The influence of salinity on the kinetics of NH inf4 sup+ uptake in Spartina alterniflora

Summary The effects of short- and long-term exposure to a range in concentration of sea salts on the kinetics of NH _inf4 ^sup+ uptake by Spartina alterniflora were examined in a laboratory culture experiment. Long-term exposure to increasing salinity up to 50 g/L resulted in a progressive increase in the apparent K_m but did not significantly affect V_max (mean V_max=4.23±1.97 mole·g^–1·h^–1). The apparent K_m increased in a nonlinear fashion from a mean of 2.66±1.10 mole/L at a salinity of 5 g/L to a mean of 17.56±4.10 mole/L at a salinity of 50 g/L. These results suggest that the long-term effect of exposure to total salt concentrations within the range 5–50 g/L was a competitive inhibition of NH _inf4 ^sup+ uptake in S. alterniflora. No significant NH _inf4 ^sup+ uptake was observed in S. alterniflora exposed to 65 g/L sea salts. Short-term exposure to rapid changes in salinity significantly affected both V_max and K_m. Reduction of solution salinity from 35 to 5 g/L did not change V_max but reduced K_m by 71%. However, exposing plants grown at 5 g/L salinity to 35 resulted in an decrease in V_max of approximately 50%. Exposure of plants grown at 35 g/L to a total sea salt concentration of 50 g/L for 48h completely inhibited uptake of NH _inf4 ^sup+ . For both experiments, increasing salinity led to an increase in the apparent K_m similar to that found in response to long-term exposure. Our data are consistent with a conceptual model of changes in the productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH _inf4 ^sup+ uptake kinetics.     

Different Ks values for hydrogen of methanogenic bacteria and sulfate reducing bacteria: An explanation for the apparent inhibition of methanogenesis by sulfate

Desulfovibrio vulgaris (Marburg) and Methanobrevibacter arboriphilus (AZ) are anaerobic sewage sludge bacteria which grow on H₂ plus sulfate and H₂ plus CO₂ as sole energy sources, respectively. Their apparent K_s values for H₂ were determined and found to be approximately 1 M for the sulfate reducing bacterium and 6 M for the methanogenic bacterium. In mixed cell suspensions of the two bacteria (adjusted to equal V _max) the rate of H₂ consumption by D. vulgaris was five times that of M. arboriphilus, when the hydrogen supply was rate limiting. The apparent inhibition of methanogenesis was of the same order as expected from the different K_s values for H₂. Difference in substrate affinities can thus account for the inhibition of methanogenesis from H₂ and CO₂ in sulfate rich environments, where the H₂ concentration is well below 5 M.     

Effects of temperature and CO2 enrichment on kinetic properties of NADP+-malate dehydrogenase in two ecotypes of Barnyard grass (Echinochloa crus-galli (L.) Beauv.) from contrasting climates

Summary The apparent energy of activation (E _a), Michaelis-Menten constant (K _mfor oxaloacetate), V _max/K _mratios and specific activities of NADP⁺-malate dehydrogenase (NADP⁺-MDH; EC 1.1.1.82) were analyzed in plants of Barnyard grass from Québec (QUE) and Mississippi (MISS) acclimated to two thermoperiods 28/22°C, 21/15°C, and grown under two CO₂ concentrations, 350 l l^-1 and 675 l l^-1. E _avalues of NADP⁺-MDH extracted from QUE plants were significantly lower than those of MISS plants. K _mvalues and V _max/K _mratios of the enzyme from both ecotypes were similar over the range of 10–30°C but reduced V _max/K _mratios were found for the enzyme of QUE plants at 30 and 40°C assays. MISS plants had higher enzyme activities when measured on a chlorophyll basis but this trend was reversed when activities were expressed per fresh weight leaf or per leaf surface area. Activities were significantly higher in plants of both populations acclimated to 22/28°C. CO₂ enrichment did not modify appreciably the catalytic properties of NADP⁺-MDH and did not have a compensatory effect upon catalysis or enzyme activity under cool acclimatory conditions. NADP⁺-MDH activities were always in excess of the amount required to support observed rates of CO₂ assimilation and these two parameters were significantly correlated. The enhanced photosynthetic performance of QUE plants under cold temperature conditions, as compared to that of MISS plants, cannot be attributed to kinetic differences of NADP⁺-malate dehydrogenase among these ecotypes.     

Transport of glucose,fructose and sucrose by Streptanthus tortuosus suspension cells

Streptanthus tortuosus Kell. suspension cells will grow in a medium with sucrose as carbohydrate source. It was investigated whether the cells are able to take up sucrose or whether sucrose has to be hydrolyzed to glucose and fructose which eventually are taken up. The detailed quantitative analysis of sugar-uptake rates in the low concentration range up to 1 mM showed the following features: (i) There is definitely no sucrose-uptake system working in the low concentration range; any uptake of radioactivity from labelled sucrose proceeds via hydrolysis of sucrose by cell-wallbound invertase. (ii) Hexoses are taken up by two systems, a glucose-specific system with a K _m of 45 M and a high V _max for glucose and a K _m of 6 mM and a low V _max for fructose, and a fructosespecific system with a K _m of 500 M and high a V _max for fructose and a K _m of 650 M and a low V _max for glucose. (iii) There is a more than tenfold preference for uptake of the fructose derived from sucrose versus uptake of free fructose, with the result that the kinetic disadvantage of the fructoseuptake system compared to the glucose-uptake system is diminished if sucrose is supplied as the carbon source. It is speculated that invertase might work as an enzyme aiding in fructose transport.Abbreviations FCCP carbonylcyanide-p-trifluoromethoxyphenylhydrazone - FW fresh weight     

Competition between sulfate-reducing and methanogenic bacteria for H2 under resting and growing conditions

The basis for the outcome of competition between sulfidogens and methanogens for H₂ was examined by comparing the kinetic parameters of representatives of each group separately and in co-culture. Michaelis-Menten parameters (V _max and K _m) for four methanogens and five sulfate-reducing bacteria were determined from H₂-depletion data. Further, Monod growth parameters (_max, K _s, Y _H2) for Desulfovibrio sp. G11 and Methanospirillum hungatei JF-1 were similarly estimated. H₂ K _m values for the methanogenic bacteria ranged from 2.5 M (Methanospirillum PM1) to 13 M for Methanosarcina barkeri MS; Methanospirillum hungatei JF-1 and Methanobacterium PM2 had intermediate H₂ K _m estimates of 5 M. Average H₂ K _m estimates for the five sulfidogens was 1.2 M. No consistent difference among the V _max estimates for the above sulfidogens (mean=100 nmol H₂ min^-1 mg^-1 protein) and methanogens (mean=110 nmol H₂ min^-1 mg^-1 protein) was found. A two-term Michaelis-Menten equation accurately predicted the apparent H₂ K _m values and the fate of H₂ by resting co-cultures of sulfate-reducers and methanogens. Half-saturation coefficients (K _s) for H₂-limited growth of Desulfovibrio sp. G11 (2–4 M) and Methanospirillum JF-1 (6–7 M) were comparable to H₂ K _m estimates obtained for these organisms. Maximum specific growth rates for Desulfovibrio sp. G11 (0.05 h^-1) were similar to those of Methanospirillum JF-1 (0.05–0.06 h^-1); whereas G11 had an average yield coefficient 4 x that of JF-1. Calculated _max and V _max/K _m values for the methanogens and sulfidogens studied predict that the latter bacterial group will process more H₂ whether these organisms are in a growing or resting state, when the H₂ concentration is in the first-order region.     

A novel isolate of Desulfovibrio sp. with enhanced ability to reduce Cr(VI)

Kinetic analysis of the reduction of Cr(VI) by resting cell suspensions of Desulfovibrio vulgaris ATCC 29579 and a new isolate, Desulfovibrio sp. (`Oz7') was studied using lactate as the electron donor at 30 °C. The apparent K <sub>m</sub> (K <sub>m app</sub>) and V <sub>max</sub> with respect to Cr(VI) reduction was compared for both strains. Desulfovibio sp. `Oz7' had a K _{m app} of 90 M (threefold lower than that of D. vulgaris ATCC 29579) and a V _max of 120 nmol h^–1 mg^–1 biomass dry wt (approx. 30% lower than for the reference strain). The potential of the new isolate for bioremediation of Cr(VI) wastewaters is discussed.     

Properties of the phosphate and phosphite transport systems of Phytophthora palmivora

Germlings of Phytophthora palmivora possess at least two systems for the uptake of inorganic phosphate (P_i). The first is synthesized on germination in medium containing 50 M P_i and has a K_m of approx. 30 M (V_max=7–9 nmol P_i/h·10⁶ cells). The second is synthesized under conditions of P_i-deprivation and has a higher affinity for P_i (K_m=1–2 M), but a lower V_max (0.5–2 nmol P_i/h·10⁶ cells). The fungicide phosphite likewise enters the germlings via two different transport systems, the synthesis of which also depends on the concentration of P_i in the medium. The K_m of the lower affinity system is 3 mM (V_max=20 nmol phosphite/h·10⁶ cells) and that of the higher affinity system is 0.6 mM (V_max=12 nmol/h·10⁶ cells). P_i and phosphite are competitive inhibitors for each other's transport in both systems. However, whereas mM concentrations of phosphite are necessary to inhibit P_i transport, only M concentrations of P_i are required to inhibit phosphite transport. A third system of uptake for P_i also exists, since when phosphate-deprived cells are presented with mM concentrations of P_i, they transport the anion at a very high rate (around 100 nmol/h·10⁶ cells). High rates of transport of phosphite are also observed when these cells are presented with mM concentrations of this anion.     

The uptake of serotonin and dopamine by homogenates of frozen rat and human brain tissue

A recently described procedure of freezing and thawing, which allows retention of metabolic and functional integrity, has been applied in the study of serotonin and dopamine uptake into frozen rat and post mortem human frozen tissue. TheK _m andV _max for the serotonin uptake into human hypothalamus were estimated to be 0.12 M and 0.03 nmol/g/min respectively. TheK _m andV _max for the dopamine uptake into human putamen were estimated to be 0.28 M and 0.13 nmol/g/min respectively. The results indicate that the freezing procedure does not affect the uptake sites for these transmitters. The storage time before freezing is however of importance for theV _max value. TheK _m value for the uptake, on the other hand, seems to be rather resistant to storage time before freezing.     

Presence of an X AG − carrier in frog (Rana esculenta) red blood cells

Evidence is presented that the high levels of internal l-glutamic and l-aspartic acid in frog Rana esculenta red blood cells are due to the existence of a specific carrier for acidic amino acids of high affinity K _m = 3 m and low capacity (V_max) 0.4 mol l-Glu · Kg^–1 dry cell mass · 10 min^–1. It is Na+ dependent and the incorporation of l-glutamic acid can be inhibited by l and d-aspartate and l-cysteic acid, while d-glutamic does not inhibit. Moreover, this glutamic uptake shows a bell-shaped dependence on the external pH. All these properties show that this carrier belongs to the system X _AG ^– family. Besides the incorporation through this system, l-glutamic acid is also taken up through the ASC system, although, under physiological conditions, this transport is far less important, since it has relatively low affinity K _m 39 m but high capacity (V _max) 1.8 mol l-Glu · Kg^–1 dry cell mass · 10 min^–1.     

Uptake of iron byGeotrichum candidum,a non-siderophore producer

Summary Geotrichum candidum (isolate 1–9) pathogenic on citrus fruits, appears to lack siderophore production. Iron uptake byG. candidum is mediated by two distinct iron-regulated, energy-and temperature-dependent transport systems that require sulfhydryl groups. One system exhibits specificity for either ferric or ferrous iron, whereas the other exhibits specificity for ferrioxamine-B-mediated iron uptake and presumably other hydroxamate siderophores. Radioactive iron uptake from⁵⁹FeCl₃ showed an optimum at pH 6 and 35° C, and Michaelis-Menten kinetics (apparentK _m = 3 m,V _max = 0.054 nmol · mg^–1 · min^–1). The maximal rate of Fe²⁺ uptake was higher than Fe³⁺ (V _max = 0.25 nmol · mg^–1 · min^–1) but theK _m was identical. Reduction of ferric to ferrous iron prior to transport could not be detected. The ferrioxamine B system exhibits an optimum at pH 6 and 40° C and saturation kinetics (K _m = 2 M,V _max = 0.22 nmol · mg^–1 · min^–1). The two systems were distinguished as two separate entities by negative reciprocal competition, and on the basis of differential response to temperature and phenazine methosulfate. Mössbauer studies revealed that cells fed with either⁵⁷FeCl₃ or⁵⁷FeCl₂ accumulated unknown ferric and ferrous binding metabolites.     

Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, K _m and I _max (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in I _max (up to four times), without concomitant change in K _m, which remained between 5 and 7 M. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the K _m (i.e. an increased apparent affinity for potassium) from 53 M to 11 M, without alteration to I _max. After longer periods of potassium starvation, I _max increased (about two times) while the K _m remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.Symbol I _max the maximum rate of absorption at saturating concentrations     

Regulation of sulfate uptake and xylem loading of poplar roots (Populus tremula x P. alba)

Sulfate transport processes and its regulation were studied in roots of poplar trees (Populus tremula x P. alba). From the exponential increase in sulfate uptake with temperature an activation energy (E_a) of 9.0±0.8 kJ mol^–1 was calculated. In the concentration range 0.005–10 mM sulfate uptake showed biphasic Michaelis-Menten kinetics with a K_m of 3.2±3.4 M and a V_max of 49±11 nmol SO₄^2– g^–1 FW h^–1 for the high-affinity uptake system (phase 1) and a K_m of 1.33±0.41 mM and a V_max of 255±25 nmol SO₄^2– g^–1 FW h^–1 for the low-affinity system (phase 2). Xylem loading decreased linearly with temperature and remained unchanged within the sulfate concentration range studied. Regulation of sulfate uptake and xylem loading by O-acetyl serine (OAS), Cys, reduced glutathione (GSH), Met and S-methylmethionine (SMM) were tested by perfusion into the xylem sap with the pressure probe and by addition to the incubation medium. When added directly to the transport medium, Cys and GSH repressed, and OAS stimulated sulfate uptake; xylem loading was stimulated by Cys, repressed by GSH and only slightly affected by OAS. When perfused into the xylem, none of the compounds tested affected sulfate uptake of excised roots, but xylem loading was stimulated by SMM and OAS and repressed by Met. Apparently, the site of application strongly determined the effect of regulatory compounds of sulfate transport processes.     

Formation of thionates by freshwater and marine strains of sulfate-reducing bacteria

The formation of thionates (thiosulfate, trithionate and tetrahionate) during the reduction of sulfate or sulfite was studied with four marine and four freshwater strains of sulfate-reducing bacteria. Growing cultures of two strains of the freshwater species Desulfovibrio desulfuricans formed up to 400 M thiosulfate and 100 M trithionate under conditions of electron donor limitation. Tetrathionate was observed in lower concentrations of up to 30 M. Uncoupler-treated washed cells of the four freshwater strains formed thiosulfate and trithionate at low electron donor concentrations with sulfite in excess. In contrast, only one of four marine strains formed thionates. The freshwater strain Desulfobulbus propionicus transformed sulfite almost completely to thiosulfate and trithionate. The amounts produced increased with time, concentration of added sulfite and cell density. Tetrathionate was detected only occasionally and in low concentrations, and was probably formed by chemical oxidation of thiosulfate. The results confirm the diversity of the sulfite reduction pathways in sulfate-reducing bacteria, and suggest that thiosulfate and trithionate are normal by-products of sulfate reduction.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone     

Uptake of [14C]sucrose in isolated minor-vein networks of Commelina benghalensis L

Maceration with pectinase (4.5h) of Commelina benghalensis L. leaves stripped at either side yielded isolated vein networks consisting of four to five secondary veins and tertiary cross veins (=minor veins). Examination with Evans Blue and injection of Fluorescein F showed that 80% of the veins were viable. Proof of normal functioning of isolated minor veins was that [¹⁴C]sucrose fed to an apical vein network attached to the remaining intact part of the leaf was absorbed and finally arrived in the petiole. Sucrose uptake by veins obeyed Michaelis-Menten kinetics (K _m 5·10^-4 mol l^-1; V _max (light) 3.2 mol h^-1 g^-1 fresh weight, V _max (dark) 1.5 mol h^-1 g^-1 fresh weight). A linear component, not inhibited by carbonylcyanide m-chlorophenylhydrazone and p-chloromercuribenzenesulfonic acid, was present. Maximal uptake took place at 5 mmol l^-1 K⁺; concentrations of K⁺ higher than 10 mmol l^-1 decreased the rate of uptake. The uptake rates by isolated veins and veins in situ (in disks) were in the same order of magnitude. Altogether, isolated veins promise to be a useful system for the study of loading.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - EDTA ethylenediamine tetraacetic acid - PCMBS p-chloromercuribenzenesulfonic acid     

Transport of organic sulfur and nitrogen in the roots of young mycorrhizal pedunculate oak trees (Quercus robur L.)

Uptake and xylem loading of organic sulfur and nitrogen were analyzed in detached mycorrhizal (Laccaria laccata L.) roots of pedunculate oak (Quercus robur L.) seedlings using radiolabeled reduced glutathione (GSH) and glutamine (Gln) for transport analyses. The experiments showed for the first time that GSH is taken up by plant roots from the nutrient solution and is partially allocated to the shoot. Apparently, GSH produced during mineralization processes in the soil can be used by plant roots as a sulfur source. GSH uptake into the roots showed biphasic kinetics within the concentration range studied (0–500 M) with maximum transport velocities (v _max) and substrate affinities (K _m) that were similar to the kinetics of Gln uptake. GSH uptake kinetics were also in the same range as previously reported for sulfate uptake by mycorrhizal roots of pedunculate oak. It may therefore be assumed that GSH and sulfate uptake can be of comparable significance for sulfur nutrition, provided both sulfur sources are available at similar concentrations at the sites of uptake. Xylem loading of GSH and Gln showed monophasic transport kinetics with v _max significantly lower than observed for the two respective uptake systems and, as indicated by the K _m-values, a substrate affinity between the high and the low affinity uptake systems. The possible nature of the transport systems for GSH and Gln is discussed.     

Magnesium uptake kinetics in loblolly pine seedlings

Recent studies have suggested that the growth of loblolly pine (Pinus taeda L.) has declined in the southern United States and it has been hypothesized that foliar Mg deficiency may play an important role in the perceived decline. Quantitative nutrient uptake models such as the Barber-Cushman model have been used successfully to investigate nutrient uptake by crop species under a variety of field and experimental conditions and may provide one approach to evaluating this question. However, in order to use this approach it is necessary to develop, for the plant species and nutrient of interest, values for maximal nutrient influx rate at high solution concentrations (I_max), the solution concentration where net influx is 0.5 I_max (K_m), and the nutrient concentration below which influx ceases (C_min). As a first step in evaluating the potential of such an approach, two sets of experiments using established solution nutrient depletion techniques were used to define these values for loblolly pine seedlings 180, 240, 365, and 425 days in age. Observed I_max values for Mg range from 7.90E-8 to 1.29E-7 mol cm^–2 s^–1 with younger seedlings having higher values. Values of K_m for all seedling ages were quite similar ranging from 8.69 to 8.58E-3 mol cm^–3. Most importantly, the results of both experiments indicate that during a growth flush, seedlings will withdraw Mg from solution until the concentration is essentially zero (C_min=0). During non-flush periods uptake rates appear to be greatly reduced. Therefore, efforts to model Mg uptake will need to take these differences as well as seedling age influences into consideration.     

Acetate uptake by the unicellular cyanobacteria Synechococcus and Aphanocapsa

Acetate uptake by strains of Synechococcus and Aphanocapsa in short experiments required light, and was strongly inhibited by m-dichlorocarbonyl cyanide phenylhydrazone and dichlorophenyl dimethyl urea. Acetate carbon was distributed in amino acids and in the acyl portion of lipids in the same way as during growth experiments when CO₂ was available, but the reduced incorporation in the absence of CO₂ was primarily into the lipid fraction. An apparent K _m for uptake by Synechococcus and for Aphanocapsa 6308 of 20 and 180 M at pH 7.4 was obtained; corresponding V _max values were 6 and 11 nmol x min^-1 x mg protein^-1. Uptake with Synechococcus was affected by pH, with affinity decreased and maximal rate increase with rising pH. Acetate uptake was not affected by propionate or butyrate when both were added at the same time, but a light and concentration dependent inhibition developed if suspensions were preincubated with propionate. Acetate carbon moved rapidly into acid insoluble material, but after 10–15 s 75% or more of the recovered intracellular counts were in acetyl CoA. Counts in this compound were reduced by preincubation with propionate.Kinetic measurements of acetyl CoA synthetase in fractionated cell extracts gave values for K _m of about 50 M for acetate, 5 mM for propionate, 100 M for CoA and 0.38 mM for ATP. The internal pool of free CoA was measured to be about 20 M, and was reduced by preincubation with propionate. This suggests that the activity of CoA-mediated reactions may be regulated by the availability of this cofactor.Abbreviations Used CCCP m-Dichlorocarbonyl cyanide phenyl hydrazone - DCMU dichlorophenyl dimethyl urea - TCA trichloroacetic acid - Tris trishydroxymethyl amino methane - HEPES N-2-hydroxyethylpiperazine-N-2-ethane-sulfonic acid     

Sodium dependency of GABA uptake into glial cells in bullfrog sympathetic ganglia

The kinetics of sodium dependency of GABA uptake by satellite glial cells was studied in bullfrog sympathetic ganglia. GABA uptake followed simple Michaelis-Menten kinetics at all sodium concentrations tested. Increasing external sodium concentration increased bothK _m andV _max for GABA uptake, with an increase in theV _max/K _m ratio. The initial rate of uptake as a function of the sodium concentration exhibited sigmoid shape at 100 M GABA. Hill number was estimated to be 2.0. Removal of external potassium ion or 10 M ouabain reduced GABA uptake time-dependently. The effect of ouabain was potentiated by 100 M veratrine. These results suggest that at least two sodium ions are involved with the transport of one GABA molecule and that sodium concentration gradient across the plasma membrane is the main driving force for the transport of GABA. The essential sodium gradient may be maintained by Na⁺, K⁺-ATPase acting as an ion pump.     

Evidence for proton/sulfate cotransport and its kinetics inLemna gibba G1

Sulfate uptake into duckweed (Lemna gibba G1) was studied by means of [³⁵S]sulfate influx and measurements of electrical membrane potential. Uptake was strongly regulated by the intracellular content of soluble sulfate. At the onset of sulfate uptake the membrane potential was transiently depolarized. Fusicoccin stimulated uptake up to 165% of the control even at pH 8. It is suggested that sulfate uptake is energized in the whole pH range by a 3H⁺/sulfate cotransport mechanism. Kinetics of sulfate uptake and sulfate-induced membrane depolarization in the concentration range of 5 M to 1 mM sulfate at pH 5.7 was best described by two Michaelis-Menten terms without any linear component. The second system had a lower affinity for sulfate and was fully active only at sufficiently high proton concentrations.Abbreviations c _o extracellular sulfate concentration - c _i intracellular sulfate concentration - E _m electrical membrane potential difference - E _m sulfate-induced, maximal membrane depolarization - electrochemical proton gradient - FW fresh weight     

Phosphate-limited growth of Chromatium vinosum in continuous culture

Chromatium vinosum DSM 185 was grown in continuous culture at a constant dilution rate of 0.071 h^-1 with sulfide as the only electron donor. The organism was subjected to conditions ranging from phosphate limitation (S _R-phosphate=2.7 M and S _R-sulfide=1.8 mM) to sulfide limitation (S _R-phosphate=86 M and S _R-sulfide=1.8 mM). At values of S _R-phosphate below 7.5 M the culture was washed out, whereas S _R-phosphate above this value resulted in steady states. The saturation constant (K ) for growth on phosphate was estimated to be between 2.6 and 4.1 M. The specific phosphorus content of the cells increased from 0.30 to 0.85 mol P mg^-1 protein with increasing S _R-phosphate. The specific rate of phosphate uptake increased with increasing S _R-phosphate, and displayed a non-hyperbolic saturation relationship with respect to the concentration of phosphate in the inflowing medium. Approximation of a hyperbolic saturation function yielded a maximum uptake rate (V _max) of 85 nmol P mg^-1 protein h^-1, and a saturation constant for uptake (K _t) of 0.7 M. When phosphate was supplied in excess 8.5% of the phosphate taken up by the cells was excreted as organic phosphorus at a specific rate of 8 nmol P mg^-1 protein h^-1.Non-standard abbreviations BChla bacteriochlorophyll a - D dilution rate; _max, maximum specific growth rate - maximum specific growth rate if the substrate were not inhibitory - K saturation constant for growth on phosphate - V _max maximum rate of phosphate uptake - K _i saturation constant for phosphate uptake - K _i inhibition constant for growth in the presence of sulfide - S _R concentration of substrate in the inflowing medium