Virioplankton dynamics and virally induced phytoplankton lysis versus microzooplankton grazing southeast of the Kerguelen (Southern Ocean)

Viral dynamics, community structure, and the impact of viruses on phytoplankton mortality in comparison with microzooplankton grazing were determined in the natural iron-fertilized waters southeast of the Kerguelen Islands, Southern Ocean, during the austral summer (January–February 2005). The study area was characterized by a phytoplankton bloom above the Kerguelen Plateau and the high-nutrient low-chlorophyll waters surrounding it. During the Kerguelen Ocean and Plateau compared Study (KEOPS), viral abundance was relatively high (1–19×107 mL−1) as compared to the few other studies in the Southern Ocean, significantly correlating with depth and system productivity. Viral abundance showed a strong positive relationship with the numerically dominant bacterial hosts, which in turn were correlated to phytoplankton biomass. In total, 13 different viral genome sizes were detected, with the lower-sized genomes 34 and 68 kb dominating at all stations. The viral community at the low chlorophyll C-transect grouped apart from the more productive transects A and B. Potential algal viruses were recorded for all stations, but only at very low intensities. Virally induced lysis of the smaller-sized (<10 μm) phytoplankton was a minor loss factor as compared to microzooplankton grazing (up to 6% and 45% of total <30 μm algal standing stock per day, respectively). Grazing was phytoplankton population-specific, but was in all cases able to keep the standing stock of the small-sized phytoplankton low (net growth rates between −0.2 and 0.2 d−1). Microzooplankton regenerated on average 1.1 pM Fe d−1 (present study), which represented approximately 30% of the total regeneration rate and at least 15% of the total biogenic Fe demand as calculated by [Sarthou, G., Vincent, D., Christaki, U., Obernosterer, I., Timmermans, K.R., Brussaard, C.P.D., 2008. The fate of biogenic iron during a phytoplankton bloom induced by natural fertilization: impact of copepod grazing. Deep-Sea Research II].