Ecophysiological aspects of iron nutrition and domoic acid production in oceanic and coastal diatoms of the genus Pseudo-nitzschia

In vast regions of the world's oceans, phytoplankton, in particular diatoms, are growth-limited by low concentrations of the micronutrient, iron. SERIES (Subarctic Ecosystem Response to Iron Enrichment Study) was performed near Ocean Station Papa (144.45°W, 50.20°N) to determine the phytoplankton response to iron enrichment in high nutrient, low chlorophyll (HNLC) waters of the NE subarctic Pacific. Chlorophyll a (chl a) increased 21 times from 0.3 mg m-3 to a peak of 6.3 mg m-3 18 days after the initial addition of iron. Over the duration of the iron-induced phytoplankton bloom, all macronutrient concentrations were drawn down with silicic acid (Si(OH)4) being depleted to low and possibly diatom growth-limiting concentrations due to a >2 Si(OH) 4: 1 nitrate (NO3) drawdown ratio in the later stages of the bloom. Microplankton (>20 mum), which initially accounted for 25% of the phytoplankton biomass and increased by a factor of 60 after iron addition, consisted primarily of the pennate diatom genera Pseudo-nitzschia, Neodenticula and Thalassiothrix and the centric diatom genera Chaetoceros, Rhizosolenia, and Proboscia . The magnitude, duration and composition of the phytoplankton response to the iron enrichment clearly depicted a major shift in the structure of the algal assemblage to a dominance of large diatoms.; The ecophysiology and toxicity of the pennate diatom Pseudo-nitzschia spp. were also investigated in the laboratory and field. Five Pseudo-nitzschia species, P. cf. fraudulenta, P. granii. P. cf. heimii, P. cf. inflatula and P. turgidula were isolated from the NE subarctic Pacific. Under low iron conditions, Pseudo-nitzschia spp. had slower specific growth rates and reduced photochemical efficiencies, cell volumes and chl a when compared to high iron conditions. Oceanic Pseudo-nitzschia spp. had higher iron use efficiencies compared to coastal variants due to a combination of faster growth rates and lower iron-to-carbon (Fe : C) ratios. In iron-replete conditions, Fe : C ratios ranged from 115--181 mumol Fe mol-1 C, with no apparent differences between oceanic and coastal isolates. Under iron-limiting conditions, the Fe : C ratios dropped substantially in all species and ranged from 1.9--3.8 mumol Fe mol-1 C. The broad range in iron quotas between high-iron (Fe-Qmax) and low-iron (Fe-Q min) growth conditions resulted in oceanic Pseudo-nitzschia spp. having a large Fe-Qmax : Fe-Qmin ratio relative to other oceanic diatoms reported in the literature. Pseudo-nitzschia 's ability to exhibit the greatest physiological response to iron enrichment as well as inhabit environments with a wide distribution of iron concentrations is likely a result of their exceptional capacity to accumulate intracellular iron when iron concentrations are high, yet reduce their iron requirements to a minimum when iron concentrations are low. (Abstract shortened by UMI.)...