Phytoplankton community response to a manipulation of bioavailable iron in HNLC waters of the subtropical Pacific Ocean (original) (raw)
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Bioavailability of Iron Sensed by a Phytoplanktonic Fe-Bioreporter
Environmental Science & Technology, 2006
This study describes a short-term (12 h) evaluation of iron (Fe) bioavailability to an Fe-dependent cyanobacterial bioreporter derived from Synechococcus PCC 7942. Several synthetic ligands with variable conditional stability constants for Fe(III) (K* of 10 19.8 to 10 30.9), in addition to several defined natural Fe-binding ligands and a fulvic acid of aquatic origin (Suwannee River), were used to elucidate the forms of Fe that are discerned by this phytoplanktonic microbe: Fe-HEBD (log conditional stability constant, K*,) 28.1, HEBD) N,N′-di(2-hydroxybenzyl)ethylenediamine-N,N′-diacetic acid monohydrochloride hydrate), Fe-HDFB (K*) 30.9, DFB) desferroxamine B), Fe-ferrichrome (K*) 23.2), Fe-DTPA (K*) 21.1, DTPA) diethylenetrinitrilopentaacetic acid), Fe-(8HQS) 2 (K*) 20.4, 8HQS) 8-hydroxyquinoline-5-sulfonic acid), Fe-CDTA (K*) 19.8, CDTA) trans-1,2-cyclohexylenedinitrilotetraacetic acid), and Fe-EDTA (K*) 19.2). Iron bioavailability sensed by the bioreporter was related to diffusion limitation and activity of high-affinity transporters rather than by siderophore secretion. Iron complexed with a K* < 23.2 contributes to the bioavailable pool; bioavailability could be explained by disjunctive ligand exchange considerations and fully, partially, and nonbioavailable complexes could be distinguished according to their conditional stability constant. The use of Fe-bioreporters provides a relevant measurement of bioavailability to an important group of primary producers in freshwaters (cyanobacteria) and is thus a promising technique for understanding Fe cycling in aquatic systems.
Iron bioavailability to phytoplankton: an empirical approach
The ISME Journal, 2014
Phytoplankton are often limited by iron in aquatic environments. Here we examine Fe bioavailability to phytoplankton by analyzing iron uptake from various Fe substrates by several species of phytoplankton grown under conditions of Fe limitation and comparing the measured uptake rate constants (Fe uptake rate/ substrate concentration). When unchelated iron, Fe 0 , buffered by an excess of the chelating agent EDTA is used as the Fe substrate, the uptake rate constants of all the eukaryotic phytoplankton species are tightly correlated and proportional to their respective surface areas (S.A.). The same is true when FeDFB is the substrate, but the corresponding uptake constants are one thousand times smaller than for Fe 0 . The uptake rate constants for the other substrates we examined fall mostly between the values for Fe 0 and FeDFB for the same S.A. These two model substrates thus empirically define a bioavailability envelope with Fe 0 at the upper and FeDFB at the lower limit of iron bioavailability. This envelope provides a convenient framework to compare the relative bioavailabilities of various Fe substrates to eukaryotic phytoplankton and the Fe uptake abilities of different phytoplankton species. Compared with eukaryotic species, cyanobacteria have similar uptake constants for Fe 0 but lower ones for FeDFB. The unique relationship between the uptake rate constants and the S.A. of phytoplankton species suggests that the uptake rate constant of Fe-limited phytoplankton has reached a universal upper limit and provides insight into the underlying uptake mechanism.
Aquatic Microbial Ecology, 2007
It is now established that iron (Fe) availability controls phytoplankton productivity and community structure in ca. 50% of the Pacific Ocean's surface waters and that heterotrophic bacterioplankton may also be either directly or indirectly Fe-limited. Proxy indicators of Fe-stress are available for the phototrophic community (e.g. ferredoxin/flavodoxin ratios) but are lacking for the heterotrophic bacterioplankton. While current analytical tools provide valuable information with regard to micronutrient chemistry and speciation, they do not provide insight into the relative bioavailability of different Fe sources. We present the results of a field trial in an oceanic system of a tool that allows for the assessment of Fe bioavailability in natural systems: the Fe-responsive bioluminescent heterotrophic bacterial reporter. Fe bioavailability was monitored with this tool at the scale of the Eastern Pacific Basin during the mature phase of the El Niño event of 2002. The results demonstrate significant spatial variance, highlighted by regions of decreased Fe availability at equatorial stations along the transect. Using this tool in combination with radiotracer studies of bacterial growth and community Fe uptake, we provide insight into system Fe chemistry and the status of the heterotrophic bacterial community. Our results indicate that different environments with similar concentrations of total Fe can demonstrate different Fe bioavailabilities. Moreover, the small particulate size fraction (0.2 to 0.8 µm) appears to buffer artificially induced variations in Fe bioavailability, implying that studies of Fe bioavailability need to be extended beyond those in the dissolved (< 0.2 µm) size class.
Inducing phytoplankton iron limitation in iron-replete coastal waters with a strong chelating ligand
Limnology and Oceanography, 1999
Dissolved iron (Fe) concentrations in the California coastal upwelling regime vary over two orders of magnitude (from Ͻ0.05 to Ͼ5 nM), which leads to a wide range in Fe effects on phytoplankton growth. Fe-addition experiments are appropriate to use to assess the biological role of Fe in low-Fe areas, but other methods are needed in Fe-replete regions. We present experiments that use additions of the exogenous siderophore desferrioxamine B (DFOB, obtained from a terrestrial actinomycete fungus) to sequester ambient Fe and to markedly decrease its availability to the biota. DFOB additions resulted in artificial Fe limitation of the phytoplankton community in high-Fe areas of the upwelling region. Results of these ''Fe-removal'' experiments mirror those of Fe-addition experiments in low-Fe, high-nutrient, low-chlorophyll (HNLC) waters. When DFOB is added to Fe-replete waters, changes in nutrient concentrations, biomass, and other biological parameters closely resemble those seen in Fe-limited controls in HNLC areas, while the controls without DFOB behave much like HNLC Fe-addition bottles. DFOB additions in high-Fe waters greatly reduced biological Fe uptake and, consequently, nitrate, silicic acid, and carbonuptake rates as well as particulate production. Diatoms and other phytoplankton bloomed profusely in unamended controls but not in Fe-limited ϩDFOB bottles. Bacterial numbers and zooplankton grazing activity were also severely reduced in DFOB-addition bottles. These experiments demonstrate that artificially lowering Fe availability can induce limitation of autotrophic and heterotrophic plankton and can prevent utilization of the high ambient levels of upwelled nutrients along the California coast. Our results suggest that DFOB-bound Fe is highly unavailable to the plankton community, a result that offers researchers an important tool to use to probe the influence of Fe on biological community development in high-Fe regimes.
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.
A chemical method for estimating the availability of iron to phytoplankton in seawater
Marine Chemistry, 1991
A technique that employs the complexing agent 8-hydroxyquinoline (oxine) to measure a labile portion of total Fe in seawater was used to test the hypothesis that the biological availability of colloidal Fe is a direct function of its chemical lability. Three colloidal ferrihydrites (amorphous FeOOH) were examined for their lability in seawater and for their ability to support growth of three species of neritic phytoplankton (Thalassiosira pseudonana, Isochrysis galbana and Dunaliella tertiolecta). Earlyphase (freshly precipitated) ferrihydrite was ~ 65% oxine-labile and proved to be an excellent source of available Fe, as determined by organism growth. Limited heating of this ferrihydrite, which accelerates low-temperature aging processes similar to those operating in nature, caused sharp decreases in both the algal growth responses and in the oxine-lability of the ferrihydrite. The correspondence between colloid lability and organism growth in culture suggests that Fe availability is related strongly to its chemical lability. It thus appears that the oxine technique provides an operational method for estimating the biological availability of Fe in seawater.