Viruses, driving primary production?

Virus-driven nitrogen cycling enhances phytoplankton growth

Viruses lyse between 20% to 40% of bacterial production on a daily basis liberating the cells contents into the surrounding environment, this has been termed ‘The Viral Shunt’. This can be described as ‘the viral mediated transfer of nutrients from particulate to dissolved material’.  The viral lysates are  rich in both free and combined amino acids, thus are a potential source of organic nitrogen, particularly in conditions of limited nutrients bacteria uptake Dissolved Organic Nitrogen (DON). Moreover, carbon limited bacteria hydrolyse amino acids along with other nitrogenous substances, to access the carbon, and produce ammonium as a by-product. It is this utilization of the lysates by uninfected bacteria that brings viruses into a role in nutrients and carbon cycling. The lysates nitrogenous content is in excess of bacterial requirement, thus metabolites produced by the bacteria should provision a source of inorganic nitrogen to the phytoplankton.

In March this year Shelford, E et al published a paper that put forward the hypothesis:

“Viral lysis of bacteria releases cellular debris to the dissolved organic matter (DOM) pool, accessible to uninfected bacteria. The subsequent turnover is potentially an important source of N for phytoplankton growth”.

This hypothesis was tested experimentally at two sites; The Indian Ocean (IO) and False Creek (FC), Vancouver. The aim of the experiments were to examine the impact of viruses on ammonium production and how this would affect phytoplankton growth. This, in essence, was done by maintaining bacterial abundances close to in situ while reducing (-V) or maintaining (+V) near in situ viral concentrations. Cell and virus counts were enumerated via use of a flow cytometer, where as chlorophyll and ammonia were measured fluorometrically.

The prominent findings were that chlorophyll and ammonium production decreased in both temperate and tropical seawater in which viral abundance had been reduced. This would be because at lower viral concentration there would be fewer viral lysates for uninfected bacteria to metabolise, consequently lower concentrations of inorganic nitrogen would be available for phytoplankton production.  The aforementioned  is implicit that viruses are important in ammonium regeneration.

I think this paper begins to illuminate the importance of viruses in ammonium regeneration and nitrogen cycling, hopefully opening a doorway to further research into this area of study. Furthermore the fact that treatments were applied to both tropical oligotrophic and temperate nutrient rich waters is commendable as it starts to display that viral cycling of ammonium and nitrogen may hold global significance. Though I do find it a shame that key players remain nameless in as far as viral strains, bacterial strains and species of phytoplankton aren’t mentioned. This makes it hard to conceptualize where this study fits in with the main players of nitrogen cycling such as:  Trichodesmium, or Crenarchaeota to name a few.

 A final note: I have some confusion on the statistics used, thought haven’t had time to look into it.

Shelford, E. et al., 2012. Virus-driven nitrogen cycling enhances phytoplankton growth. Aquatic Microbial Ecology, 66(1), pp.41–46. Available at: http://www.int-res.com/abstracts/ame/v66/n1/p41-46/ [Accessed November 2, 2012].