Death by grazing or viral lysis?

Before Proctor and Fuhrman (1991) started to research viral infection and mortality of cyanobacteria, protozoan grazing was considered the main cause of cyanobacteria mortality, although the mechanisms of this were poorly understood. Their findings in this paper showed uncertain results as to just how much mortality viruses were responsible for, due to the difficulty in differentiating between viruses simply being attached to the hosts, or viruses infecting their hosts via the lytic cycle. Due to this, many scientists have used this paper as a basis to try and find out just how many microbes are killed via viral lysis or grazing.

One such paper published by Tsai, Gong and Chao (2016) looked at nanoflagellate grazing and viral lysis on bacterial mortality in the Subtropical Western Pacific. Using two different depths and a modified dilution technique, their results showed that nanoflagellate grazing was the highest cause of bacterial mortality at the shallowest depth (81-87%), whereas viral lysis was the highest cause of bacterial mortality at the deepest depth (67-75%). 

Whilst this study was successful in identifying the percentages of bacterial mortality due to viral lysis and nanoflagellate grazing, and therefore how the nutrients are flowing within the food web, it only gives an image of what is happening at the two depths that were studied in the one specific area of the subtropical western pacific. Whilst other studies have been conducted to look at how this changes due to season, host organism and environmental conditions, these factors are still poorly understood, particularly in this region. This suggests that this study is a good starting point in determining how nutrients enter the food web at the different depths, as many of the previous studies to date have focused on shallow waters. However, further study is needed to understand the carbon cycling under these different conditions, and also on a global scale. It seems there are a lot of factors to consider when researching this topic, and so there is a lot of room to experiment. It will be interesting to see whether grazing or viral lysis is responsible for the most bacterial mortality when tested under other conditions, especially in deeper waters.

Proctor, Lita. M., and Fuhrman, Jed. A. (1990). Viral Mortality of Marine Bacteria and Cyanobacteria. Nature, 343: 60-62 http://www.nature.com/nature/journal/v343/n6253/pdf/343060a0.pdf

Reviewed Paper: Tsai, A., Gong, G. and Chao, C. F. (2016). Contribution of Viral Lysis and Nanoflagellate Grazing to Bacterial Mortality at Surface Waters and Deeper Depths in the Coastal Ecosystem of Subtropical Western Pacific. Estuaries and Coasts, 39(5): 1357-1366 http://link.springer.com/article/10.1007/s12237-016-0098-9