Adenosine triphosphate and adenylate energy charge in marine sediments

The determination of adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) concentrations in sediments from both Puget Sound and the Washington State continental shelf (USA) has been investigated. Major losses during the extraction procedure included sediment sorption, coprecipitation, and unreactivity of the soluble adenylates presumably through irreversible binding to solubilized sediment organics. Addition of phosphoric acid and the non-ionic detergent Triton X-100 to cold sulfuric acid significantly increased the extraction efficiency and sensitivity over that of sulfuric acid alone, although both methods yielded similar results when corrected for losses. Vertical profiles of ATP, total adenylates (A_T), and adenylate energy charge (EC) in sediments from both areas are presented. Profiles of A_T are similar in both areas, but EC values in the shelf sediments are significantly lower, which suggests a depressed physiological condition in the shelf population relative to that in Puget Sound.Contribution No. 513 from the University of Washington's College of Fisheries, and Contribution No. 1135 from the University of Washington's Department of Oceanography     

Adenosine nucleotide “energy charge” ratios as an ecophysiological index for microplankton communities

Energy charge (EC) ratios of microplankton samples have been measured from their adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) contents, according to a method based on enzymatic tranformations of ADP and AMP into ATP, and the subsequent quantitative analysis of the latter by the bioluminescent reaction of a firefly lantern extract (Photinus pyralis). Interference caused by other non-adenylic nucleotides and bioluminescence inhibition by various compounds were reduced, or estimated, by the use of internal standards for each sample. This was accomplished both by the injection of a small volume of a non-commercial extract of P. pyralis and by measurement of the bioluminescent flash at its maximal value. This standard method for preparation and analysis afforded good reproducibility and permitted the calibration of individual samples, thus allowing the comparison and treatment of data in the large series necessary in oceanographic studies. Studies were made on natural populations taken from a polluted marine area where phytoplankton communities live under the stressful influence of an urban sewer outlet, near Marseille (France). Three sampling strategies in surface waters (transects from the outlet, sampling during a 24-h buoydrift, and regular sampling for a day-night period) conducted at a point in front of the outlet revealed a decrease of EC in relation to urban activity or approaching proximity of the sewer outlet. These low EC values (0.2 to 0.5) were caused by a high AMP content, perhaps originating from degraded biogenic particles and dead or metabolically impeded bacteria of terrestrial orgin stressed by ecologic factors such as increased salinity and decreased temperature. Further away from the sewage outlet, the EC increased as the bacterial population decreased, and the phytoplanktonic communities reappeared parallel to the dilution of the effluent. Although in mature and unperturbed ecosystems EC values of 0.70 to 0.80 were recorded, the values were generally lower than those measured in growing bacterial or phytoplankton cultures. This fact may be related to differences in the various metabolic states of multispecific populations. Some EC measurements from deeper microplankton samples presented in this paper were difficult to interpret, reflecting perhaps unsolved problems concerning the treatment of samples. However, it was possible to associate the range of variation in EC values to differences in composition and structure of the ecosystem of microplankton populations in neritic surface waters.     

Temperature adaptation in sea anemones: Physiological and biochemical variability in geographically separate populations of Metridium senile

Latitudinally separate populations of the sea anemone Metridium senile (L.) are very similar genetically by electrophoretic criteria, yet respond differently to temperature. Anemones from southern and northern California (USA) have different oxygen consumption patterns in response to acclimatory and acute changes in temperature. Northern anemones show a pronounced increase in Q₁₀ at temperatures just above the normal environmental range, but southern anemones do not. The two populations also differed in the extent of metabolic compensation to temperature following several weeks of acclimation. This acclimation regime resulted in changes in the activities of several enzymes of intermediary metabolism, yet the extent and direction of these changes did not display a consistent trend with regard to acclimation temperature or population. The biochemical concomitants of acclimatory and acute temperature effects were studied further by measuring the concentrations of adenylates (ATP, ADP, and AMP) in anemones from the two populations exposed to different temperature regimes. During cold acclimation for several weeks, total adenylate concentrations (A_T) increased in both the southern and northern populations, possibly due to metabolic rate compensation, since A_T is positively correlated with tissue metabolic rate in many species. Moreover, the extremely low weight-specific oxygen consumption rates of M. senile are probably related to its very low A_T values. Acute temperature decreases had no effect on adenylate concentrations and adenylate energy charge (AEC); in contrast, acute temperature increases led to large changes in adenylate concentrations. The effects of starvation on adenylate concentrations are pronounced, and the effect is temperature-dependent. In starved individuals held at 20°C, AEC values fell to 50% of normal values after 8 d, while those held at 10°C maintained normal AEC values.     

Improved methodology for ATP determination in marine environments

A simple technique is described for de-salting and concentrating adenosine triphosphate (ATP) extracted from seawater or marine sediment samples prior to assays with the standard luciferin-luciferase procedure. The technique involves chromatography of H₂SO₄ extracts on columns of activated carbon. The efficiency of ATP recovery from marine sediments using this pre-treatment was superior to that attained with either boiling Tris extraction or with H₂SO₄ extraction without subsequent purification. All ions which interfere with the luciferase reaction or precipitate ATP upon neutralization of acid extracts are removed with this procedure, thus eliminating the 50- to 100-fold dilutions required with other acid-extraction procedures. In addition, the purified ATP extracts may be concentrated up to 100-fold, thus greatly improving the sensitivity of ATP measurement in samples containing very low biomass. This procedure has been applied to ATP determination in marine sediments in the upwelling area off northwest Africa. The average ATP concentrations in the upper 6 cm of sediment from 1000 and 200 m were 195 and 545 ng per ml wet sediment, respectively.     

Effect of reduced salinity on adenylate energy charge in three estuarine molluscs

The value of adenylate energy charge as a biological indicator of the severity of departure from normal environmental conditions was examined in the gastropod Pyrazus ebeninus (Bruguière, 1792) and the bivalves Anadara trapezia (Deshayes, 1840) and Saccostrea commercialis (Iredale and Roughley, 1933). Mean energy charge for each species decreased by 17% or more when salinity was reduced from 35 to 10. Changes with reduced salinity were also found in the concentrations of individual adenylates and several adenylate ratios other than energy charge. Energy charge was calculated from the relative concentrations of adenosine 5-tri-, di- and monophosphate (ATP, ADP, AMP) in the columellar muscle of P. ebeninus and in the adductor muscle of A. trapezia and S. commercialis. Mean energy charge values for individuals in the low-salinity condition (10) were 0.61 for P. ebeninus, 0. 69 for A. trapezia and 0.53 for S. commercialis, compared with mean energy charge values in the control individuals (35 S) of 0.85 to 0.87 for P. ebeninus, 0.84 to 0.85 for A. trapezia and 0.64 to 0.76 for S. commercialis. The decrease in energy charge occurred within 24 h; no further change was found with exposure to low salinity for 48 h (A. trapezia) or no further change was found with exposure to low salinity for 48 h (A. trapezia) or 20 h (P. ebeninus, S. commercialis). Total adenylate concentrations (ATP+ADP+AMP) differed between the three species, with mean values (kg^-1 wet weight) of 5.0 mmol in P. ebeninus, 3.4 mmol in A. trapezia and 3.0 mmol in S. commercialis. No trends with time were found in total adenylate concentrations in any species. The changes in energy charge that occurred were not consistent with the differing sensitivity of the three species to reduced salinity, and do not support the use of energy charge as an absolute measure of stress in molluscs generally.     

Ecobiochemical studies on the deep-water pelagic community of Korsfjorden,Western Norway. Adenine nucleotides in zooplankton

Adenine nucleotides of zooplankton were extracted from freeze-dried individuals killed in liquid nitrogen. 96% of the ATP in frozen samples stored at-26°C degraded to AMP in 8 days, while ATP was stable for the same period in freeze-dried samples. A strong metabolic stress caused by capturing the animals was evidenced by a marked lowering of the energy charge [(ATP)+1/2(ADP)/(ATP)+(ADP)+(AMP)], the natural level of energy charge being restored in about 24 h. The concentration of ATP showed no significant correlation with the dry weight of the individuals of a species nor with the mean dry weight of the species. The ATP concentration showed marked seasonal changes with increasing values during periods of reproduction, which is assumed to reflect an inheretn seasonal rhythm in the metabolism. ATP cannot be used as a general estimator of metabolism as it did not reflect differences in metabolic activity between different-sized individuals or species. The mean energy charge for 13 species was 0.7.Contribution from the Biological Station, University of Bergen, N-5065 Blomsterdalen, Norway.     

Measurement of the adenylate energy charge in Nereis diversicolor and Nephtys sp. (Polychaeta: Annelida):

ATP-content and adenylate energy charge (AEC) ratios were determined in two polychaete species (Neries diversicolor and Nephytys sp.), sampled in ten stations along the heavily polluted Western Scheldt estuary (N. Belgium, S. Holland). The samples were taken between 27 December 1982 and 6 January 1983. Nereis diversicolor was also sampled in an unpolluted brackish water pond, and subjected to artificial stress by drying the organisms on filter paper. Adenine nucleotide levels were determined using the firefly bioluminescence reaction. The light emission was measured with the integration mode. As the Constant Light Signal reagent used contained AMP and probably ADP itself, both myokinase and pyruvatekinase had to be inactivated before the assay. The use of pepsin in acidic conditions to denaturate these enzymes is discussed. For both N. diversicolor and Nephtys sp., a significant relation between ATP-content and fresh weight is found in most samples. Within each species, ATP-content per g fresh weight is dependent on the sampling method, but it is not significantly different between stations. It is higher in Nephtys sp. from the W. Scheldt (130. 8 10^-5 g ATP/g fresh weight) than in N. diversicolor from the Dievengat (83.9 10^-5 g ATP/g fresh weight). This value in turn is significantly higher than that for N. diversicolor in the W. Scheldt (64.0 10^-5 g ATP/g fresh weight). In the drying experiment, AEC gradually decreases with increasing drying time. In the field, AEC is generally high and not significantly different between the three populations. AEC ratios also do not differ between stations within each population. It is argued that field monitoring of AEC in invertebrates is not valuable as a pollution assessment tool for two reasons. In polluted stations, only pollutant-resistant species will be found, and there is evidence that the maintenance of a stable population is impossible when the individuals constantly have low AECs.     

Community metabolism of adenylates by microheterotrophs from the Los Angeles Harbor and Southern California coastal waters

The natural concentration (S _n) of dissolved total adenylates TA(=AMP+ADP+ATP) in coastal seawater from a depth of 1 m at 5 stations (California, USA) sampled periodically for 1 yr had a mean value ±1 SD of 2.8±1.7 nmol TA1^-1. The specific uptake rates of TA by microheterotrophs at a station inside the Los Angeles Harbor and at a station 1.5 km offshore in the San Pedro Channel were studied by simple uptake and saturation-type kinetic analysis using ³H-AMP as a tracer. Within the harbor, the specific uptake rate (nmol TA 10⁹ cell^-1 h^-1) at S _n ranged 10-fold from 0.028 in December to 0.28 in August. K _t (half-saturation constant) values always exceeded theS _n concentrations in any given month, and were greater in the harbor than in the channel. Generally, over 80% of biological uptake of ³H-AMP was associated with organisms <1.0 m, a size class accounting for about 20% of the total particulate adenylate concentration in the 0.2 to 203 m size fraction. Assuming steady-state conditions for the dissolved adenylate pool, we propose a model in which losses from this pool are balanced by inputs to the pool through inefficient feeding, lysis and decomposition of particulate adenylates.     

ATP concentration and adenylate energy charge of tropical zooplankton from waters inside the great barrier reef

The adenylate energy charge ratio (EC) of 8 species of zooplankton ranged from 0.70 to 0.89. Eucalanus subcrassus had significantly (P<0.05) lower EC when captured during a 30-min tow than during a 3-min tow, indicating stress at capture. The concentration of ATP in the 8 species ranged from 9.1 to 36.0 g/mg protein and from 0.17 to 15.7 g/mg dry weight. Excluding a low value for the ctenophore Pleurobrachia sp., the average concentration was 8.1 g ATP/mg dry weight, which is equivalent to an approximate C/ATP ratio of 50.Contribution from the Australian Institute of Marine Science, Australia, and the Institute of Marine Biology, University of Bergen, Norway     

A rapid,highly sensitive technique for the determination of ammonia in seawater

A reliable and sensitive technique for the rapid determination of ammonia in seawater is described. The procedure is based on the conversion of NH ₄ ⁺ in seawater to NH₃ and the subsequent diffusion of NH₃ across a hydrophobic membrane using flow-injection analysis. Sixty determinations can be made per hour on a flowing stream of seawater or discrete samples. The lower limit of detection is 0.05 M. Results of two applications that demonstrate the potential of this technique are presented, a laboratory excretion experiment employing the crabPachygrapsus crassipes, and small-scale vertical mapping of ammonia in the ocean.     

Ammonium enhancement of dark carbon fixation and nitrogen limitation in zooxanthellae symbiotic with the reef coralsMadracis mirabilis andMontastrea annularis

The nutrient status (limitation vs sufficiency) of dinoflagellates (zooxanthellae) symbiotic with reef corals in Bermuda was assessed in 1989 and 1990 by measuring the enhancement of dark carbon fixation with 20 M ammonium by isolated symbionts. A colony ofMadracis mirabilis was kept in the laboratory and fed daily or starved for one month. Symbionts from fed portions of the colony had ammonium-enhancement ratios (NH _4dark ⁺ ; SW_dark;SW=seawater without added ammonium) similar to those of the original field population (1.2 to 1.3). Ammonium-enhancement ratios increased with starvation of the host (x1.7) as did values forV _D:V _L [(ammonium dark rate-seawater dark rate): light rate in seawater]. Both parameters indicated decreasing nitrogen sufficiency of the algae when the host was not fed, but starvation appeared to affect these algae less than symbionts of sea anemones. Field samples of zooxanthellae fromM. mirabilis (Three Hill Shoals and Bailey's Bay Flats) yielded results similar to those for fed corals, but those taken from Bailey's Bay Flats in May 1990 yielded exceptionally high values for enhancement (>3) andV _D:V _L indicating pronounced nitrogen limitation at the time of sampling. We sampled zooxanthellae from populations ofMontastrea annularis at 8 m (Three Hill Shoals) and 24 m (Soldier's Point) depths. Enhancement andV _D:V _L values for zooxanthellae from the 8 m corals were density-dependent: symbionts from corals with normal symbiont densities displayed the most nitrogen limitation (enhancement values=1.4 to 2.0), while those from bleached corals with lower density exhibited enhancement andV _D:V _L values typical of nitrogen-sufficient algae. Symbionts isolated from the 25 m corals yielded the highest values, and appeared to exhibit the least nitrogen-sufficiency for this species.     

Anaerobic metabolism of the common cockle Cardium edule

Changes in the level of the adenylates (ATP, ADP, AMP), arginine phosphate, arginine, octopine, D-lactate, succinate, and aspartate were determined in the foot of Cardium edule, after 12 h of anoxia and after aerobic recovery for up to 12 h. Control levels of the adenylates, the calculated energy charge and the arginine phosphate content were restored after about 1 h of recovery. On the other hand, the recovery of the D-lactate, succinate, and aspartate pools was a slow process, taking place over about 12h.     

Hydrogeochemistry study and groundwater quality assessment in the north of Isfahan,Iran

This study presents the groundwater quality assessment in the north of Isfahan, Iran. In the study area, assessment and measurement of groundwater hydrochemical parameters such as pH, total dissolved solids (TDS), electrical conductivity (EC), sodium absorption ratio (SAR), total hardness, major cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺) and major anions (Cl^?, \({\text{HCO}}_{ 3}^{ - } ,{\text{CO}}_{3}^{2 - }\) and \({\text{SO}}_{4}^{2 - }\)) concentrations were performed. Accordingly, the 66 water samples from different locations were collected during April and May 2015. Water samples collected in the field were analyzed in the laboratory for cations and anions using the standard methods. In this research, the analytical results of physiochemical parameters of groundwater were compared with the standard guideline values as recommended by the world health organization (WHO) for drinking and public health purposes. The pH values of groundwater samples varied from 7.05 to 8.95 with a mean of 7.78, indicating a neutral to slightly alkaline water. TDS values showed that 14% of the samples exceeds the desirable limit given by WHO. EC values varied from 213 to 4320 µS/cm, while 23% of the samples were more than the standard limit. Gibbs diagram had shown that 90% of the samples in the study area fall in the rock weathering zone, and this means that chemical weathering of rock-forming minerals is the main factor controlling the water chemistry in the study area. Irrigation suitability and risk assessment of groundwater are evaluated by measuring EC, %Na, SAR and RSC. According to the dominant cations and anions, five types of water were identified in the water samples: Ca-HCO₃, Ca-SO₄, Na-Cl, Na-HCO₃ and Na-SO₄. The results show that the majority of samples (30 samples, 45%) belongs to the mixed Na-SO₄ water type. Correlation analysis and principal component analysis was used to identify the relationship between ions and physicochemical parameters. Results indicated that 18 stations of the study area had the best quality and can be used for irrigation and drinking purposes in the future.     

Photosynthesis and respiration in the alga Ahnfeltia plicata in a flow-through system

Photosynthesis and respiration in Ahnfeltia plicata (Huds.) Fries (Gigartinales) was measured in a seawater flowthrough system at different temperatures, salinities and photon flux densities (PFD). The exchanges of dissolved oxygen and inorganic carbon were continuously recorded with an oxygen probe and a pH electrode measuring variation in CO₂–HCO ₃ ^- equilibrium as pH changes. Highest apparent photosynthesis at moderate photon flux density (PFD 50 E m^-2 s^-1) was found at 15°C and 33 S. Photosynthesis was measured up to PFD 500 E m^-2 s^-1 and no light saturation was documented. In the present experimental set-up, with continuous supply of fresh seawater, the number of limiting factors during photosynthesis measurements is reduced.     

Environmental information stored in otoliths: insights from stable isotopes

The present study compares the stable oxygen-and carbon-isotope ratios (¹⁸0:¹⁶O;¹³C:¹²C) in the otoliths of Atlantic cod,Gadus morhua, with those expected at equilibrium with seawater. Otoliths from juveniles reared for a 3 mo period under controlled conditions indicate that otoliths are formed in isotopic disequilibrium with seawater. This is probably due to positive metabolic fractionating of the heavier isotopes. This vital effect remains constant over the temperature range studied here (9 to 16°C) but may differ among other species. Our data indicate that the concentration of¹⁸O in calcium carbonate is inversely related to temperature and is described as ¹⁸O_a – _w – 3.79 – 0.200(T°C). The¹³C:¹²C ratios of otoliths and body tissues are related to the carbon ratio in the food source, although we found that the¹³C concentration is considerably higher in the otoliths relative te, the body tissues and the diet.     

Actions of 17β-estradiol on carbohydrate metabolism in liver,gills, and brain of gilthead sea bream Sparus auratus during acclimation to different salinities

The influence of gonadal maturation on seawater adaptability at the metabolic level was assessed in the euryhaline teleost Sparus auratus by treatment of immature fish with 17ß-estradiol (E₂) followed by acclimation to different environmental salinities. Fish were injected with coconut oil alone (sham) or containing E₂ (10 µg g^–1 body weight) and maintained in seawater (40). After 5 days, fish from oil- and E₂-implanted groups were sampled. At the same time, fish of both groups were transferred to brackish water (5, hypoosmotic test), seawater (40, transfer test), and hypersaline water (55, hyperosmotic test); 4 days after transfer (9 days post-implant) all groups were sampled. Data obtained from liver suggest that E₂ treatment produced effects comparable to those already reported in literature, including decreased glycogen levels, increased glycolytic potential, and decreased gluconeogenic potential. This, together with the fact that many changes displayed in shams among salinities disappeared in E₂-treated fish allow us to suggest that the response of liver carbohydrate metabolism to osmotic acclimation is exceeded by the response elicited by E₂ treatment. In gills, E₂ treatment produced increased lactate levels, decreased capacity for use of exogenous glucose, and decreased the potential of the pentose phosphate pathway. These findings suggest that the energy demand occurring in gills during osmotic acclimation should be increasingly fuelled by substrates other than exogenous glucose. Finally, data obtained in brain of E₂-treated fish suggest a lower necessity of exogenous glucose, increased lactate levels, and decreased glycolytic potential.Communicated by S.A. Poulet, Roscoff     

Physiological responses of Nodularia harveyana to osmotic stress

The effects of salinity stress on biomass yield, photosynthetic O₂ evolution and nitrogenase activity were investigated using axenic cultures of Nodularia harveyana (Thwaites) Thuret originally isolated from a salt marsh at Gibraltar Point, Lincolnshire, UK in 1971 and studied in this laboratory in 1983. Biomass yields, as chlorophyll a per culture, were highest in the 0 to 100% seawater (0 to 35 sea salt) range with negligible growth in 200% seawater; growth on NH ₄ ⁺ was greater than on N₂ and NO ₃ ^- , which did not differ significantly from each other. In short-term experiments, photosynthetic O₂ evolution remained high at salinities up to 150% seawater (52.5 sea salt); nitrogenase activity remained high at salinities up to 100% seawater (35 sea salt). The major internal low molecular weight carbohydrate which accumulated in response to increased salinity was sucrose, the levels of which fluctuated markedly and rapidly in response to salinity change.     

Characteristics of chemical components in PM<Subscript>2.5</Subscript> at a plateau city,South-west China

A sampling campaign including summer, autumn and winter of 2014 and spring of 2015 was accomplished to obtain the characteristic of chemical components in PM_2.5 at three sites of Kunming, a plateau city in South-west China. Nine kinds of water-soluble inorganic ions (WSI), organic and element carbon (OC and EC) in PM_2.5 were analyzed by ion chromatography and thermal optical reflectance method, respectively. Results showed that the average concentrations of total WSI, OC and EC were 22.85±10.95 µg·m^-3, 17.83±9.57 µg·m^-3 and 5.11±4.29 µg·m^-3, respectively. They totally accounted for 53.0% of PM_2.5. Secondary organic and inorganic aerosols (SOA and SIA) were also assessed by the minimum ratio of OC/EC, nitrogen and sulfur oxidation ratios. The annual average concentrations of SOA and SIA totally accounted for 28.3% of the PM_2.5 concentration. The low proportion suggested the primary emission was the main source of PM_2.5 in Kunming. However, secondary pollution in the plateau city should also not be ignorable, due to the appropriate temperature and strong solar radiation, which can promote the atmospheric photochemical reactions.

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Seasonal variations in the stable isotopic composition and the skeletal density pattern of the coralPorites lobata (Gulf of Eilat,Red Sea)

The stable carbon and oxygen isotopic compositions of the massive coralPorites lobata collected from the Gulf of Eilat, Red Sea, in May 1987 were measured along the vertical growth axis. The isotopic compositions show annual periodicity. The variations along the isotopic profile, compared with the X-radiography, indicate that the high-density bands are enriched in¹⁸O and deposited during the season of lowest water temperature (winter). In contrast, the low-density bands are depleted in¹⁸O and deposited during the warmest seawater temperatures (summer). The stable carbon and the oxygen isotopic ratios are negatively correlated with a shift in phase. During the season of higher water temperatures and light intensity values (summer), the skeleton is depleted in¹⁸O and enriched in¹³C and vice versa for the winter time. The shift between the carbon and the oxygen isotopic curves reflects the shift between the seasonal light intensities and seasonal temperature variations in the shallow water. The oxygen isotope ratio was used to detect the seasonal variations in seawater temperatures. The coral aragonitic skeleton is depleted in¹⁸O compared to apparent equilibrium with ambient seawater. The disequilibria range from (ca.) -3.10 to -3.50 with an average value of 3.40. The isotope fractionation behavior during skeleton precipitation is discussed in light of the environmental variables.     

Bioavailable iron species in seawater measured by macroalga (Laminaria japonica) uptake

The bioavailability of iron in seawater filtered through a 0.025-m filter was investigated using ⁵⁹Fe-labeled iron uptake by the macroalga Laminaria japonica (Areschoug: Phaeophyta) (collected in the northern Japan Sea 1993) as an assay. About 80% of the iron in the 0.025-m filtered coastal seawater was soluble and/or small colloidal organically bound iron, associated with natural organic ligands forming complexes with ferric ion. After decomposition of the organic matter by ultraviolet (UV) irradiation, 55% of the iron addition [or 0.6 nM, nearly the concentration of Fe(OH) ₂ ⁺ in equilibrium with amorphous hydrous ferric oxide in seawater at pH 8.0] in the 0.025-m filtered coastal seawater was taken up by the macroalga. Since the iron concentrations in the 0.025-m filtered coastal seawater are 0.1 to 2.0 nM and only 0.6 nM of the iron is likely available to biota over 1 to 2 d, we suggest that only small amounts of bioavailable iron exist in coastal seawater not affected by inflow from land and that a significant fraction of dissolved (<0.025 ) iron occurs in forms, such as organic iron complexes, other than the simple hydroxo-complex species predicted by thermodynamic models.