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The response of heterotrophic bacteria in a mesoscale iron (Fe) enrichment was measured in the northeast subarctic Pacific Ocean in July 2002. Addition of FeSO4 increased the dissolved Fe concentration in the fertilized patch to 2–3 nmol L−1 and triggered an increase in concentration of Fe(III)-binding ligands that complexed all of the dissolved Fe. Two to three days later, leucine incorporation rate and specific growth rate of bacteria doubled. Physiological markers of bacterial Fe nutritional state varied during the experiment as the microbial community assimilated the added Fe. Cellular uptake rate of an iron-siderophore complex, 55Fe-ferrioxamine B (FB), increased twofold to fourfold over background values and then declined by day 4.5. The fastest rate of Fe-FB uptake on day 2.5 coincided roughly with a transient increase in outer-membrane, 55Fe-FB-binding protein(s) in bacteria and with the peak in ligand concentration. Maximum potential uptake rate of inorganic Fe (Vmax) was 8 zmol Fe bacterium−1 h−1 prior to Fe enrichment and then decreased by a factor of four within 2.5 d of fertilizing the patch as bacteria became Fe sufficient. Vmax gradually increased by day 6.5 as the bacterial community re-entered iron deficiency. Similar changes in growth and Fe uptake kinetics were observed after a second Fe addition. Heterotrophic bacteria in the subarctic Pacific were Fe-deficient and responded directly to Fe addition by up-regulating pathways for Fe-siderophore acquisition and assimilating complexed Fe. The observation that increases in Fe uptake pathways and production were synchronous is consistent with the hypothesis that bacterial growth was directly limited by Fe.

Ice-retreat from the Cape Bathurst Polynya occurred in late May and early June 2004, and was quickly followed by a decrease in dissolved nutrient concentration at the sea sur- face. Concentration plots of NO3 � versus PO4 3� , and Si(OH)4 versus NO3 � showed the ratios of nutrient uptake in the surface layer were in proportions expected for diatom growth (N:P 13.1:1; Si:N 1.8:1) and that water-column NO3 � was depleted before PO4 3� and Si(OH)4. The temporal changes in integrated nitrate, phosphate and silicate concentrations were well described by logis- tic models that showed maximum consumption rates of 11.8, 0.82, and 17.8 mmol m �2 d �1 , respec- tively, and a total seasonal draw-down of 210 ± 19 mmol NO3 � m �2 . If we include estimates of NO3 � supply by advection, nitrification and freshwater dilution, then the amount of NO3 � consumed could be 25�33% higher. Uptake of NO3 � above the 1% isolume was balanced by an equivalent (94%) increase in particulate N over a 15 d period beginning at ice break-up. Thus, the amount of particulate spring new production associated with NO3 � disappearance was estimated to be 16.1 ± 1.5 g C m �2 .

Uptake of 15 N-labelled nitrate, ammonium, and urea was measured over a quasi-annual cycle in the Cape Bathurst Polynya in the Amundsen Gulf and on the Mackenzie Shelf, during the Canadian Arctic Shelf Exchange Study (CASES) in 2003 and 2004. Before the phytoplankton bloom and in autumn, nitrogen uptake was slow, representing less than 5% of annual consumption. Uptake rates increased exponentially after ice retreat and within 3 wk reached a maximum of 38.6 mmol N m ―2 d ―1 . During spring, NO 3 ― uptake supported new production of 166 mmol N m ―2 and f-ratios rose from 0.1―0.2 to 0.6―0.9. Filter fractionation showed that GF/F filters retained 93.1 ± 1.3% of the 15 N incorporated into particulate matter, suggesting that phytoplankton were responsible for the majority of the N uptake. Although free-living bacteria took up relatively more 15 N in autumn and in the lower part of the euphotic zone than phytoplankton, their assimilation of inorganic N had little effect on water column integrated f-ratios or new production. Urea supplied almost half the N assimilated by phytoplankton annually and about 80 % of the regenerated production during the spring bloom. Total new production, estimated from water column integrated 15 N-nitrogen uptake rates and linear models that interpolated rates over unsampled periods, was 342―415 mmol N m ―2 yr ―1 . Total annual N production for the region was 1.24-1.48 mol N m ―2 yr ―1 .

Assessments of carbon and nitrogen (N) assimilation in Canadian Arctic waters confirmed the large contribution of subsurface chlorophyll maxima (SCM) to total water-column production from spring to late fall. Although SCM communities showed acclimation to low irradiance and greater nitrate (NO3−) availability, their productivity was generally constrained by light and temperature. During spring–early summer, most of the primary production at the SCM was sustained by NO3−, with an average f-ratio (i.e., relative contribution of NO3− uptake to total N uptake) of 0.74 ± 0.26. The seasonal decrease in NO3− availability and irradiance, coupled to the build up of ammonium (NH4+), favoured a transition toward a predominantly regenerative system (f-ratio = 0.37 ± 0.20) during late summer and fall. Results emphasize the need to adequately consider SCM when estimating primary production and to revisit ecosystem model parameters in highly stratified Arctic waters.

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) biological cycling rates were determined during SERIES, a mesoscale iron-fertilization experiment conducted in the high-nutrient low-chlorophyll (HNLC) waters of the northeast subarctic Pacific. The iron fertilization resulted in the rapid development of a nanoplankton assemblage that persisted for 11 days before abruptly crashing. The nanoplankton bloom was followed by a diatom bloom, accompanied by an important increase in bacterial abundance and production. These iron-induced alterations of the plankton assemblage coincided with changes in the size and biological cycling of the DMSP and DMS pools. The initial nanoplankton bloom resulted in increases in particulate DMSP (DMSPp; 77–180 nmol L−1), dissolved DMSP (DMSPd; 1–24 nmol L−1), and biological gross (0.11–0.78 nmol L−1 h−1) and net (0.04–0.74 nmol L−1 h−1) DMS production rates. During the nanoplankton bloom, DMSPd consumption by bacteria exceeded their sulfur demand and the excess sulfur was probably released as DMS, consistent with the high gross DMS production rates observed during that period. The crash of the nanoplankton bloom was marked by the rapid decline of DMSPp, DMSPd, and gross DMS production to their initial values. Following the crash of the nanoplankton bloom, bacterial production and estimated sulfur demand reached transient maxima of 9.3 μg C L−1 d−1 and 14.2 nmol S L−1 d−1, respectively. During this period of high bacterial production, bacterial DMSPd consumption was also very high (6 nmol L−1 h−1), but none of the consumed DMSPd was converted into DMS and a net biological DMS consumption was measured. This transient period initiated a rapid decrease in DMS concentrations inside the iron-enriched patch, which persisted during the following diatom bloom due to low biological gross and net DMS production that prevented the replenishment of DMS. Our results show that the impact of Fe fertilization on DMS production in HNLC waters result from a complex interplay between the dynamics of the algal blooms and their influence on bacterial DMSP and DMS metabolism.

We measured the isotopic composition and accumulation of particulate organic matter (POM) and the uptake of carbon (C) and nitrogen (N) in an early bloom of the most productive recurring polynya of the Arctic Ocean. The estimated compensation irradiance at the onset of the bloom was similar to the average for the North Atlantic Ocean, implying that shallow mixing was of critical importance for the bloom’s early initiation. Planktonic POM had a much lower d 13 C than ice POM, suggesting that ice-algae contributed little to the pelagic biomass. The overall isotopic fractionation of pelagic N during bloom development was consistent with in situ diatom growth under saturating irradiance and limiting NO . Soon after the ice cleared, rapid physiological changes induced an order of 2 3 magnitude increase in the C and NO uptake capacity of diatoms, leading to very high f ratios (NO uptake : total 2 2 3 3 N uptake). Most of the NO taken up appeared in the POM, so that little net release of reduced N occurred during 2 3 the period of active growth. Given the tight coupling between photosynthesis and NO uptake under N limitation, 2 3 the magnitude of primary production in the Arctic Ocean is expected to respond to changes in N supply.

Centric diatoms isolated from open ocean environments require higher concentrations of Cu for growth than their coastal counterparts. In artificial seawater medium containing ,1 nmol L 21 Cu, three coastal species maintained near maximum rates of growth, but the oceanic clones were unable to survive. Copper limitation was more severe in the diatoms grown in low- than in high-Fe seawater, suggesting that Cu and Fe were interacting essential resources. The interactive effect was in part the result of a Cu requirement for Fe transport. Thalassiosira weissflogii and Thalassiosira oceanica had lower Fe quotas and slower rates of Fe uptake when [Cu] was reduced in the medium. Brief exposure of Cu-limited cells to 10 nmol L 21 Cu increased the instantaneous Fe uptake rate by 1.5 times in T. oceanica. Steady-state uptake rates of both species at high, growth-saturating concentrations of Fe were also Cu dependent. Oceanic species appeared to have an additional Cu requirement that was independent of Fe acquisition and likely responsible for their higher requirements compared to coastal species. Evidence for the importance of Cu in natural communities of phytoplankton was obtained from an incubation experiment performed in the Fe-limited basin of the Bering Sea. Addition of 2 nmol L 21 Cu doubled the phytoplankton net growth rate compared to the untreated controls and, in the presence of extra Fe, increased the growth rate compared to the samples amended with Fe alone. The results suggest that Cu may be an important micronutrient for phytoplankton growth in low-Fe regions of the sea because of its role in Fe acquisition. Paradoxically, oceanic diatoms may be more susceptible to the effects of low Cu concentrations than coastal species.

We grew Thalassiosira weissflogii to steady state over a range of Fe-limiting conditions with nitrate or ammonium as the N source. Nitrate-dependent cells had faster Fe-uptake rates, contained significantly higher intracellular Fe quotas, and grew faster than cells cultivated with NH4 1 when Fe was most limiting. Under these conditions, carbon (C) : chlorophyll a ratios and the minimum fluorescence yield per chlorophyll a increased, but N source had no effect on either parameter. The ratio of variable to maximum fluorescence (F v F ) declined little with Fe limitation 21 m even when T. weissflogii was grown at 25% of its maximum rate (mmax). C : N ratios were higher in nitrate than in ammonium-grown cells and were constant at all Fe levels. Protein was independent of Fe and N, and amino acids were lowest in cells using NO . The P content of T. weissflogii (mol P L 21 cell volume) increased by 1.5 times as 2 3 Fe became most limiting to growth, causing N : P and C : P ratios to decline significantly. The elemental stoichiometry for Fe-limited new production of T. weissflogii (0.25mmax) was thus 70C : 10N : 5.9Si : 1P : 0.00074Fe (by mols) compared with 97C : 14N : 4.7Si : 1P : 0.029Fe for nutrient-replete conditions. Uptake rate ratios of NO : PO showed the same dependence on Fe as the cellular N : P quotas, decreasing as [Fe] decreased. Iron 2 32 34 limitation influenced the elemental composition of this marine diatom and will alter the assimilation ratios of C, N, and P in the high nitrate, low chlorophyll regions of the sea.

We have discovered that coastal and oceanic diatoms require more manganese (Mn) to grow in iron (Fe) -deficient than in Fe-sufficient seawater. At low inorganic concentrations, like those of the open sea, Fe and Mn can thus colimit Thalassiosira pseudonana and T. oceanica so that maximum rates of cell division are achieved only when both resources are added simultaneously to cultures. Colimited diatoms amended with either Fe or Mn alone show unique physiological responses, which implies that the observed interaction between Fe and Mn is not caused by a substitution of one metal for the other. Iron deficiency increases the Mn quota of T. pseudonana by three times compared with controls and enhances the production of reactive oxygen species by 1.7 times in T. weissflogii. Both diatoms respond to this oxidative stress by increasing the activities of the antioxidant enzyme superoxide dismutase (SOD). The Mn content of the SODs increases by 1.8 to 2.8 times when Fe is limiting, which suggests that the SODs contain Mn and may account for part of the observed increase in the Mn quota. Such an increased biochemical requirement may elevate the Mn content of low Fe diatoms, and possibly other phytoplankton, resulting in high Mn: Fe ratios in surficial particulate matter in Fe-limited regions of the sea.

Pathogenic gram-negative bacteria use specific receptors to transport ferric siderophore complexes across their outer membrane during iron (Fe)-limited growth. Receptors such as these have not yet been characterized in oceanic heterotrophic bacteria. We examined four species of g-proteobacteria for the presence of Fe-siderophore receptors with the use of a nondenaturing polyacrylamide gel electrophoresis binding assay and the siderophore ferrioxamine B (FB) labeled with 55 Fe. Small-subunit rRNA sequence analysis assigned these bacteria to the genera Pseudoalteromonas and Alteromonas. Two oceanic species, Pseudoalteromonas haloplanktis (Neptune) and Alteromonas macleodii (Jul88), which were shown previously to transport and assimilate Fe bound to FB during growth, synthesized an outer-membrane FB receptor under Fe-limiting conditions. Only low concentrations of the receptors were detected in these bacteria when they were grown with high concentrations of Fe. The FB receptor of P. haloplanktis (Neptune) had an apparent molecular mass of 79 kDa and an externally oriented binding site. The molecular mass of the receptor of A. macleodii (Jul88) was 100 kDa. No FB receptors were detected by our methods in two coastal species, Pseudoalteromonas rubra(LMG1) and Pseudoalteromonas piscicida(PWF3). P. haloplanktis (Neptune) and A. macleodii (Jul88) also bound 55 Fe-ferrichrome, a trihydroxamate siderophore like FB. Binding assays conducted with 115 nmol L 2 15 5 Fe-FB in the presence of increasing concentrations of desferrioxamine B showed a progressive decrease in the amount of 55 Fe-FB bound by the receptor protein, suggesting strong affinity of the receptor for the Fe-free siderophore. Our results provide the first demonstration of Fe-siderophore receptors in oceanic heterotrophic bacteria. Heterotrophic bacteria have evolved Fe(III) transport systems that enable them to grow in environments containing extraordinarily low concentrations of Fe. When Fe is scarce, many of these organisms excrete low‐molecular mass Febinding compounds, called siderophores, that bind free Fe(III) in solution. The Fe-siderophore complex attaches to its cognate receptor on the outer membrane of the cell and is subsequently internalized (Braun and Killman 1999; Ratledge and Dover 2000). Some bacterial receptors recognize more than one siderophore, not necessarily of the same structural type (e.g., FhuE of E. coli) (van der Helm 1998). Many bacteria express receptors for siderophores released by other species or for Fe sources contained in their hosts (Braun et 1

New, net, and diatom production in the North Water were estimated during May to July 1998 from in vitro measurements of nitrate uptake and mesoscale temporal changes in the inventories of nitrate, silicate, oxygen, and inorganic carbon (DIC). Sampling stations were divided into two domains according to the position of the dominant water types: the silicate-rich Arctic water (SRAW) and Baffin Bay Water (BBW). BBW dominated in the southeast and was associated with relatively shallow upper mixed layers (UMLs) and weak horizontal advection. A phytoplankton bloom started in late April in BBW and grew slowly over 7 weeks, during which time the build-up of particulate organic nitrogen and carbon accounted for ca. 80% of the nitrate and DIC deficit, respectively. Over half of the new production (1.37 g C m −2 d −1 ) during this period was attributed to wind-driven replenishment of nitrate in the euphotic zone. The bloom culminated when seasonally declining winds and rising temperatures severed the UML from the deep nutrient reservoir. The same change in weather induced ice melt, stratification, and bloom development in northern SRAW, which had previously been characterized by deep UMLs. Collectively, the results imply that the timing and magnitude of blooms in the North Water are controlled by a succession of oceanic and climatic forcings. New C production in the North Water during April to July (1.11 g C m −2 d −1 ) was an order of magnitude higher than in adjacent waters and up to 8 times higher than in the Northeast Water polynya. As much as 80% of this production was mediated by diatoms >5 μm, suggesting potentially high and efficient C transfer to the herbivorous food web and deep waters.

Thalassiosira spp., large-chain forming centric diatoms, typically dominate the biomass during phytoplankton blooms in the North Water Polynya (76 to 79°N, centred on ca. longitude 75°W), which is the largest recurring polynya in the Canadian Arctic. We used an experimental method based on semi-continuous cultures to investigate mechanisms responsible for bloom mainte- nance and associated changes in microbial food web constituents. We compared 2 treatments: (1) a new nutrient system in which the cultures were partially enriched every 2 d with nutrient-rich sea- water from depth to simulate horizontal or vertical advection, and (2) a recycled nutrient system in which large particles (> 2.0 µm) were partially removed every 2 d to simulate grazing and sinking losses without nutrient replacement. The experiment lasted 8 d. In the new nutrient treatment, large diatoms, particularly Thalassiosira spp. and to a lesser extent Chaetoceros spp., consumed the added nutrients and continued to dominate production and biomass of the protist community. The total eukaryotic community production in the 'recycled' community shifted to one dominated by dino- flagellates and ciliates in the absence of diatom growth. These 2 end points corresponded to 2 types of communities observed in the North Water Polynya in June 1998. Net production rates for viruses and bacteria were not significantly different between treatments. These results demonstrate the importance of advective processes in maintaining a prolonged diatom bloom. An underlying micro- bial food web dominated by large (> 20 µm) ciliates and dinoflagellates was able to maintain similar rates of net production respective of new versus recycled nutrient supply. Dominance of the protist communities by large cells under both conditions is likely to favour the sustained high productivity of zooplankton and megafauna that characterize the North Water Polynya.

Three species of photosynthetic flagellates capable of phagotrophy (mixotrophic species) were tested for their abilities to use inorganic iron colloids for growth. Ochromonas sp., Chrysochromulina ericina (a coastal strain), and C. ericina (an oceanic strain) were grown in iron-free seawater supplemented with 1 mM Fe-ferrihydrite (amorphous hydrous ferric oxide), magnetite (Fe3O4)/maghemite (g-Fe2O3), hematite (a-Fe2O3), or goethite (a-FeOOH). Desferrioxamine B, an iron-binding siderophore, was used to reduce the concentration of dissolved iron in the colloid-amended media, and none of the flagellates were able to use its iron complex as an iron source under the conditions of the experiments. Both strains of Chrysochromulina grew at 35%‐70% of their maximum rates with goethite, hematite, and magnetite/maghemite but were unable to use ferrihydrite. Ochromonas grew well with ferrihydrite but could not use any of the other forms. Thalassiosira oceanica (clone 1003) and Thalassiosira pseudonana (clone 3H), diatoms that could only take up dissolved forms of iron, were unable to use any of the colloids tested. The mechanism of iron acquisition by the flagellates appeared to involve ingestion of the iron colloids, because bacteria resident in the cultures were too iron poor to be a significant source of iron and were unable to use the iron contained in the colloids themselves. Variations in the sizes of the colloids were hypothesized to account for differences in their availability, independent of colloid chemical stability. The results provide the first strong evidence for direct use (i.e., without prior dissolution) of colloidal iron by mixotrophic phytoplankton and document a new pathway of iron acquisition that may be important for their survival in low-iron waters of the sea.

Under Fe-limiting conditions, nitrate (NO3 2 )-grown marine diatoms have higher intracellular Fe requirements, but divide as fast or faster than ammonium (NH4 1 )-grown cells by maintaining faster steady-state Fe uptake rates. Here we report that Thalassiosira oceanica, clone 1003, possesses an Fe reductase that reduces Fe(III) bound to a variety of organic ligands, including the siderophore desferrioxamine B (DFB), a high affinity, Fe(III)-specific ligand. Reduction is mediated extracellularly and is induced by Fe deficiency. Cellular rates of Fe(III) reduction are significantly faster in NO 3 2 - than in NH4 1 -grown cultures suggesting a link with N metabolism. At subsaturating Fe concentrations, short- and long-term Fe uptake rates are also significantly faster in NO3 2 - than in NH4 1 -grown cells. The results suggest that when Fe is limiting, faster rates of reduction of organically bound Fe(III) by phytoplankton promote faster rates of Fe transport and growth. The implications of these findings could be significant for understanding phytoplankton Fe nutrition in oceanic waters where organic complexation dominates the speciation of Fe. We hypothesize that the reductive Fe transport pathway may enable phytoplankton to directly utilize Fe bound to strong organic ligands in the sea. Iron plays a catalytic role in many biochemical reactions as a cofactor of enzymes and proteins involved in chlorophyll synthesis, detoxification of reactive oxygen species, respiratory and photosynthetic electron transport, and N assimilation. Changes in the activity of these reactions or their replacement by functionally equivalent Fe-deficient pathways can greatly influence cellular Fe requirements of organisms. Because the NO3 2 assimilatory pathway is highly Fe dependent, utilization of NO3 2 by marine centric diatoms (Thalassiosira spp.) imparts a higher metabolic demand for Fe than the use of NH4 1 . The demand for Fe is such that the Fe quotas (Fe : C) of NO3 2 -grown cells are 1.8 times higher than those of phytoplankton using NH4 1 (Maldonado and Price 1996). Despite the higher Fe requirement for NO3 2 assimilation, under moderately Fe-limiting conditions (ca. 0.7‐0.85 m/mmax), growth rates of NO3 2 -amended cultures are not slower than those of NH4 1 -amended ones (Maldonado and Price 1996). Nitrate-grown cells apparently compensate for their extra Fe requirement by sustaining faster steadystate Fe uptake rates (Maldonado and Price 1996). The

Experiments were conducted along a coastal-oceanic transect in the NE subarctic Pacific to examine acquisition of organically complexed Fe by autotrophic and heterotrophic plankton. During short-term experiments, plankton took up Fe bound to the siderophores desferrioxamine B and E, microbial Fe chelates with a high affinity for Fe. Uptake occurred in all size fractions: 0.2–1, 1–3, and >3 μm. Heterotrophic bacteria had higher Fe : C ratios (1.5 to 2 times) than phytoplankton, and accounted for 70±8% of the total Fe uptake by the community (mol Fe ml −1 h −1 ). This latter result was partially explained by the higher C biomass of bacteria, but was not related to their productivity. Carbon-specific uptake rates of Fe were also faster (1.6±1.5 times) in bacteria than phytoplankton. When the rates were normalized per cell surface area, however, phytoplankton were observed to transport Fe at a rate more than 30 times that of bacteria. Large phytoplankton greater than 3 μm reduced Fe bound to organic ligands extracellularly. Their Fe : C ratios and rates of uptake and reduction of organically bound Fe were very similar at all stations along the transect and were characteristic of Fe-stressed phytoplankton. A strong seasonal trend of Fe uptake and reduction was apparent. The results suggest that heterotrophic bacteria are responsible for a large fraction of dissolved Fe uptake and that the indigenous plankton of the subarctic Pacific are able to acquire Fe bound to strong organic ligands, the predominant form of dissolved Fe in the sea.

Phytoplankton acclimate to low irradiance by increasing their cellular demand for Fe, to allow synthesis of additional light-harvesting pigments and Fe-containing redox proteins involved in photosynthesis. In the open NE subarctic Pacific, Fe concentrations limit primary productivity and irradiances may be suboptimal, particularly during winter. Phytoplankton thus may be unable to fulfill their extra Fe requirements for growth under these low-light conditions and become effectively co-limited. We tested this hypothesis by manipulating Fe and light in in vitro experiments at OSP (Ocean Station PAPA, 50°N 145°W) during winter 1997. The results show that metabolic rates, growth, and photosynthetic parameters of phytoplankton are enhanced in winter by increasing either irradiance or Fe. The greatest response occurs when Fe and light are amended concomitantly, confirming that the community is indeed co-limited by both resources. Analysis of environmental conditions (i.e. incident irradiance, mixed layer depth and Fe concentrations) in winter at OSP reveals that they are similar to those observed in the austral spring and fall at three sites in the Southern Ocean. Extrapolating our experimental field results to the Southern Ocean illustrates that co-limitation by light and Fe also may play an important role in regulating phytoplankton growth in this region.

Recent studies demonstrate that dissolved iron in seawater is bound to strong organic complexes that have stability constants comparable to those of microbial iron chelates. We examined iron acquisition by seven strains of heterotrophic marine bacteria from a number of siderophore‐iron complexes, including desferrioxamine B (DFB) and marine siderophores partially purified from iron-limited cultures. Hydroxamate siderophores were detected in the supernatants of four strains, one of which also produced a catechol. All strains transported iron bound to siderophores regardless of whether or not they produced their own, and the majority took up iron bound to DFB. Uptake rates of Fe siderophores were similar among iron-limited strains and among ligands. Transport of FeDFB by strain Neptune was enhanced 20 times by iron limitation, whereas uptake of unchelated iron (Fe9) did not saturate at the highest concentration tested and was not regulated by the iron nutritional status of the cells. The half-saturation constant for uptake of FeDFB by Neptune was 15 nM, the lowest reported for an Fe siderophore in any microorganism. Iron uptake by the catechol-producing strain, LMG1, differed markedly in two respects from the other strains: LMG1 could not take up iron bound to DFB; furthermore, transport of Fe9 by iron-limited LMG1 was 10 times faster than the other strains and was upregulated 46 times compared to Fe-sufficient cells. Experimental evidence suggests that iron transport by LMG1 may be mediated by surface-associated catechol siderophores that scavenge inorganic ferric species as well as iron bound to weaker complexes, such as EDTA (ethylenediaminetetraacetic acid). The combined results of the study highlight the importance of siderophores in iron transport by heterotrophic marine bacteria and suggest, by inference, that bacteria may rely on siderophores to acquire iron in situ.

Phytochelatin, an intracellular metal-binding polypeptide synthesized in eucaryotic algae in response to metals such as Cd and Cu, was measured in particulate samples collected from the equatorial Pacific. The concentrations in these samples (normalized to total particulate chl a ) were unexpectedly high compared to laboratory culture data and were on average slightly more than in coastal areas where the metal concentrations are typically much greater. In part, the high field concentrations can be explained by the low cellular concentrations of chlorophyll a resulting from very low ambient Fe, but laboratory experiments provide a possible explanation for the rest of the difference. At low concentrations of inorganic Cd (Cd′=3 pM), increasing amounts of phytochelatin were induced by decreasing Zn concentrations in the culture medium of two diatoms: Thalassiosira weissflogii , a coastal species, and T. parthenaia , an isolate from the equatorial Pacific. In all previous studies, phytochelatin production has been directly correlated with increasing metal concentrations. Decreasing Co also resulted in higher phytochelatin concentrations in T. weissflogii and Emiliania huxleyi . Replicating the field concentrations of Zn, Co, and Cd in the laboratory results in cellular concentrations (amol -1 cell) that are very similar to those estimated for the field. Contrary to the expectation that high metal concentrations in the equatorial upwelling would cause elevated phytochelatin concentrations, there was no increase in phytochelatin concentrations from 20° S to 10° N—near surface samples were roughly the same at all stations. Also, most of the depth profiles had a distinct subsurface maximum. Neither of these features is readily explained by the available Zn and Cd data. Incubations with additions of Cd and Cu performed on water sampled at four separate stations induced significantly higher concentrations of phytochelatins than those in controls in a majority of the samples. The differences between metal additions and controls were greater within the upwelling zone, where water presumably has had less time to accumulate biogenic complexing agents. However, the uniformity of phytochelatin concentrations in and out of the upwelling region suggests that the phytoplankton or the biota are successfully maintaining a tight control over the trace metal chemistry both intra- and extracellularly across a wide range of oceanic conditions.

This work was funded by the Natural Sciences and Engineering Research Council of Canada and the McGill University Faculty of Graduate Studies and Research. Z.C. was supported by a fellowship from the Fonds pour la Formation de Chercheurs et l’Aide a la Recherche (Quebec).

In coastal and shelf waters, substantial external inputs of iron come from riverine sources and bottom sediments, leading to markedly higher dissolved and particulate iron concentrations. Much of the particulate iron in nearshore waters is inorganic and processes that solubilize this reservoir, making it accessible to phytoplankton, are especially relevant (see below). The greater iron inputs to coastal and shelf regions compared to the open ocean are accompanied by high iron requirements of neritic phytoplankton species (Brand et al., 1983; Sunda et al., 1991). These systems therefore might be strongly influenced as the comparatively rich iron resource is diminished by phytoplankton blooms. The single largest reservoir of iron in the surface waters of HNLC regions may be the biota itself. New evidence indicates that this biological pool of iron is recycled on the time scale of days, much like N and P (Hutchins et al., 1993). This “input” of regenerated iron to surface waters is estimated to be more than an order of magnitude greater than the external supply rate of iron (Bruland, this meeting; Morel, this meeting) and may largely satisfy the iron-demand of phytoplankton in these systems. This view is supported by recent results of Price et al. (1994) showing that iron uptake rates of plankton in the equatorial Pacific are sufficient to entirely turn over the dissolved iron pool within half a day or less. Presently, there is no indication of the chemical forms of this regenerated iron or whether these forms are directly reassimilated by phytoplankton. B. What are the sinks of iron? The removal of iron from surface waters is fairly well constrained within a geochemical (i.e. mean residence time) perspective, however, the mechanisms and dynamics of this removal is not well understood. Mechanisms for removing iron from surface waters include: * sorption and precipitation, 0 biological assimilation, aggregation of inorganic or organic colloids, and . sinking of mineral and biogenic particles. While much of the particulate iron introduced via rivers, sediment resuspension, or as mineral aerosols will be removed by settling, ascertaining the underlying basis for the removal of “dissolved” iron forms is much more difficult. In regions with a high sinking flux of inorganic mineral particles (e.g. in some coastal and well mixed shelf waters), dissolved iron may be removed abiotically by sorption to surfaces of these particles. Similarly, sinking organic particles also can scavenge soluble iron from surface waters (Morel and Hudson, 1985). Dissolved iron also is “removed” via direct assimilation by phytoplankton. The subsequent sinking of live cells or fecal matter will transport a portion of this biogenic iron from surface waters. In addition to direct assimilation and sorption onto sinking (mineral and biogenic) particles, iron may sorb to colloidal organic matter which is abundant in surface waters (Wells and Goldberg, 1992, 1994). The stability of this colloidal phase is a topic of much dispute (Honeyman and Santschi, 1989; Bauer et al., 1992; Moran and Buesseler, 1992; Wells and Goldberg, 1993), but the extremely large colloidal surface area combined with the particle reactive nature of iron suggests that aggregation of organic colloids could be important for removing iron. Significant unresolved issues regarding iron removal include (1) identifying the specific mechanisms of iron sorption to abiotic and biotic sinking particles, which will shed light on how changes in iron speciation may affect this removal pathway, (2) changes in the iron “export efficiency” of the assimilation pathway with shifts in primary productivity or species assemblage, and (3) the abundance and reactivity of iron in the colloidal reservoir. The relative importance of abiotic, bioM.L. Wells et al.iMarine Chemistry 48 (199.5) 157-182 163 tic and colloid aggregation removal processes will vary from regime to regime and with season. (2) What are the chemical speciation and forms of iron among the soluble, colloidal and particulate fractions, including the rates and mechanisms of transformations among these forms? There are some large gaps in our knowledge of iron chemistry in seawater. The development of trace metal clean techniques for seawater collection and analysis over the past decade (Bruland et al., 1979; Gordon et al., 1982; Landing and Bruland, 1987) has given us reliable profiles for iron in the traditional categories of particulate (> 0.4 pm) and “dissolved” (< 0.4 pm) fractions; however, the chemical forms of iron within these fractions has been largely speculative. For example, there now is evidence that iron exists, at least partially, in the small colloidal phase (Wells and Goldberg, 1991; Wu and Luther, 1994; Powell and Landing, abstract) which is included in operationally defined “dissolved” fractions. Determining how iron is partitioned among these phases, and among various chemical forms within each phase, is central to understanding iron speciation in seawater.

Iron enrichments in bottle experiments in high nutrient, low chlorophyll (HNLC) surface waters typically stimulate the net growth of nanophytoplankton (2.0–20 µm) but not picophytoplankton (

Experiments were conducted in the equatorial Pacific Ocean to assess the role of Fe and grazing in regulating use of N03- by the phytoplankton community. Nitrate uptake rates in situ were slow because NH,+ concentrations were inhibitory and because phytoplankton biomass was kept low by grazing. When feeding of grazers was artificially suppressed, phytoplankton net growth rate increased, biomass accumulated, and NO, - was consumed. Rapid rates of Fe uptake [40 pmol Fe (g Chl a)-’ h-l] decreased by an order of magnitude in l-2 d after Fe was added, demonstrating that these rates were under physiological regulation and were elevated in response to low Fe concentrations. Addition of Fe increased carbon uptake and the short-term N-specific NO,- uptake rate by 2-9 times. These physiological stimulations were confined to large phytoplankton (> 3 pm), which thus must have been Fe-limited in situ. N03- uptake rate and biomass of small phytoplankton were unaffected by Fe enrichment. The results thus suggest that the low biomass, N03- -rich condition of the equatorial Pacific Ocean exists because low Fe concentrations limit use of N03- by large phytoplankton and favor growth of small phytoplankton that are grazed efficiently and use NH,+ preferentially.

Some trace metals such as Fe, Ni, Cu, and Zn are essential for the growth OF phytoplankton. The concentrations of these essential trace elements in seawater are so low as to limit their availability to aquatic microbiota. Trace element uptake is ultimately limited by kinetics of reaction with transport ligands or by diffusion to the cell. From what we know of the characteristics of the uptake systems of phytoplankton and their trace metal requirements we can estimate that Fe and Zn may at some times in some places limit phytoplankton productivity, which is in accord with available field data on trace metal enrichmeuts: Although the founding fathers of the field discussed many elements, including Fe, Mn, Zn, and Cu, as potentially limiting algal growth in seawater (Harvey 1945), for the past several decades biological oceanographers have focused almost exclusively on N and, to a lesser extent, on P and Si. Over that period we have learned that surface seawater concentrations of biologically interesting trace elements are much lower than previously thought (Bruland 1983). The pervasive contamination that distorted early measurements of trace elements would have also obfuscated their possible biological role. The application of so-called clean techniques to biological experiments has rekindled oceanographers’ interest in the biological and ecological function of trace elements in the sea, particularly the possible role of Fe in limiting primary production in oceanic regions where surface seawater is relatively rich in N and P. Several years ago we proposed that many bioactive elements may be colimiting phytoplankton growth in oceanic waters, that the stoichiometric concepts of Redfield

Axenic cultures of Thalussiosiru M;eissflogii need nickel when urea is the source of nitrogen for growth, but have no measurable Ni requirement when growing on NH,‘. This result is due to the Ni-containing enzyme, urease. which is necessary for urea catabolism in this diatom. In Ni-deficient cultures, where urea is the sole source of N, urease activity is low and growth rate is reduced. In Ni-replete cultures, urease activity is enhanced and the maximum growth rate achieved. Saturated uptake rates of Ni by urea-grown cells are faster than by NH,+-grown cells and are enhanced under Ni-limiting conditions. Ni-limited T. weiss&‘ogii cells are also N limited, even with high concentrations of urea in the medium, because they have insufficient urease activity to provide adequate N for growth. In these cultures, maximum growth rates are thus obtained by supplying either Ni or NH,+. These results demonstrate colimitation of T. M;eissflogjii growth by Ni and N under laboratory conditions and provide an archetypal example of a trace metal and macronutrient interaction that may be applicable to other nutrients.

[1] The distributions of nitrate, phosphate, and silicate in northern Baffin Bay were determined from 90 bottle casts taken between April 11 and July 21, 1998. During late spring, low-salinity Arctic water entered northern Smith Sound and mixed with Baffin Bay water (BBW) within the North Water Polynya. The Arctic water originated from the Bering Sea and contained high concentrations of phosphate and silicate (referred to as silicate-rich Arctic water (SRAW)). The distribution of the two water masses was established using a new tracer, Siex, which showed moderate penetration of SRAW into Smith Sound during April and a very strong incursion in May and June, consistent with the intensification of southward current velocities. Biological depletion of macronutrients in BBW began in April and continued until nitrate was exhausted from the upper mixed layer in early June. Beneath the Polynya the deep waters (>450 m) showed a marked increase in nutrient concentration toward the bottom, which was most pronounced in the south and much stronger for silicate than nitrate and phosphate. The silicate enrichment was consistent with dissolution of diatom-derived biogenic silica in deep waters. The results indicate that the North Water acts as a silicate trap in which the biota differentially transports surface silicate to depth, thereby influencing local and downstream nutrient signatures.

Over 250 samples of phytoplankton, zooplankton, other invertebrates, fish and macroalgae were collected from coastal waters of the southeast China Sea and analyzed for their selenium (Se) content. Phytoplankton (1·2 ppm) contained about twice as much Se as seaweeds (0·08 to 0·6 ppm) and both of these algal groups accumulated Se to concentrations three to four orders of magnitude above the ambient concentration in seawater. The Se content of zooplankton (∼ 4 ppm) was three to four times higher than phytoplankton, and juvenile stages (nauplii) and small zooplankton had the highest Se content. Fish often contained less Se than zooplankton on a dry weight basis. The Se content of various tissues and organs in fish varied from a maximum in liver (1·4–28·3 ppm) to a minimum in muscle and hard tissues (1–3 ppm and 0·1–6 ppm, respectively). There was little evidence for the bioaccumulation of Se in organisms of higher trophic status. The major concentrating step in the food web was Se incorporation by phytoplankton and zooplankton, up to four orders of magnitude above ambient seawater concentrations.

Uptake rates, determined with saturating additions (10 μg-at. N l−1) of 15N-labeled ammonium, nitrate, and urea were measured at two stations in the western Sargasso Sea and at one station in continental slope water off Cape Hatteras in August, 1985. Daily rates of nitrogen uptake were determined during 24 h incubations in water samples collected within the euphotic zone. 15N-Urea uptake was roughly constant during 24 h incubations in the Sargasso Sea, but uptake rates were faster during the day and slower at night in continental slope water. In slope water, NH4+ uptake rates were 2–3 times faster than urea uptake rates and 3–3.5 times faster than NO3− uptake rates. Urea uptake rates were 5 times faster than NH4+ uptake rates in surface water of one Sargasso Sea station, but below 30 m, uptake of NH4+ was more rapid than urea. At the other Sargasso Sea station, NH4+ uptake rates were faster than urea uptake rates. 14C-Urea and 15N-urea uptake rates were not equivalent: the median 14C-urea rate/15N-urea rate ratio was 1.5. In situ urea turnover times, determined using trace additions of 14C-urea, were ca. 13 h in the surface mixed layer and ca. 2 days in water samples collected from the base of the euphotic zone in the Sargasso Sea. The ratio of saturated urea uptake rate/in situ urea uptake rate was near unity in four of the six samples. These results show that in situ urea uptake rates are near the maximum potential uptake rates, and that phytoplankton in these regions have a high affinity for dissolved urea.

Urea uptake rates of Thalassiusiru pseudonana (clone 3H) were determined using [14C]urea, [15N]urea, and by measuring disappearance of dissolved urea from the medium after adding 10 lg-atoms urea-N liter-‘. In nitrate-sufficient cultures, the average [14C]urea uptake rate was 60% of the urea disappearance rate. Nitrate uptake continued in the presence of urea at a reduced rate, and only 15% of the urea N taken up was retained by the phytoplankton. The increase in PON during incubation was roughly equal to the total NO, and urea N taken up. Average urea uptake rates measured by all methods in 24-h nitrate-starved cultures were in excellent agreement with the rate of increase in PON during a l-h incubation. Uptake rate of [14C]urea and disappearance rate of urea were constant and equal. In neither nitrate-sufficient nor nitrate-starved cultures were [15N]urea uptake rates constant, with maximal rates measured 5.-l 5 min after the addition of urea. Ammonium was released by T. pseudonana following uptake of urea and was then taken up. A model of urea uptake and assimilation by T. pseudonana that involves efflux of urea N as NH, and its rapid reabsorption is proposed. These results can explain previous observations and have implications for utilization and cycling of urea N by phytoplankton in nature.