the surface mixed layer (ca. 3 x 10⁵ ml^-1) but ranged from < 10² to ca. 3 x 103 ml^-1 below the mixed layer, at 20m, indicating a stable coexistence between the cyanophages and Synechococcus spp. Observations using TEM indicated that ca. 1.1 % (sd 0.32) of the cyanobacteria contained virus particles, and that the number of viruses produced per cell lysed was 81 (sd 17.3), suggesting some viral induced mortality. In an effort to quantify the effect of viruses on Synechococcus we diluted triplicate natural samples with seawater filtered through a 30,000 MW ultrafiltration cartridge (virus-free) or a 0.2-µm pore-size membrane filter (with viruses). The final range of dilutions was 10, 30, 60, 80 and 100 % natural seawater. The abundances of Synechococcus spp. and cyanophages, as well as the frequency of dividing cyanobacteria (FDC) were followed over 24h in simulated in situ on-deck incubations. Abundances of cyanobacteria remained relatively constant over the first 10h (dark period), and approximately doubled the following morning, irrespective of treatment. At the onset of the next dark period there was a strong treatment effect. Cyanobacterial growth rates as indicated by FDC were much higher in samples diluted with seawater containing viruses than in samples to which viral-free seawater was added. This was followed by a decrease in cyanobacterial numbers at the onset of darkness in the samples in which virus concentrations were greatly reduced. There was no strong or consistent treatment effect on the concentration of infectious cyanophages. These results suggest that cyanobacteria growth rates were stimulated by viruses, presumably by enhancing the recycling of nutrients.