Results and discussion

Hex:Chl a ratios

Our experimental results show that sub-nanomo-lar additions of dissolved Fe had a significant effect on the pigment ratios of our Ross Sea P. antarctica culture. The ratio of Hex to chlorophyll a (Hex:Chl a ratio) decreased from approximately 1.10 ± 0.20 under low-iron (0.22 nM dissolved Fe) conditions to 0.56 ± 0.02 under high-iron (2.02 nM dissolved Fe) conditions (Fig. 1a). The range of Hex:Chl a ratios observed in this experiement are consistent with the overall mean Hex:Chl a ratio measured in water samples collected in the Ross Sea during all three ROAVVERS expeditions (DiTullio et al. 2003b). In addition, the trend of decreasing Hex:Chl a ratios with increasing dissolved Fe concentration observed in our laboratory experiment is qualitatively consistent with the data trend for the ROAVERRS field samples: during the three ROAVERRS cruises, the highest Hex:Chl a ratio (1.08) was observed during the

Fig. 1 (a) Hex:Chl a and (b) Fuco:Chl a ratios of Ross Sea colonial P. antarctica culture as a function of initial dissolved iron concentration, after incubation for 25 days (duplicate bottles sampled) and 31 days (triplicate bottles sampled) at an irradiance of 20 iE m-2 s-1. The data points show mean values ± one standard deviation

Fig. 1 (a) Hex:Chl a and (b) Fuco:Chl a ratios of Ross Sea colonial P. antarctica culture as a function of initial dissolved iron concentration, after incubation for 25 days (duplicate bottles sampled) and 31 days (triplicate bottles sampled) at an irradiance of 20 iE m-2 s-1. The data points show mean values ± one standard deviation austral summer period (DiTullio et al. 2003b), when dissolved Fe concentrations in the upper water column of the southern Ross Sea polynya are typically low (<0.2 nM; Fitzwater et al. 2000; Sedwick et al. 2000; Coale et al. 2005); whereas the lowest Hex:Chl a ratio (0.62) was observed in the southern Ross Sea during the early spring (DiTullio et al. 2003b), when dissolved Fe concentrations in the upper water column are typically higher than the summer values (Fitzwater et al. 2000; Sedwick et al. 2000; Coale et al. 2005). Although these field samples were collected in regions dominated by P. antarctica (as verified by microscopy; DiTullio et al. 2003b), the likely cooccurrence of diatoms, which will contribute to Chl a but not Hex to the field pigment samples, means that these field ratios represent lower limits for the Hex:Chl a ratio in native P. antarctica.

Fuco:Chl a ratios

The concentrations of Fuco in our Ross Sea P. antarctica isolate remained relatively low for all iron treatments used in our laboratory iron-addition experiment. However, the ratio of Fuco to chlorophyll a (Fuco:Chl a ratio) increased eightfold in response to iron addition, from approximately 0.01 ± 0.02 under iron-limited conditions (0.22 nM dissolved Fe) to 0.08 ± 0.02 under high-iron (2.02 nM dissolved Fe) conditions (Fig. 1b). In comparison, the mean Fuco:Chl a ratio in the P. antarctica-dominated region of the Ross Sea during the ROAVERRS expeditions was somewhat higher than our experimental values, at 0.12 ± 0.05 (DiTullio et al. 2003b). In bulk field samples, however, this pigment ratio is sensitive to the presence of diatoms, since these organisms contain Fuco but not Hex. Whereas Hex in the Ross Sea is diagnostic for the occurrence of P. antarctica, the presence of Fuco is not necessarily highly correlated with the presence of diatoms, since P. antarctica are known to produce Fuco.

Chl c3:Chl a ratios

In our laboratory experiment, iron addition had mediated a decrease in the ratio of chlorophyll c3 to chlorophyll a (Chl c3:Chl a ratio) at day 25 (from -0.27 at low iron to -0.13 at high iron). This pigment ratio, however, had reached a relatively constant value of -0.25 in all iron treatments during the late exponential growth phase (Fig. 2a). In comparison, analyses of the Chl c3:Chl a ratio in P. antarctica-dominated regions in the Ross Sea during the three ROAVERRS cruises showed an average Chl c3:Chl a ratio of 0.24 ± 0.11 (DiTullio et al. 2003b). The data trend for our laboratory experiment at day 25 are qualitatively consistent with the seasonal trend in the ROAVERRS field data: the highest mean Chl c3:Chl a ratio (0.34) was observed during the summer, and the lowest mean Chl c3:Chl a ratio (0.12) was observed during the spring (DiTullio et al. 2003b), when dissolved Fe concentrations

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