Viruses Inhibit CO2 Fixation in the Most Abundant Phototrophs on Earth

This week, we published a paper in Current Biology, titled “Viruses Inhibit CO2Fixation in the Most Abundant Phototrophs on Earth”. The paper is a product of work done by Rich Puxty in the course of his Phd thesis under the supervision ofDave Scanlan, Dave Evans and myself. During his postgraduate research, Rich was looking into the effect of cyanophage infection on host photosynthesis and carbon fixation, using a model system of marine cyanophages S-PM2 and S-RSM4, with their host cyanobacterium Synechococcus WH7803.

Cyanophages are a group of viruses that specifically infect cyanobacteria –  photoautotrophic bacteria  which  utilise light energy to  synthesize sugars and drive metabolic processes. The two most interesting phyla of marine cyanobacteria are Synechococcus and Prochlorococcus. They are widespread in the global oceans and play an important role in the fixation of CO2 and production of oxygen. It has been known for two decades that cyanophages infect cyanobacteria and for over a decade we have known that cyanophages carry genes that can maintain the photosynthetic apparatus of their host.

Our latest published research has shown that whilst photosynthesis is maintained during infection, CO2 fixation is not, thus effectively decoupling CO2 fixation from photosynthesis. If we use these findings with the current data for the number of infected marine cyanobacterial cells per day, the abundance of cyanobacteria and the data from model system of S-PM2/S-RSM4 and WH7803 ,  we can extrapolate that up to 5.39 petagram (1015 grams) of carbon per year is lost to viral-induced inhibition of marine CO2 fixation. Just to put this number in some perspective, that ~ 10% of total marine primary production. More importantly, this huge impact that viruses have on global carbon fixation has not been taken into account in all of the global models and we are hoping that our research will allow us to get a far more accurate picture about global carbon balances.
Another surprising finding in this work was difference in “fitness ” between cyanophages S-PM2 and S-RSM4. These phages contain a number of auxiliary metabolic genes (AMGs) – genes which are suspected to participate in bacterial metabolic processes of the infected host. Our assumption was that since AMGs are supposed to provide the cyanophage with more metabolites required for successful infection cycle, and thus with selective advantage, the phage which carries more AMGs, would possess greater “fitness”.

By using  infection physiological parameters – burst size (number of phage particles produced per cell) and latent period (time from initial attachment to production of new phage particles), we discovered that surprisingly the phage which contained fewer putative AMGs – S-PM2, had both a larger burst size and a shorter latent period, suggesting selective advantage over AMG-replete S-RSM4.

Puxty, RJ., Millard, AD., Evans, DJ., Scanlan, DJ., Viruses inhibit CO2 fixation in the most abundant phototrophs on Earth. Current Biology. doi: 10.1016/j.cub.2016.04.036.

More comments on this work in the general press can be found here