Ecological Importance of Viral Lysis as a Loss Factor of Phytoplankton in the Amundsen Sea

Abstract

Whether phytoplankton mortality is caused by grazing or viral lysis has important implications for phytoplankton dynamics and biogeochemical cycling. The ecological relevance of viral lysis for Antarctic phytoplankton is still under-studied. The Amundsen Sea is highly productive in spring and summer, especially in the Amundsen Sea Polynya (ASP), and very sensitive to global warming-induced ice-melt. This study reports on the importance of the viral lysis, compared to grazing, of pico- and nanophytoplankton, using the modified dilution method (based on apparent growth rates) in combination with flow cytometry and size fractionation. Considerable viral lysis was shown for all phytoplankton populations, independent of sampling location and cell size. In contrast, the average grazing rate was 116% higher for the larger nanophytoplankton, and grazing was also higher in the ASP (0.45 d^-1 vs. 0.30 d^-1 outside). Despite average specific viral lysis rates being lower than grazing rates (0.17 d^-1 vs. 0.29 d^-1), the average amount of phytoplankton carbon lost was similar (0.6 µg C L^-1 d^-1 each). The viral lysis of the larger-sized phytoplankton populations (including diatoms) and the high lysis rates of the abundant P. antarctica contributed substantially to the carbon lost. Our results demonstrate that viral lysis is a principal loss factor to consider for Southern Ocean phytoplankton communities and ecosystem production.

Keywords: Amundsen Sea Polynya; Antarctic phytoplankton; Southern Ocean; carbon flux; microzooplankton grazing; viral lysis.

Figure 1

Location of the Amundsen Sea in the Southern Ocean (A) and a closeup of stations sampled within the Amundsen Sea (B). GIS = West Getz ice shelf region, DIS = Dotson ice shelf region. The black line shows the rough outline of the Amundsen Sea Polynya (ASP) during the period of sampling. Stations within the ASP are in black and stations outside the ASP are in white. Based on ice cover, density, and autofluorescence depth profiles, intermediate station 49 was treated as a non-ASP station (see Section 2).

Figure 2

Chlorophyll a concentrations and taxonomic composition of the phytoplankton. (A) total and <20 µm Chl a concentrations. (B) Phytoplankton community composition for <20 µm size fraction. Stations 31–45 lie within the Amundsen Sea Polynya (ASP) and stations 49–57 outside of it.

Figure 3

Phytoplankton populations, clustered in picophytoplankton (<3 um average cell diameter; Phyto 1 and 2), nano-sized phytoplankton populations 3-10 um (Phyto 3–13), and >10 um (Phyto 14–17). Stations 31–45 lie within the Amundsen Sea Polynya (ASP) and stations 49–57 outside of it.

Figure 4

Viral lysis (V) and microzooplankton grazing (G) of phytoplankton for different size ranges. (A) Phytoplankton < 7 µm (n = 26). (B) Phytoplankton ≥ 7 µm (n = 20). The black line shows the median, the boxes show the interquartile range (IQR), and the whiskers show minimum and maximum values without outliers. Outliers are defined as outside 1.5 × IQR. The colored dots show the data points.

Figure 5

Viral lysis (A) and microzooplankton grazing (B) rates from stations outside and within the Amundsen Sea Polynya (ASP). The black line shows the median, the boxes show the interquartile range (IQR), and the whiskers show minimum and maximum values without outliers. Outliers are defined as outside 1.5 × IQR. The colored dots show the direct data points. n-numbers were 30 and 16 for outside and within the ASP, respectively.

Figure 6

Mortality rates for all phytoplankton populations and all stations. (A) Mean specific viral lysis (V) and microzooplankton grazing (G) rates. (B) Mean viral lysis and grazing mediated phytoplankton carbon losses. The error bar shows the standard deviation (only positive SD is shown). The n-number for both specific viral lysis and grazing rates as well as carbon loss was 46.