Do Viruses play a vital role in the food chain of marine ecosystems?

In marine ecosystems viruses are known to cause phytoplankton cells to lyse which releases the cell contents into the water as dissolved organic matter (DOM), this in turn provides nutrients for heterotrophic bacteria which may then be consumed by dinoflagellates, linking in to the traditional ocean food chain. Some nutrients from lysed cells may also become mineralised, therefore providing nutrients to phytoplankton of the same species that are resistant to the virus and other species of phytoplankton. This study looks at marine microorganism population dynamics in more detail and puts the role of viruses in nutrient cycling into practice. They performed two experiments within their study. In the first one they studied two phytoplankton species, Phaeocystis pouchetii and Rhodomonas salina, and the effects that a virus specific to P. pouchetii (PpV) had on both populations. In the second the populations of bacteria and heterotrophic nanoflagellates (HNFs) were also recorded, along with the nutrient levels.

In the first experiment it was found that the populations of both species of algae didn’t start growing until 3 or 4 days into the experiment, at which point the populations started increasing exponentially until it levelled off due to nutrient availability or viral infection. The results showed that the population of P. pouchetii was negatively impacted even when low concentrations of the virus were present, with the population decreasing to zero after just 13 days. The population of R. salina on the other hand was effected much less drastically by the virus and rather than killing off the species completely it levelled off the population more quickly, having a lower population at the end of the experiment than had the viruses not been there. On top of this the levels of R. salina were in fact higher than when the algae had to compete with P. pouchetii.

The results of experiment two showed that the bacterial population increased considerably after lysis of the P. pouchetii took place. This increase was higher when P. pouchetii was the only algal species (culture 2) than when R. salina was present too (culture 3). After peaking the bacterial populations fell and then stabilised with the bacteria in culture 2 stabilising at a higher population than the bacteria in culture 3. The HNF population initially rose more quickly in culture 3 than in culture 2 and as the HNF numbers rose the bacterial levels fell as there were larger numbers of HNFs to consume the bacteria. The populations of HNFs in culture 3 fell and stabilised at a lower population than that in culture 2 but the reason for this was that there was initially a higher concentration of dinoflagellates in the R. salina than the P. pouchetii. The measurements of nutrient levels showed why the increase in the bacteria population took place as levels of nutrients generally rose due to being released during lysis of the P. pouchetii. As there was a higher concentration of P. pouchetii to lyse in culture 2 the nutrient levels rose higher than in culture 3, explaining the differences in bacteria numbers between the 2 cultures.

This experiment has shown that viruses are able to release nutrients into the water, by lysing algal cells, to allow bacteria to grow more quickly as well as allowing other species of resistant algae to grow without competition, showing that the presence of viruses can allow two species that occupy a similar niche to coexist in some cases.

Reference: Haaber, J and Middelboe, M (2009), Viral lysis of Phaeocystis pouchetii:Implications for algal population dynamicsand heterotrophic C, N and P cycling. International Society for Microbial Ecology, Vol 3, Issue 4, pg 430-441