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. / Science, Faculty of / Botany, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/16981 |
| Date | 05 1900 |
| Creators | Marchetti, Adrian |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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