Changes in cell size upon iron limitation vary among the four <i>P</i>. <i>antarctica</i> clones.

<p>These graphs plot the cell volume (<b>a</b>) and surface area to volume ratios (<b>b</b>) of solitary cells from the four <i>P</i>. <i>antarctica</i> clones grown under iron limiting and iron replete conditions. The iron limiting conditions, described in the Materials and Methods, were chosen so that each clone had approximately the same degree of iron limitation for these measurements, i.e. half of the iron replete growth rate. The error bars represent the standard error of the mean of three measurements from each of three biological replicates (n = 3). The data are included in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179751#pone.0179751.s001" target="_blank">S1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179751#pone.0179751.s002" target="_blank">S2</a> Tables.</p

Cultured <i>P</i>. <i>antarctica</i> clones respond differently to iron limitation.

<p>Cultured <i>P</i>. <i>antarctica</i> clones respond differently to iron limitation.</p

Absolute growth rates of <i>P</i>. <i>antarctica</i> clones under various low iron conditions.

<p>These graphs show the absolute growth rates (day^-1) of the four <i>P</i>. <i>antarctica</i> clones grown in iron replete, high EDTA and high DFB media under low (a) and high (b) light conditions. For clone W51, “ng” (no growth) indicates that the clone did not grow in the high DFB medium. For clone AA1, “nd” (no data) were collected in the high EDTA medium at low light. The error bars represent the standard error of the mean of the growth rate (n is provided for each treatment in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179751#pone.0179751.t001" target="_blank">Table 1</a>). The data from clone AA1 have been published previously (Strzepek <i>et al</i>. 2011 [high light], Strzepek <i>et al</i>. 2012 [low light]).</p

Intraspecific variability in <i>Phaeocystis antarctica's</i> response to iron and light stress

<div><p><i>Phaeocystis antarctica</i> is an abundant phytoplankton species in the Southern Ocean, where growth is frequently limited by iron and light. Being able to grow under low iron conditions is essential to the species’ success, but there have been hints that this ability differs among clones. Here, we compare the growth, cell size and chlorophyll <i>a</i> concentrations of four <i>P</i>. <i>antarctica</i> clones cultured under different iron and light conditions. Iron was provided either as unchelated iron (Fe′) or bound to the bacterial siderophore desferrioxamine B, representing, respectively, the most and least bioavailable forms of iron which phytoplankton encounter in the marine environment. The growth rate data demonstrate that the clones vary in their ability to grow using organically bound iron, and that this ability is not related to their ability to grow at low inorganic iron concentrations. These results are consistent at low and high light. Physiologically, only three of the four clones shrink or decrease the concentration of chlorophyll <i>a</i> in response to iron limitation, and only one clone decreases colony formation. Together, our data show that <i>P</i>. <i>antarctica</i> clones 1) respond to the same degree of iron limitation using different acclimation strategies, and 2) vary in their ability to grow under the same external iron and light conditions. This physiological diversity is surprisingly large for isolates of a single phytoplankton species.</p></div

Growth rates of <i>P</i>. <i>antarctica</i> clones under different combinations of iron, EDTA and DFB.

<p>Growth rates of <i>P</i>. <i>antarctica</i> clones under different combinations of iron, EDTA and DFB.</p

Information about the <i>P</i>. <i>antarctica</i> clones used in this study.

<p>Information about the <i>P</i>. <i>antarctica</i> clones used in this study.</p

Dissolved Fe concentration (nmol L^-1) measured in cultures of a) <i>Eucampia antarctica</i> or b) <i>Proboscia inermis</i> under different conditions (control, +EDC Dust, +FeCl₃) by Flow Injection Analysis.

<p>The initial values were measured in the 8 L canister, prior to subdivision into five 1 L bottles, and these values are shown for time = 0. After this, the mean of measurements in the replicate bottles is shown (n = 3 or n = 5 for final measurement), with error bars denoting ± 1SD (where bigger than the size of the symbol).</p

Photomicrographs of <i>Eucampia antarctica</i> cells, showing higher number and density of chloroplasts in amended treatments.

<p>Panels are labeled control, +dust or +FeCl₃ to denote the incubation conditions.</p

Measured <i>Eucampia antarctica</i> and <i>Proboscia inermis</i> growth parameters (cell number, chlorophyll <i>a</i>, particulate organic carbon [POC] and nutrient uptake) under different conditions (control, + EDC dust, +FeCl₃).

<p>Mean values are shown, calculated from 5 replicate bottles, ±1 SD. Initial chlorophyll <i>a</i> concentration was calculated from a single dilution of initial cell cultures. Nutrient uptake was calculated from the reduction in nutrient concentration in solution from beginning to end of experiment. Statistical difference from the control was tested with Analysis of Variance (ANOVA; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158553#sec002" target="_blank">Methods</a>), and p values and Least Significant Difference (LSD) are shown.</p

Final composition of <i>Eucampia antarctica</i> and <i>Proboscia inermis</i> cells, and calculated nutrient uptake ratios under different conditions (control, + EDC dust, +FeCl₃).

<p>Mean values are shown, calculated from 5 replicate bottles, ±1 SD. Nutrient uptake ratios were calculated from the reduction in nutrient concentration in solution from beginning to end of experiment (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158553#pone.0158553.t001" target="_blank">Table 1</a>). Statistical difference from the control was tested with Analysis of Variance (ANOVA; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158553#sec002" target="_blank">Methods</a>), and p values and Least Significant Difference (LSD) are shown.</p