Iron enrichment and photoreduction of iron under UV and PAR in the presence of hydroxycarboxylic acid: implications for phytoplankton growth in the Southern Ocean (original) (raw)
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Phytoplankton response to natural and experimental iron addition
Deep Sea Research Part II: Topical Studies in Oceanography, 1998
Primary production, chlorophyll biomass, and the maximum quantum yield of photosynthesis were measured during a survey around the Galapagos Islands and during an open-ocean iron addition experiment. Dissolved iron concentrations exceeded 1 nmol kg\ over the Galapagos shelf, and a plume of chlorophyll was observed to the west of the islands. The quantum yield of photosynthesis was greater than 0.02 mol C mol photons\ at the surface in the base of the plume, and less than 0.01 mol C mol photons\ outside the plume. Primary production was two-fold higher at the base of the plume relative to surrounding waters. The distributions of quantum yield, primary production and chlorophyll are consistent with a direct effect of iron on phytoplankton growth rate. The experimental iron addition was performed 300 km south of the Galapagos and raised surface water dissolved iron concentrations from 0.05 to approximately 3 nmol kg\. Quantum yield increased from less than 0.008 to 0.015 mol C mol photons\ within 24 h of the addition, and remained elevated throughout the experiment. Chlorophyll concentrations were initially near 0.2 mg m\, and reached 0.8 mg m\ in the iron-enriched waters. Primary production increased from 2.0 to 3.1 mmol C m\ d\. These results show that iron directly regulates photosynthesis of the dominant phytoplankton in surface waters of the high-nutrient, low-chlorophyll eastern equatorial Pacific. It appears, however, that the dominant phytoplankton were unable to escape grazing control, limiting the impact of a transient increase in iron availability on surface water nutrient concentrations.
Deep Sea Research Part II: Topical Studies in Oceanography, 2000
Eight shipboard iron-enrichment experiments were carried out during the late summers of 1997 and 1998 in the Ross Sea and the Polar Front, respectively, as part of the US JGOFS Southern Ocean program. Using active #uorescence techniques (pump-during-probe #ow cytometry/micro#uorometry and fast repetition rate #uorometry) and #ow cytometry, we examined responses of phytoplankton to iron enrichment over time scales of days. Results of both individual cell and bulk water measurements suggest that physiological iron limitation was widespread in the Ross Sea gyre in the late summer, but that in the region just south of the Polar Front other factors were limiting phytoplankton growth. In the "ve experiments in which responses to enrichment occurred, all the phytoplankton groups we examined, with the exception of cryptophytes, responded to iron enrichment by increasing normalized variable #uorescence (F /F) over several days. Normalized variable #uorescence of cryptophyte cells was typically higher than that of other cells and often near the maximum observed. Signi"cant correlations were observed between ambient iron concentrations and normalized variable #uorescence at the beginning of each experiment, and also between ambient iron and the response of normalized variable #uorescence to enrichment. These relationships, which have not been previously documented, support the use of ambient active #uorescence measurements to predict iron-limiting conditions without conducting incubations.
Photosynthesis Research, 1994
Iron supply has been suggested to influence phytoplankton biomass, growth rate and species composition, as well as primary productivity in both high and low NO 3 surface waters. Recent investigations in the equatorial Pacific suggest that no single factor regulates primary productivity. Rather, an interplay of bottom-up (i.e., ecophysiological) and top-down (i.e., ecological) factors appear to control species composition and growth rates. One goal of biological oceanography is to isolate the effects of single factors from this multiplicity of interactions, and to identify the factors with a disproportionate impact. Unfortunately, our tools, with several notable exceptions, have been largely inadequate to the task. In particular, the standard technique of nutrient addition bioassays cannot be undertaken without introducing artifacts. These so-called 'bottle effects' include reducing turbulence, isolating the enclosed sample from nutrient resupply and grazing, trapping the isolated sample at a fixed position within the water column and thus removing it from vertical movement through a light gradient, and exposing the sample to potentially stimulatory or inhibitory substances on the enclosure walls. The problem faced by all users of enrichment experiments is to separate the effects of controlled nutrient additions from uncontrolled changes in other environmental and ecological factors. To overcome these limitations, oceanographers have sought physiological or molecular indices to diagnose nutrient limitation in natural samples. These indices are often based on reductions in the abundance of photosynthetic and other catalysts, or on changes in the efficiency of these catalysts. Reductions in photosynthetic efficiency often accompany nutrient limitation either because of accumulation of damage, or impairment of the ability to synthesize fully functional macromolecular assemblages. Many catalysts involved in electron transfer and reductive biosyntheses contain iron, and the abundances of most of these catalysts decline under iron-limited conditions. Reductions of ferredoxin or cytochrome f content, nitrate assimilation rates, and dinitrogen fixation rates are amongst the diagnostics that have been used to infer iron limitation in some marine systems. An alternative approach to diagnosing iron-limitation uses molecules whose abundance increases in response to iron-limitation. These include cell surface iron-transport proteins, and the electron transfer protein flavodoxin which replaces the Fe-S protein ferredoxin in many Fe-deficient algae and cyanobacteria.
Iron-light interactions differ in Southern Ocean phytoplankton
Limnology and Oceanography, 2012
In laboratory experiments we examined the interplay of light and iron availability on the intracellular iron concentrations, specific growth rates, and photosynthetic physiology of Southern (S.) Ocean diatoms (Eucampia antarctica and Proboscia inermis) and the haptophyte Phaeocystis antarctica. Intracellular iron concentrations and iron : carbon (Fe : C) molar ratios increased with decreasing irradiance in temperate coastal (Thalassiosira weissflogii) and oceanic (Thalassiosira oceanica) diatoms, in support of the well-established antagonistic iron-light relationship. In contrast, S. Ocean species required lower cellular iron concentrations and Fe : C ratios than temperate diatoms to grow at comparable rates, and their iron requirements decreased or remained relatively constant with decreasing light. These results suggest that the current paradigm that low light increases algal cellular iron requirements (supplied through ''biodilution'') is not applicable to S. Ocean phytoplankton. Although iron use efficiencies decreased at sub-saturating light in all species, these reductions were due primarily to lower growth rates, but not higher intracellular Fe : C ratios, in S. Ocean species. We propose that S. Ocean species have overcome the antagonistic iron-light relationship by increasing the size, rather than the number, of photosynthetic units under low irradiances, resulting in an acclimation strategy that does not increase their cellular iron requirements.
Controlling iron availability to phytoplankton in iron-replete coastal waters
Marine Chemistry, 2004
Recent work demonstrates that the micronutrient iron may strongly influence the magnitude and character of algal production in nearshore waters due in part to the higher but variable iron requirements of neritic phytoplankton. However, ascertaining the direct effects of iron nutrition in coastal waters has been forestalled by our inability to experimentally regulate ambient iron availability independent of other factors. We present here results from size-fractionated iron uptake experiments showing that increasing concentrations of the siderophore Desferriferrioxime B (DFB) progressively decreases the biological availability of iron tracer added to natural seawater. These findings extend those of previous studies showing that high concentrations of DFB induce iron limitation of phytoplankton in coastal waters. Similar tests with two siderophores (P1P and PCC7002 No. 1) isolated from marine prokaryotes showed little or no impact on short-term iron uptake in these natural population cultures. DFB additions did not influence the short-term uptake of carbon indicating that its inhibitory effect was not due to general toxicity to the cells. Uptake rates of iron tracer in the large (>5.0 Am) phytoplankton fraction decreased linearly with increasing DFB concentrations, becoming undetectable at z 3 nM DFB, or f 5 Â over ambient dissolved iron concentrations. The decrease in iron availability with DFB addition was equally dramatic for the ultraplankton (0.2-5.0 Am), but in this case low-level tracer uptake (f 10%) persisted even at high DFB concentrations (3-500 nM). Our experimental findings are combined with a preliminary kinetic model to suggest that iron equilibration among the natural ligand classes (L 1 , L 2) and DFB may require an adjunctive (or associative) ligand substitution mechanism to explain the very rapid effect that DFB exerts on iron uptake when added to seawater. Even so, several hours to days likely are needed for the equilibration of added iron among DFB and natural ligands when low-level (e.g., 0.5 nM) DFB concentrations are employed. Model results provide indirect support to earlier suggestions that large eukaryotic phytoplankton extract iron from the weaker class of natural ligands (Fe(III)L 2). The combination of iron enrichment and DFB amendments provides a practical means for studying how iron influences algal production, carbon cycling and phytoplankton species composition in nearshore waters.
Availability of iron for phytoplankton growth in the north-east Atlantic
Because of recent findings that Fe is a limiting factor for phytoplankton activity even at relatively high dissolved iron (DFe) concentrations, the potential importance of Fe limitation was revisited in the northeast Atlantic Ocean (39-45ЊN, 17-21ЊW). We report data gathered during deck incubation experiments performed at three stations in February-March 2001 with surface seawater containing DFe concentrations of ϳ0.40 nmol L Ϫ1. At all stations, Fe addition enhanced phytoplankton growth. Fe limitation was moderate and occurred simultaneously with limitation by major nutrients. This was clearly demonstrated for diatoms that were colimited by orthosilicic acid. Micro-, nano-, and picoplankton benefited from Fe enrichment. Experiments performed with the trihydroxamate siderophore desferrioxamine mesylate B (DFOB) indicated that Fe reserves exist within the cells, especially within the larger cells. This reserve could result from luxurious storage of Fe by colimited cells during episodic atmospheric deposition of Saharan dust. Simulating concentrations of dust resulting from aerosol deposition in well-stratified surface waters, we determined that the solubility of Saharan dust was very low (Ͻ0.1% w/w) but the amount of DFe released in seawater was sufficient to relieve the Fe limitation of the ambient phytoplankton community.