IRON STIMULATION OF PHOTOSYNTHESIS AND NITROGEN FIXATION IN ANABAENA 7120 AND TRICHODESMIUM (CYANOPHYCEAE)1

Iron availability may limit carbon and nitrogen fixation in the oceans. The freshwater cyanobacterium, Anabaena, was used as a laboratory model for the biochemical and physiological effects of iron. Increased iron nutrition, in the range of 10^?8 M to 10^?6 M resulted in increases of approximately four fold in carbon and nitrogen fixation rates. Chlorophyll concentration increased, and the relative amount of in vivo fluorescence was reduced with more iron. Natural samples of Trichodesmium, collected off Barbados and incubated with increased iron for two days, showed similar effects. Trichodesmium responded to iron additions indicating that it may be Fe limited in its natural environment. These responses to iron are consistent with the biochemical roles of iron in photosynthesis and nitrogen fixation. The results are discussed in the geochemical context of the sporadic total iron input to tropical oceans and possible implications to spatial and temporal patterns of productivity.     

INTERACTIONS BETWEEN NITRATE UPTAKE AND NITROGEN FIXATION IN CONTINUOUS CULTURES OF THE MARINE DIAZOTROPH TRICHODESMIUM (CYANOBACTERIA)1

Diazotrophic cyanobacteria can take up combined nitrogen (nitrate, ammonium, amino acids, dissolved organic nitrogen) from solution, but the interaction between N₂ fixation and uptake of combined nitrogen is not well understood. We studied the effects of combined nitrogen ) additions on N₂ fixation rates in the cyanobacterium Trichodesmium erythraeum (IMS‐101) maintained in continuous culture in an N‐free medium (YBCII) and a 12:12‐h light:dark cycle. We measured acetylene reduction rates, nutrient concentrations, and biomass throughout the 12 h of illumination after the addition of nitrate (0.5–20 μM) at the start of the light period. Compared with unamended controls, Trichodesmium showed strong inhibition of acetylene reduction (up to 70%) in the presence of , with apparent saturation of the inhibition effect at an initial concentration of approximately 10 μM. The inhibition of acetylene reduction persisted through much of the light period as concentration in the culture vessel decreased. Recovery of N₂ fixation was observed late in the light period in cultures amended with low concentrations of (<5 μM) when ambient concentrations had decreased to 0.3–0.4 μM in the culture vessel. Nitrate uptake accounted for as much as 86% of total N uptake and, at the higher treatment concentrations, more than made up for the observed decrease in N₂ fixation rates. We conclude that Trichodesmium can obtain significant quantities of N through uptake of nitrate and does so in preference to N₂ fixation when sufficient is available.     

NITROGEN FIXATION,HYDROGEN CYCLING,AND ELECTRON TRANSPORT KINETICS IN TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA) STRAIN IMS1011

This study describes the relationships between dinitrogen (N₂) fixation, dihydrogen (H₂) production, and electron transport associated with photosynthesis and respiration in the marine cyanobacterium Trichodesmium erythraeum Ehrenb. strain IMS101. The ratio of H₂ produced:N₂ fixed (H₂:N₂) was controlled by the light intensity and by the light spectral composition and was affected by the growth irradiance level. For Trichodesmium cells grown at 50 μmol photons · m^?2 · s^?1, the rate of N₂ fixation, as measured by acetylene reduction, saturated at light intensities of 200 μmol photons · m^?2 · s^?1. In contrast, net H₂ production continued to increase with light levels up to 1,000 μmol photons · m^?2 · s^?1. The H₂:N₂ ratios increased monotonically with irradiance, and the variable fluorescence measured using a fast repetition rate fluorometer (FRRF) revealed that this increase was accompanied by a progressive reduction of the plastoquinone (PQ) pool. Additions of 2,5‐dibromo‐3‐methyl‐6‐isopropyl‐p‐benzoquinone (DBMIB), an inhibitor of electron transport from PQ pool to PSI, diminished both N₂ fixation and net H₂ production, while the H₂:N₂ ratio increased with increasing level of PQ pool reduction. In the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), nitrogenase activity declined but could be prolonged by increasing the light intensity and by removing the oxygen supply. These results on the coupling of N₂ fixation and H₂ cycling in Trichodesmium indicate how light intensity and light spectral quality of the open ocean can influence the H₂:N₂ ratio and modulate net H₂ production.     

INTERACTION OF NITROGEN FIXATION AND PHOSPHORUS LIMITATION IN APHANIZOMENON FLOS-AQUAE (CYANOPHYCEAE)1

The effect of simultaneous nitrogen fixation and phosphorus limitation on the physiological adaptation and growth performance of Aphanizomenon flos-aquae (L.) Ralfs PCC 7905 was studied in continuous culture. In the absence of ammonia, N₂ fixation occurred and the maximum growth rate (as determined in diluted batch cultures) was lower. However, no distinction could be made between the steady-state N uptake rates (based on cellular N contents) of N₂-fixing cells and cells grown with ammonia. At the higher dilution rates, the residual P concentration increased with increasing dilution rate, more so under N₂-fixing conditions, compared to the cultures grown in the presence of ammonia. More generally, the yield of biomass per consumed P, as the biomass concentration itself, decreased with increasing dilution rate, and both were lower under N₂-fixing conditions. The restricted biomass production under N₂-fixing conditions suggests that reduction of N loading may benefit lake restoration projects. The influence of N₂-fixation on the severity of P limitation is discussed in terms of metabolic control analysis. From the increase of the residual P concentration on switching from ammonium to N₂-fixing conditions, it is deduced that under N₂-fixing and P-limited conditions, control of growth is shared by N and P metabolism.     

GROWTH AND NITROGEN FIXATION OF THE DIAZOTROPHIC FILAMENTOUS NONHETEROCYSTOUS CYANOBACTERIUM TRICHODESMIUM SP. IMS 101 IN DEFINED MEDIA: EVIDENCE FOR A CIRCADIAN RHYTHM1

Trichodesmium sp. IMS 101, originally isolated from coastal western Atlantic waters by Prufert-Bebout and colleagues and maintained in seawater-based media, was successfully cultivated in two artificial media. Its characteristics of growth, nitrogen fixation, and regulation of nitrogen fixation were compared to those of natural populations and Trichodesmium sp. NIBB 1067. Results indicate that the culture grown in artificial media had nitrogen fixation characteristics similar to those when the culture is grown in seawater-based medium and to those of Trichodesmium sp. in the natural habitat. The study provides practical artificial media to facilitate the physiological studies of these important diazotrophic cyanobacteria, as well as the cultivation of other Trichodesmium species in future studies. Manipulations of the light/dark cycle were performed to determine whether or not the daily cycle of nitrogen fixation is a circadian rhythm. Cultures grown under continuous light maintained the cycle for up to 6 days. We demonstrated that the daily cycle of nitrogen fixation in Trichodesmium sp. IMS 101 was at least partially under the control of a circardian rhythm.     

OXYGEN-INDUCED CHANGES IN MORPHOLOGY OF AGGREGATES OF APHANIZOMENON FLOS-AQUAE (CYANOPHYCEAE): IMPLICATIONS FOR NITROGEN FIXATION POTENTIALS1

The filamentous nitrogen-fixing cyanobacterium Aphanizomenon flos-aquae (L.) Ralfs forms bundle or fake shaped aggregates which can provide buoyancy control, protection against intense illumination, enhancement of phycosphere nutrient regeneration, and which may result from size-selective herbivory by zooplankton. The dimensions of aggregates can change quickly. In this study, after a period of darkness, illumination caused aggregates to elongate approximately five-fold over a 10-15 min period. The metamorphosis was reversible upon cessation of illumination and through successive light-dark cycles. Manipulations of environmental oxygen concentration and photosystem Ü activity (via DCMU amendment), together with measurements made inside flakes with O₂-sensitive microelectrocles, showed that the metamorphosis was a response to oxygen concentration and operated to enhance diffusive efflux of photosynthetically produced oxygen during illumination. During darkness oxygen concentration within contracted aggregates became severely depleted relative to the environment. We propose that metamorphic minimization of local oxygen concentration is an adaptation that enhances the ability of Aphanizomenon flos-aquae to fix atmospheric nitrogen via the oxygen-labile nitrogenase enzyme system.     

RESPONSE OF MARINE SYNECHOCOCCUS (CYANOPHYCEAE) CULTURES TO IRON NUTRITION1

Three clones of marine Synechococcus (WH6501, WH7803, and WH8018) were grown through at least three transfers, at 6-day intervals, in synthetic medium with total iron concentrations from 10^?9 to 10^?6 M. After 6 days of exponential growth, these cultures were harvested, and the cell density and protein and pigment concentrations were measured. Aliquots of the culture were assayed for their carbon fixation rates at two light intensities. Cell density and protein concentration increased by up to 7.8 times over a range of iron from the lowest (10^?9 M) to the highest concentrations (10^?6 M). The concentration of chlorophyll-a and phycobiliproteins showed a wider range of response, increasing by up to 48 times. The carbon fixation rate (per mL of culture) also increased approximately 40 times over the total range of iron concentration. The ranges of these biochemical and physiological responses were much lower than the range of total available iron, which was 1000-fold, and the range of total cellular iron, which was estimated to be about 160-fold. This “less-than-linear” relationship indicates that the cells are adapting to make more efficient use of iron under limiting conditions. Our results demonstrate characteristics of iron-limited Synechococcus that may be important in understanding the relationships between primary productivity and iron availability in the oceans.     

INTERACTIVE EFFECTS OF IRRADIANCE AND CO2 ON CO2 FIXATION AND N2 FIXATION IN THE DIAZOTROPH TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA)1

The diazotrophic cyanobacteria Trichodesmium spp. contribute approximately half of the known marine dinitrogen (N₂) fixation. Rapidly changing environmental factors such as the rising atmospheric partial pressure of carbon dioxide (pCO₂) and shallower mixed layers (higher light intensities) are likely to affect N₂‐fixation rates in the future ocean. Several studies have documented that N₂ fixation in laboratory cultures of T. erythraeum increased when pCO₂ was doubled from present‐day atmospheric concentrations (～380 ppm) to projected future levels (～750 ppm). We examined the interactive effects of light and pCO₂ on two strains of T. erythraeum Ehrenb. (GBRTRLI101 and IMS101) in laboratory semicontinuous cultures. Elevated pCO₂ stimulated gross N₂‐fixation rates in cultures growing at 38 μmol quanta · m^?2 · s^?1 (GBRTRLI101 and IMS101) and 100 μmol quanta · m^?2 · s^?1 (IMS101), but this effect was reduced in both strains growing at 220 μmol quanta · m^?2 · s^?1. Conversely, CO₂‐fixation rates increased significantly (P < 0.05) in response to high pCO₂ under mid‐ and high irradiances only. These data imply that the stimulatory effect of elevated pCO₂ on CO₂ fixation and N₂ fixation by T. erythraeum is correlated with light. The ratio of gross:net N₂ fixation was also correlated with light and trichome length in IMS101. Our study suggests that elevated pCO₂ may have a strong positive effect on Trichodesmium gross N₂ fixation in intermediate and bottom layers of the euphotic zone, but perhaps not in light‐saturated surface layers. Climate change models must consider the interactive effects of multiple environmental variables on phytoplankton and the biogeochemical cycles they mediate.     

PHYSIOLOGICAL PROFILES OF SYNECHOCOCCUS (CYANOPHYCEAE) IN IRON-LIMITING CONTINUOUS CULTURES1

We examined the physiology and biochemistry associated with the iron-limited continuous culture of the halotolerant cyanobacterium Synechococcus PCC 7002. Biomass production, photosynthetic pigment levels, photosynthetic efficiency, and the production of hydroxamate- and catechol-type siderophores are reported for cells grown over a range of available iron concentrations. The relationship between the yield of Synechococcus PCC 7002 in iron-limited chemostats and the concentration of available iron was not linear. Synechococcus PCC 7002 expressed an inducible physiological response that led to alterations either in the cellular iron quotient or, more likely, in levels of available iron due to induced iron-scavenging processes. During iron limitation these cyanobacteria produced components consistent with the activation of a high-affinity iron transport system; both hydroxamate- and catechol-type siderophores were detected. Iron-limited Synechococcus PCC 7002 also reduced CO₂ fixation rates from luxury levels to a rate that matched the cellular growth rate, presenting interesting implications for oceanic carbon flux models.     

GROWTH AND PHOTOSYNTHESIS OF MARINE SYNECHOCOCCUS (CYANOPHYCEAE) UNDER IRON STRESS

Prokaryotic picoplankton such as Synechococcus are relatively abundant in putatively Fe-limited high-nutrient, low-chlorophyll (HNLC) regions of the oceans. The physiology of Synechococcus under Fe stress has been studied less than eukaryotic algae. Recent evidence suggests that although biomass and growth rates of Synechococcus are not typically Fe limited in situ, cells may still exhibit symptoms of Fe stress. We grew Synechococcus A2169 and WH7803 in laboratory batch cultures in the artificial medium Aquil and enriched natural seawater, at a series of Fe concentrations and Fe:macronutrient ratios, and with either nitrate or ammonium as the sole nitrogen source. Cell yields, and in some experiments exponential specific growth rate (μ), were more readily Fe limited in the Atlantic isolate WH7803 than in the equatorial Pacific isolate A2169. In both strains, final cell yields spanned about an order of magnitude and decreased continuously with Fe concentration from 900 to 3.6 nM (150 μM N, 10 μM P), whereas μ decreased much less and only at Fe concentrations below 90 nM. Synechococcus yield was controlled by both absolute Fe concentration and Fe:macronutrient ratio, but μ was determined primarily by absolute Fe concentration. Contrary to theoretical predictions, neither yield nor μ was higher in Fe-limited cells grown in ammonium compared to nitrate. Under severe Fe stress, cellular chlorophyll (Chl) content and light-saturated gross photosynthetic capacity (P^cell_m) decreased proportionately, and dark respiration (R^cell_d) increased, such that net P^cell_m was extremely low but gross P^Chl_m was unchanged. This is the first report of an absolute increase in R^cell_d under Fe stress in phytoplankton.     

NUTRIENT CONTROLS ON NITROGEN UPTAKE AND METABOLISM BY NATURAL POPULATIONS AND CULTURES OF TRICHODESMIUM (CYANOBACTERIA)

The effects of inorganic nutrient (ammonium [NH₄ ⁺ ] and nitrate [NO₃ ⁻ ]) and amino acid (glutamate [glu] and glutamine [gln]) additions on rates of N₂ fixation, N uptake, glutamine synthetase (GS) activity, and concentrations of intracellular pools of gln and glu were examined in natural and cultured populations of Trichodesmium. Additions of 1 μM glu, gln, NO₃ ⁻ , or NH₄ ⁺ did not affect short-term rates of N₂ fixation. This may be an important factor that allows for continued N₂ fixation in oligotrophic areas where recycling processes are active. N₂ fixation rates decreased when nutrients were supplied at higher concentrations (e.g. 10 μM). Uptake of combined N (NH₄ ⁺ , NO₃ ⁻ , and amino acids) by Trichodesmium was stimulated by increased concentrations. For NO₃ ⁻ , proportional increases in NO₃ ⁻ uptake and decreases in N₂ fixation were observed when additions were made to cultures before the onset of the light period. GS activity did not change much in response to the addition of NH₄ ⁺ , NO₃ ⁻ , glu, or gln. GS is necessary for N metabolism, and the bulk of this enzyme pool may be conserved. Intracellular pools of glu and gln varied in response to 10 μM additions of NH₄ ⁺ , glu, or gln. Cells incubated with NH₄ ⁺ became depleted in intracellular glu and enriched with intracellular gln. The increase in the gln/glu ratio corresponded to a decrease in the rate of N₂ fixation. Although the gln/glu ratio decreased in cells exposed to the amino acids, there was only a corresponding decrease in N₂ fixation after the gln addition. The results presented here suggest that combined N concentrations on the order of 1 μM do not affect rates of N₂ fixation and metabolism, although higher concentrations (e.g. 10 μM) can. Moreover, these effects are exerted through products of NH₄ ⁺ assimilation rather than exogenous N, as has been suggested for other species. These results may help explain how cultures of Trichodesmium are able to simultaneously fix N₂ and take up NH₄ ⁺ and how natural populations continue to fix N₂ once combined N concentrations increase within a bloom.     

GENOTYPIC RELATIONSHIPS IN TRICHODESMIUM (CYANOPHYCEAE) BASED ON nifH SEQUENCE COMPARISONS1

The DNA sequence of a fragment of nifH was compared to natural populations of the marine cyanobacteria Trichodesmium thiebautii and T. erythraeum from the Caribbean Sea and the unialgal culture Trichodesmium sp. NIBB 1067, which was isolated from the Kuroshio waters (Japan). Through replication Of amplification, cloning, and sequencing, four nucleotides in a 359-bp fragment were identified that were identical in sequence to Trichodesmium sp. NIBB 1067 and natural populations of T. erythraeum but were distinctly different in sequence from T. thiebautii. The data indicate that Trichodesmium sp. NIBB 1067 is more closely related to T. erythraeum than to T. thiebautii.     

SPATIAL SEGREGATION OF CO2 FIXATION IN TRICHODESMIUM SPP.: LINKAGE TO N2 FIXATION POTENTIAL1

The aggregate-forming, nonheterocystous, filamentous blue-green alga (cyanobacteria) Trichodesmium spp. is a widespread and important planktonic N₂ fixer and primary producer in tropical and subtropical oceans. It is unique among nonheterocystous genera because it conducts N₂ and CO₂ fixation (O₂ evolution) simultaneously; a notable achievement, because O₂ is a potent inhibitor of N₂ fixation. Spatial and temporal CO₂ fixation patterns were examined in trichomes and aggregates from natural and cultured populations, utilizing microautoradiographic detection of ¹⁴CO₂ incorporation. Parallel N₂ fixation (acetylene reduction) measurements were also made. Diel N₂ and CO₂ fixation patterns were similar, with co-optimization of both processes near midday. Microautoradiographs revealed several trichome-level ¹⁴CO₂ incorporation patterns: 1)uniform, heavy labeling, 2)uniform, light labeling, 3) heavier labeling in distal as opposed, to proximal regions, and 4) virtually no labeling throughout. Similar patterns were observed in natural and cultured populations. Given previous immunochemical findings that N₂ fixation potential is widespread in Trichodesmium spp. trichomes and aggregates, current results suggest a high degree of individuality, and possibly a “division of labor” in terms of CO₂ fixation, among trichomes comprising active N₂-fixing aggregates. Segregation of photosynthesis within and among trichomes facilitates simultaneous N₂ and CO₂ fixation in Trichodesmium spp. trichomes and aggregates.     

INFLUENCE OF IRON LIMITATION AND NITROGEN SOURCE ON GROWTH AND SIDEROPHORE PRODUCTION BY CYANOBACTERIA1

To characterize the mobilization and uptake of iron by cyanobacteria, 14 species were screened for ability to scavenge iron in a competitive system. The cyanobacteria exhibited a range of growth responses to iron limitation which could be separated into three groups, and a representative species from each group was chosen for further study. Effects of iron-limitation on growth and siderophore production of Anacystis nidulans R2, Anabaena variabilis ATCC 29413, and Plectonema boryanum UTEX 581 were determined. Both A. nidulans R2 and A. variabilis showed a reduced rate of growth with decreased available iron concentration (PFe 17–19). Growth rates increased with further reduction in the level of available iron (pFe 20 to pFe 21). The increase in growth rate occurred at the same available iron concentration as the initiation of extracellular siderophore production. In contrast, the growth of P. boryanum decreased with decreasing available iron levels. No siderophore production was detected from P. boryanum cultures. The growth kinetics of siderophore-producing species differ from traditional nutrient-limited growth kinetics and clearly reflect the presence of a high affinity, siderophore-mediated iron transport system in A. nidulans R2 and A. variabilis. Iron-limited growth kinetics more similar to traditional nutrient-limited growth kinetics were found in P. boryanum. The available nitrogen source influenced amount of siderophore produced and concentration of available iron which induced siderophore production. Siderophores were produced at high iron concentrations (pFe 18) when A. variablilis cultures were grown in the absence of combined nitrogen source. When nitrate was supplied to the culture, iron concentrations had to be reduced to pFe 20 before siderophores were produced. Cells grown on nitrogen also produced greater than two times the amount of siderophore compared with nitrate grown cells. This may be indicative of an increased demand for iron by nitrogen fixing A. variabilis Cultures.     

GAS VACUOLE COLLAPSE IN MARINE OSCILLATORIA (TRICHODESMIUM) THIEBAUTII (CYANOPHYTA) AND THE EFFECT ON NITROGENASE ACTIVITY AND PHOTOSYNTHESIS1

Photosynthesis and nitrogenase activity (acetylene reduction) in Oscillatoria (Trichodesmium) thiebautii were significantly reduced after its gas vesicles were collapsed by pressurizing to 6·2 MPa (62 bars). The reduction in nitrogenase activity was observed with both intact and disrupted colonies, and under both aerobic and microaerobic conditions. It is unlikely, therefore, that the reduction in either process results solely from loss of gas diffusion pathways, and may simply originate from disturbance of the cellular architecture as gas vesicles are destroyed. Since nitrogenase activity of the samples was affected by gas vesicle collapse, the process presumably resides in the gas vacuolate organism (Oscillatoria) rather than in any associated bacteria which might be present.     

EFFECT OF EDTA ADDITIONS ON NATURAL TRICHODESMIUM SPP. (CYANOPHYTA) POPULATIONS1

Despite nearly two decades of intensive research, many questions regarding the physiology and ecology of the marine, non‐heterocystous cyanobacterium, Trichodesmium, remain unresolved. We note here the effect of EDTA (ethylenediaminetetraacetate) on N₂ fixation by Trichodesmium, and the use of EDTA as a means of extending the viability of natural Trichodesmium spp. populations. We examined nitrogenase activity (NA) as a function of EDTA concentration, time of collection, light level, and iron addition. Samples collected early in the day and treated with EDTA maintain a steady rate of activity for hours longer than controls. Furthermore, samples preincubated through the night with EDTA were active the next morning, compared with controls that were inactive. The discovery that (10–50 μM) low concentrations of EDTA prolong the duration of NA of Trichodesmium during experimental manipulations without affecting the rate of acetylene reduction allows for longer term manipulative experiments to be conducted.     

A REVISED ESTIMATE OF THE IRON USE EFFICIENCY OF NITROGEN FIXATION,WITH SPECIAL REFERENCE TO THE MARINE CYANOBACTERIUM TRICHODESMIUM SPP. (CYANOPHYTA)1

Estimates of the iron use efficiency (IUE) for diazotrophic plant growth have been used to suggest iron limitation of marine N₂ fixation. However, in the course of these inferences, neither the physiological complexity of these estimates nor the specific physiological parameters of marine diazotrophs were evaluated. Here, a semiempirical prediction of the IUE of diazotrophic growth for Trichodesmium was computed from considerations of the Fe content and reaction rates of the nitrogenase complex and PSI:PSII ratios, as well as field measurements of Mehler activity, cellular Fe‐superoxide dismutase activity, and diel variability in C and N₂ fixation. With a PSI:PSII ratio of 1 and 48% Mehler activity, the instantaneous IUE (0.33 mol C fixed·mol cellular Fe ^{? 1} 1 Received 16 August 2001. Accepted 7 October 2002. ·s ^{? 1} 1 Received 16 August 2001. Accepted 7 October 2002. ) was only 4‐fold lower than that calculated for a phytoplankter growing on reduced N. We computed a range of daily integrated IUE values from 2900 to 7700 mol C·mol Fe ^{? 1} 1 Received 16 August 2001. Accepted 7 October 2002. ·d ^{? 1} 1 Received 16 August 2001. Accepted 7 October 2002. , accounting for the diel variability in C and N₂ fixation as well as the uncertainties in cyanobacterial nitrogenase biochemistry and PSI:II ratios of field‐collected Trichodesmium. The lowest observed Fe‐superoxide dismutase:C quota of 2.9 (μmol:mol) suggests a maintenance requirement for this enzyme. The maintenance Fe:C requirement of 13.5 μmol:mol (derived from cultures of Trichodesmium IMS 101) and values of the IUE yielded an Fe requirement ranging from 27 to 48 Fe:C (μmol:mol) to achieve a diazotrophic growth rate of 0.1 d ^{? 1} 1 Received 16 August 2001. Accepted 7 October 2002. . Based on these predicted requirements, the Fe:C contents of Caribbean Sea and most North Atlantic Ocean populations sampled thus far exceed that required to support the observed rates of N₂ fixation.     

BIOENERGETIC PROCESSES ARE MODIFIED DURING NITROGEN STARVATION AND RECOVERY IN PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

In the non-N₂-fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH-I-pCl₁), N starvation induced an increase in the rate of respiration and a decrease in the rate of O₂ evolution. When NO₃^? was added to illuminated N-starved cells, O₂ evolution immediately increased to levels shown by NO₃^? grown cells, even though N-starved cells had lost most of their in vitro photosynthetic activities. Stimulation of noncyclic electron flow was maximal under light-saturating conditions and after 2–3 days of N starvation. The respiratory rate of N-starved cells was stimulated by the addition of NO₃^? or NH₄⁺ and partially inhibited at very low irradiances, even in the presence of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). Results indicate that N-starved cells obtain the energy supply for N assimilation through a process different from that used by N-sufficient cells. N-starved cells were able to take up NO₃^? in the dark and when illuminated in the presence of DCMU under anaerobiosis. Following NO₃^? addition, the photosynthetic yield of the in vivo noncyclic electron transport slightly increased, whereas it decreased after NH₄⁺ addition. Addition of NO₃^? or NH₄⁺ favored photoinhibition of photosystem II, the effect being faster after NH₄⁺ addition.     

氮肥对覆膜花生固氮活性与产量的影响

不施氮肥花生固氮力为106～146公斤N/顷。施用高量氮肥对花生结瘤不利,用量10、20、40、60公斤NH_4NO_3/亩时,花生固氮率分别降低24、47、67、82%,花生约增产10%,在施低量氮肥(5公斤NH_4NO_3/亩)时不影响花生结瘤,且对花生有增产作用。有机肥对花生结瘤的抑制作用比无机氮肥轻,在2与4吨/亩(0.6％N)用量时,花生固氮作用降低7与23%,花生约增产8.0％     

NITROGEN UTILIZATION AND METABOLISM RELATIVE TO PATTERNS OF N2 FIXATION IN CULTURES OF TRICHODESMIUM NIBB1067

We measured uptake kinetics for four combined N sources, ambient rates of N uptake and N₂ fixation, glutamine synthetase activity (transferase and biosynthetic), and concentrations of intracellular pools of glutamate (glu) and glutamine (gln) in cultures of Trichodesmium NIBB1067. N dynamics and metabolism were examined to assess the relative importance of N₂ fixation and N uptake to Trichodesmium nutrition. Comparisons were made between cultures grown on medium without added N, with excess NO, or with excess urea. Of the combined N sources tested, Trichodesmium NIBB1067 had the highest affinity for NH; high uptake capacities for NH, urea, and glu; and little capacity for NO uptake. In cultures grown on medium without added N, NH accumulated in the medium during growth, resulting in high NH uptake rates relative to rates of N₂ fixation. Glu uptake rates were low but consistent throughout the diel period. In cultures grown on excess NO or urea, uptake of these compounds supplied the majority of the daily N demand, although some N₂ fixation occurred during the light period. NO uptake rates were reduced when N₂-fixation rates were high. In all of the cultures, the highest gln/glu ratios and the lowest glutamine synthetase transferase/biosynthetic ratios were observed during the period when rates of total N uptake were highest. In cultures growing exponentially on medium without added N, N₂ fixation accounted for 14%– 16% of the total daily N uptake. Uptake of NH and glu, presumably regenerated within the culture vessels, represented 84%–86% of the daily N uptake. Because these systems were closed, net growth was constrained by the rate at which N₂ could be fixed into the system. However, total daily N turnover was greater than that necessary to accommodate the observed increase in culture biomass. The rapid N turnover rates observed in these cultures may support gross productivity and balance the high rates of C fixation observed in natural populations of Trichodesmium.