Iron requirements of the pennate diatom Pseudo-nitzschia:: Comparison of oceanic (high-nitrate, low-chlorophyll waters) and coastal species

We quantified and compared physiological parameters and iron requirements of several oceanic Pseudonitzschia spp., newly isolated from the high-nitrate, low-chlorophyll waters of the northeast subarctic Pacific, with coastal Pseudo-nitzschia spp. and the oceanic centric diatom Thalassiosira oceanica at a range of iron concentrations. In iron-replete conditions, the iron (Fe): carbon (C) ratios in the six Pseudo-nitzschia isolates ranged from 157 mu mol Fe mol C-1 to 248 mu mol Fe mol C-1, with no apparent differences between oceanic and coastal isolates. In low iron conditions, all Pseudo-nitzschia spp. exhibited marked reductions in photosynthetic efficiency, whereas the extent of the reductions in specific growth rates varied among species. When iron-limited, the Fe: C ratios decreased significantly in all oceanic Pseudo-nitzschia species, with the lowest ratios ranging from 2.8 mu mol Fe mol C-1 to 3.7 mu mol Fe mol C-1. Combined with faster growth rates, lower Fe: C ratios in oceanic isolates of Pseudo-nitzschia resulted in significantly higher iron-use efficiencies relative to their coastal congeners and T oceanica. The wide range between iron-replete (Fe-Q(high)) and iron-limited (Fe-Q(low)) quotas indicates that oceanic Pseudo-nitzschia spp. have an extensive plasticity in iron contents relative to other diatoms grown at similar iron concentrations reported in the literature; the Fe-Qhigh: Fe-Q(low) ratios for oceanic species were 46 to 67, whereas for coastal Pseudo-nitzschia species they were 16 and 43. We suggest that the ability of oceanic Pseudo-nitzschia species to exhibit an extensive growth response to iron enrichment events may, in part, be a result of their extraordinary capacity to accumulate and potentially store large amounts of intracellular iron when iron concentrations are high, yet substantially reduce their iron requirements to sustain fast growth rates well after external iron concentrations are depleted.