Viruses as masters of the phytoplankton?

Virus-driven nitrogen cycling enhances phytoplankton growth

Last lecture Colin highlighted the importance of the viral shunt in the nutrient cycling and I chose this paper to go into more detail with this.

As already mentioned in several blog entries before, viruses have a great impact on the ecology of aquatic ecosystems and they are far more abundant than any other biological “beings” in the ocean. They account for roughly 20 – 40% of bacterial death through cell lysis every day and thereby make nutrients available to the environment, which is referred to as the “viral shunt”. The free organic carbon can then be used by other bacteria, which means that viruses have a major role in the recycling of carbon in the microbial community. Furthermore, the lysis of cells releases amino acids into the water column as a source of organic Nitrogen and contributes to the Nitrogen cycling.

Evidence for this has been found in the Gulf of Mexico and the Mediterranean where growth rates and division of cyanobacterial cells from Synechococcus were found as highest when viruses were present and lowest when viruses were removed or reduced. Additionally, it was discovered that the ammonium (NH₄⁺) concentration of a water column decreased with the removal of viruses which subsequently led to a decrease in phytoplankton growth.

For assessing the production of NH₄⁺ and phytoplankton growth, water samples were taken from two completely different environments; (1) Vancouver, Canada, with a salinity of ~ 25-26 ‰, low chlorophyll α, eutrophic conditions, (2) Indian Ocean, with salinity of ~ 35 ‰, low chlorophyll α, oligotrophic conditions.

The two water samples were subject to two conditions of viral concentration (+V = presence, -V = absence) and both set-ups were kept at in situ concentrations of bacterial cells as well as environmental conditions. Bacterial cells and viruses were stained with SYBR Green and counted using flow cytometry, phytoplankton cells were counted unstained. The chlorophyll α and NH₄⁺ content of the samples were established using fluorometry and results showed a much higher NH₄⁺ concentration from samples with viruses compared to the ones where the viruses had been removed. In addition to this, the +V treated samples had a much higher chlorophyll content than –V treated ones (see Figure 1).

 

Fig. 1: Difference in chlorophyll α concentration and picoalgal abundance from both water samples (IO = Indian Ocean; FC = Vancouver, Canada) when viruses present (+V) and viruses absent (-V). 

From these results, assumptions of viral-induced NH₄⁺ production resulting in higher phytoplankton growth rate (= increased chlorophyll α content) could be confirmed. This study strongly underlines the importance of viruses in the aquatic ecosystem as they greatly influence the nutrient cycling. The authors examined two very different ecosystems (Canada and Indian Ocean), however, I suggest that future studies also need to look at the most productive regions such as the Antarctic waters where phytoplankton growth during summer months is very high. In my first blog post (“Virus genes in the Arctic marine bacteria identified by metagenomic anaylsis”) I highlighted that viral mortality of bacteria in Antarctic waters is the highest of any marine environment and it would be interesting to see how reliant the phytoplankton production would be upon the presence of lysing viruses.

Shelfold, E.J., Middelboe, M., Møller, E.F., Suttle, C.A., 2012. Virus-driven nitrogen cycling enhances phytoplankton growth. Aquatic Microbial Ecology, 66, pp.41-46.