Virus Dynamics in High-Nutrient, Low-Chlorophyll Marine Surface Waters

Iron (Fe) limitation of primary productivity in high-nutrient, low-chlorophyll (HNLC) regions is relatively well-studied. Iron fertilization experiments as well as bottle incubations have been used to study changes in phytoplankton community biomass and diversity, changes in bacterial growth rates, etc. However, viral activity has been largely ignored in these studies. Viral activity was monitored during an iron budget study (FeCycle) in the HNLC waters of the Southern Ocean southwest of New Zealand as well as during a mesoscacle iron fertilization in the subarctic Pacific (SEEDS II). The goal of these studies was to evaluate the role of viruses in the lysis of bacterial cells and the subsequent regeneration of iron and other key nutrients. Two methods, a transmission electron microscopy (TEM) approach and a dilution assay, were used to measure viral production in each study and comparisons were made as to the appropriateness of each. From these studies, it appears that the viral community indirectly responds to changes in trophic production as observed by changes in virus abundance and production, while burst size and frequency of infection remain constant. These results suggest that there is a decrease in the length of lytic cycle after productivity is stimulated. Virus-induced lysis was found to regenerate up to 70 pM Fe in the Southern Ocean, and nearly 200 pM Fe in the subarctic Pacific. While there is little doubt as to the usefulness of TEM and its importance in determining lytic burst sizes in natural populations, the observations in this study suggest that there are problems associated with inferences concerning community mortality from such observations, especially during periods of trophic change.