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


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⁻³18 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 μm), 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 jamol 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 μmol Fe mol⁻¹ C. The broad range in iron quotas between high-iron ( F e – Q[sub max]) and low-iron ( F e – Q[sub min]) growth conditions resulted in oceanic Pseudo-nitzschia spp. having a large F e - Q[sub max] : Fe- Q[sub min] 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. Carbon (C), nitrogen (N) and silicon (Si) quotas normalized per cell decreased when cells were iron-limited. The resulting cellular Si : N or Si : C ratios (normalized per cell) increased significantly in low-iron conditions when compared to high-iron conditions due to the greater reductions in cellular C and N quotas compared to Si. In contrast, the Si[sub sa] : NV[sub vol] or Si[sub sa] : C[sub vol] (normalized to surface area or cell volume) did not significantly change between iron nutritional states due to a corresponding increase in surface areato- volume ratios because cell size was reduced as a result of iron limitation. None of the isolated oceanic Pseudo-nitzschia spp. produced detectable levels of domoic acid (DA) (<0.015 ng ml⁻¹) when they were Si-starved. The environmental conditions conducive to high Pseudo-nitzschia abundance and DA production were also assessed in coastal waters during summer transects in the Juan de Fuca eddy region (125.6°W, 48.5°N) in 2001. In September, the highest Pseudo-nitzschia abundance and particulate DA were measured in the eddy and corresponded to high macronutrient concentrations and NO₃ assimilation rates. These results provide one of the first indepth studies on the changes that occur in the elemental composition of an oceanic pennate diatom as a result of iron limitation, and elucidate the possible physiological conditions required for Pseudo-nitzschia to produce DA in the natural environment.

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