The impact of iron limitation on the physiology of the Antarctic diatom Chaetoceros simplex.

Katherina Petrou,Scarlett Trimborn,Björn Rost,Peter J Ralph,Christel S Hassler

doi:10.1007/s00227-014-2392-z

Abstract

Iron availability strongly governs the growth of Southern Ocean phytoplankton. To investigate how iron limitation affects photosynthesis as well as the uptake of carbon and iron in the Antarctic diatom Chaetoceros simplex, a combination of chlorophyll a fluorescence measurements and radiotracer incubations in the presence and absence of chemical inhibitors was conducted. Iron limitation in C. simplex led to a decline in growth rates, photochemical efficiency and structural changes in photosystem II (PSII), including a reorganisation of photosynthetic units in PSII and an increase in size of the functional absorption cross section of PSII. Iron-limited cells further exhibited a reduced plastoquinone pool and decreased photosynthetic electron transport rate, while non-photochemical quenching and relative xanthophyll pigment content were strongly increased, suggesting a photoprotective response. Additionally, iron limitation resulted in a strong decline in carbon fixation and thus the particulate organic carbon quotas. Inhibitor studies demonstrated that, independent of the iron supply, carbon fixation was dependent on internal, but not on extracellular carbonic anhydrase activity. Orthovanadate more strongly inhibited iron uptake in iron-limited cells, indicating that P-type ATPase transporters are involved in iron uptake. The stronger reduction in iron uptake by ascorbate in iron-limited cells suggests that the re-oxidation of iron is required before it can be taken up and further supports the presence of a high-affinity iron transport pathway. The measured changes to photosystem architecture and shifts in carbon and iron uptake strategies in C. simplex as a result of iron limitation provide evidence for a complex interaction of these processes to balance the iron requirements for photosynthesis and carbon demand for sustained growth in iron-limited waters.

Highlights

The Southern Ocean is the largest CO2 sink in the global ocean and plays a key role in the global climate (Sabine et al 2004)
We investigate the response of growth, photophysiology, carbon and iron uptake for the Antarctic diatom Chaetoceros simplex under iron limitation
Growth rates were significantly lower in the +DFB culture than in the +Fe culture (P > 0.001), with rates of 0.13 ± 0.07 and 0.33 ± 0.12 days−1, respectively (Table 2)

Summary

Introduction

The Southern Ocean is the largest CO2 sink in the global ocean and plays a key role in the global climate (Sabine et al 2004). To these HNLC areas, there are several regions of high primary productivity, reflected by the presence of large phytoplankton blooms These blooms usually occur in naturally iron-enriched regions, such as the sea ice edge (Lannuzel et al 2008), polynyas, continental margins (Lam et al 2006) and ocean upwelling or circulation fronts (e.g., Polar Frontal Zone; de Baar et al 1997). As these phytoplankton blooms develop, the overall consumption of iron increases, often to a level greater than the input of iron, causing iron limitation to occur, even in principally iron-enriched regions (Garibotti et al 2005)

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Discussion

Conclusion