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" 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 S i (OH) 4 : 1 nitrate (NO3) drawdown ratio in the later stages of the bloom. Microplankton (>20 pm), 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 |amol Fe mol"1 C. The broad range in iron quotas between high-iron ( F e Q m a x ) and low-iron ( F e Q m i n ) growth conditions resulted in oceanic Pseudo-nitzschia spp. having a large F e Q m a x : 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. Carbon (C), nitrogen (N) and silicon (Si) quotas normalized per cell decreased when cells were iron-limited. The resulting cellular S i : N or S i : 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 S i s a : N V O i or S i s a : C V O i (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.