Can viruses have a positive effect? The effect of Viral lysis on Synechococcus growth

A review of: Weinbauer, MG. Bonilla-Findji, O. Chan, AM. et al (2011) Synechococcus growth in the ocean may depend on the lysis of heterotrophic bacteria. Journal of Plankton Research, Vol. 33 (10) 1465-1476.

Carbon cycling is an important process which occurs in the oceans, Carbon is the basic unit of life and therefore all organisms rely on it for survival. Synechococcus and Prochlorococcus, two genera of unicellular Cyanobacteria are responsible for an astounding 50% of the total primary production in some parts of the ocean and understanding the fate of the carbon which they produce is important to understanding the complex nutrient and energy cycles which occur.

Synechococcus is very abundant and causes various blooms during the year, Heterotrophic nanoflagellates graze on Synechococcus, however the cyanobacteria is so abundant that this does not have much affect on the population. Mortality can also be due to viral lysis caused by Cyanophages; viral lysis results in the release of dissolved organic matter (DOM) such as nitrogenous and iron containing compounds, as well as the release of inorganic matter. However in their natural environment several studies have shown that viral lysis is only a moderate source of Synechococcus mortality compared to that of heterotrophic bacteria. This study looks into the effects of viral lysis in the ocean and concentrates on the lysis of heterotrophic bacteria and its effects on the growth and abundance of Synechococcus.

Two sampling sites were used throughout this study, one in the Gulf of Mexico and the second in the Mediterranean; both are areas where Synechococcus contributes >50% to primary production. Samples taken from these sites between September 2003 and April 2005 were filtered to remove viruses, heterotrophic bacteria and cyanobacteria (namely Synechococcus). Different methods were used with each sampling site but all followed a general consensus of filtrations, dilutions and in some heat treatments to inactivate viruses. A TEM (transmission electron microscope) was even used to make observational views on infected cells.

In both experiments Synechococcus abundance increased regardless of the treatment or dilution when viruses had contact with host cells; however when diluted with virus free seawater the growth rate of Synechococcus was much lower. It was also shown that Synechococcus growth and division was significantly higher with active viruses. The most likely explanation is that viral lysis of heterotrophic bacteria releases nutrients required for the growth of Synechococcus.

Since the abundance of viruses present in sea water has been recognised (10 to every bacterial cell), it has become important and fascinating to find out what role they play in the oceans. The idea that viruses could be beneficial to an organism is interesting as they are normally thought of in a negative light even though theories have already encompassed their use in the evolution of the eukaryotic cell. However there are many possible explanations for the above results and the idea that viruses could be removing competition is also another viable theory. I believe that it is more likely that there were various elements contributing to the abundance of cyanobacteria and that the combination of these is what is making Synechococcus growth so successful.