A Modeling Approach to Examining the Effect of Viruses on Marine Bacterial Populations in Different Nutrient-limited Environments

Abstract

Viruses are responsible for about half of the bacteria mortality in the ocean, with ten to twenty percent of bacteria lysed every day. The lysis of these bacteria releases nutrients into the water, which can then be reused by other bacteria. Previous models and experiments have shown that viruses could thereby cause an increase in the productivity and abundance of bacteria. However, not all nutrients are equal in this process. Because of a stoichiometric difference between viruses and bacteria, proportionally less phosphorus than nitrogen is released into the water during lysis. If phosphorus rather than nitrogen was limiting, this could result in a lower stimulatory effect of viruses. In this study, I use multitrophic models to compare the effect of viruses in nitrogen- and phosphorus-limited conditions. Viruses have a net stimulatory effect on heterotrophic bacteria abundance in the nitrogen-limited system but no net effect in the phosphorus-limited system. In both systems, viruses cause a decrease in cyanobacteria and zooplankton abundance, and an increase in inorganic and organic nutrients. The increase in inorganic nutrients is significantly larger in the nitrogen-limited system than in the phosphorus-limited system. These results are consistent with the hypothesis that nutrient release during lysis is lower in a phosphorus-limited system than in a nitrogen-limited system. However, viruses have the same positive effect on nutrient recycling in both the nitrogen- and the phosphorus-limited system. The presence of viruses causes an increase in primary productivity and carbon sink while causing a decrease in nutrient export in both systems. Virus abundance is a good predictor for carbon sink, which is otherwise difficult to measure. Including an explicit viral class in global models could help provide better estimates for carbon sink and thus a better understanding of the climate.