Are cyanobacteria pilfering credit for viral contributions?

High abundance (10⁹ – 10¹⁰ viruses L^-1) makes viruses huge contributors to marine microbial communities through infection of bacteria and phytoplankton. Infection of organisms may lead to cell death, but cell lysis also releases dissolved organic matter (DOM) creating a pathway for the regeneration of nutrients (the viral shunt). Synechococcus spp. are cyanobacteria which contribute up to 50% of total phososynthetic carbon fixation in the upper layer of oligotrophic waters.  In warm and summer weather Synechococcus spp. show distinct diel changes, with higher division rates and highest abundance at night. Grazing by pigmented nanoflaggelates appears to be the main regulating factor in Synechococcus spp. diel variations however, recent studies have investigated mortality resulting from viral lysis.

Tsai et al. (2014) investigated the significance of viral lysis in providing a source of recycled organic nutrients and the effect viral presence had on the daytime division frequency of Synechococcus spp. in a simple study using virus reduced and virus existing treatments. Serial filtering was used to provide a virus reduced (VD) treatment; and filtering through a 0.1 µm polycarbonate filter provided a grazer reduced virus existing (VE) treatment. In experiment 1: in-situ samples were taken every 3 hours for Aug 2012; experiment 2: incubations and in-situ samples were taken at 0, 20 and 44 hours for Sept 2012. Virus, bacteria and Synechococcus spp. were counted using epifluorescence. Virus were processed using SYBR Green I (molecular probes) while Bacteria were DAPI stained for identification.

Experiment 1 showed decreases in bacterial and Synechococcus spp. abundance during the night in VE experiments and greater increases in viral abundance in VE than VD treatments. Up to 30% of Synechococcus spp. were dividing during the day in the VE compared to approx. 5% in VD treatment. Experiment 2, was used to confirm experiment 1’s findings, together they indicate that viral presence significantly affects variations in Synechococcus spp. and bacterial abundance and the frequency of cell division in Synechococcus spp. with significantly more ammonium in VE cultures compared to VD cultures..

This study supports ideas that viral lysis leads to higher phytoplankton productivity due its important role in nutrient recycling. Moreover it is in favour of cell lysis stimulating the growth of the non-infected community. Studies in this area are vital in our understanding of the marine food web, without understanding of the norm we cannot investigate the effects of anthropogenic factors or longer term effects such as ocean acidification and warming. This study is important as it shows that although viral input causes mortality it can increase growth which could suggest nanoflagellate grazing has more of an input in phytoplankton abundance than viral activity. I think it would be interesting to complete this study during different seasons, do viruses increase in abundance seasonally reducing phytoplankton growth or does this effect still occur? 

Reference:

Tsai, A. Y., G. C. Gong, and Y. W. Huang, 2014: Importance of the viral shunt in nitrogen cycling in Synechococcus spp. growth in subtropical western Pacific coastal waters. Terr. Atmos. Ocean. Sci., 25, 839-846, doi: 10.3319/TAO.2014.06.11.01(Oc)