Influence of irradiance and iron on the growth of colonial Phaeocystis antarctica: implications for seasonal bloom dynamics in the Ross Sea, Antarctica

Abstract

Laboratory culture experiments were used to investigate the growth rate of colonial Phaeocystis antarctica as a function ot irradiance and dissolved iron concentration. The expenments were conducted with a P. antarctica strain isolated from the southern Ross Sea, Antarctica, and made use of natural, low-iron (< 0.2 nM dissolved Fe) filtered seawater as a growth medium, thereby avoiding the addition of synthetic organic ligands to regulate dissolved iron concentrations. Under iron-and nutrient-replete conditions, colonial P. antarctica attained an average maximum cell-specific growth rate of 0.37 d―1 at an irradiance of 68 μE m―2 s―1, above which growth rates decreased to 0.27 d―1 at an irradiance of 314 μE m―2 s―1. The dependence of growth rate on ambient dissolved iron concentration was examined in dose―response type bioassay experiments using realistic sub-nanomolar additions of dissolved iron. The experimental results indicate significant changes in the iron requirements for growth of colonial P. antarctica as a function of irradiance, with our estimates of the half-saturation constant for growth with respect to dissolved iron (Kμ) ranging from 0.26 nM at ∼20 μE m―2 s―1, to 0.045 nM at -40 μE m―2 s―1 and to 0.19 nM at -90 μE m―2 s―1. We interpret these variations in Kμ as reflecting an increase in the cellular iron requirements of colonial P. antarctica at sub-optimal and supraoptimal irradiance, such that the cells require higher ambient dissolved iron concentrations to attain maximum growth rates under such irradiance conditions. The experiments also provide evidence of a relationship between iron availability and the relative proportion of colonial versus solitary P. antarctica cells, whereby the colonial form appears to be favored by higher dissolved iron concentrations. Our experimental results suggest that the initiation and termination of colonial P. antarctica blooms in the Ross Sea are determined by the combined effects of irradiance-driven changes in cellular iron requirements and a seasonal decrease in dissolved iron availability.