Nitrogen Fixation of Faba Bean (Vicia faba L.) Interacting with a Non-legume in Two Contrasting Intercropping Systems

A field experiment was carried out to quantify biological nitrogen fixation (BNF) using the ¹⁵N isotope natural abundance method in maize (Zea mays L.)/faba bean (Vicia faba L.) and wheat (Triticum aestivum L.)/faba bean intercropping systems. Faba bean was yielding more in the maize/faba bean intercropping, but not in the wheat/faba bean intercropping. Biomass, grain yield and N acquisition of faba bean were significantly increased when intercropped with maize, and decreased significantly with wheat, irrespective of N-fertilizer application, indicating that the legume could gain or lose productivity in an intercropping situation. There was yield advantage of maize/faba bean intercropping, but no in wheat/faba bean intercropping. The grain yield of the faba bean intercropped with maize was greater than that of faba bean monoculture due to increases of the stems per plant and the pods per stem of faba bean. N fertilization inhibited N fixation of faba bean in maize/faba bean and wheat/faba bean intercropping and faba bean monoculture. The responses of different cropping systems to N-fertilizer application, however, were not identical, with competitive intercropping (wheat/faba bean) being more sensitive than facilitative intercropping (maize/faba bean). Intercropping increased the percentage of N derived from air (%Ndfa) of the wheat/faba bean system, but not that of the maize/faba bean system when no N fertilizer was applied. When receiving 120 kg N/ha, however, intercropping did not significantly increase %Ndfa either in the wheat/faba bean system or in the maize/faba bean system in comparison with faba bean in monoculture. The amount of shoot N derived from air (Ndfa), however, increased significantly when intercropped with maize, irrespective of N-fertilizer application. Ndfa decreased when intercropped with wheat, albeit not significantly at 120 kg N/ha. Ndfa was correlated more closely with dry matter yield, grain yield and competitive ratio, than with %Ndfa. This indicates that that total dry matter yield (sink strength), not %Ndfa, was more critical for the legume to increase Ndfa. The results suggested that N fixation could be improved by yield maximization in an intercropping system.     

A simple method for identification of S phase nuclei in Vicia faba root meristems using BrdUrd labeling and indirect immunofluorescence (comparison with 3H-thymidine incorporation)

A modified immunocytochemical method for identification of S-phase cells in root meristems is desribed. The technique combines incorporation of BrdUrd, rapid isolation of cell nuclei and indirect immunocytochemical detection of BrdUrd-labeled areas of chromatin using monoclonal anti-BrdUrd (I) and FITC-conjugated (II) antibodies. The labeling indexes calculated using ³H-thymidine autoradiography and BrdUrd-immunofluorescence in root meristems of Vicia faba subsp. minor were found comparable. However, the increased sensitivity of fluorescent staining reveals an enhanced complexity of visible structure emerging during successive periods of S-phase, allowing for discriminating not only quantitative but also qualitative aspects of nuclear labeling patterns characteristic for specific periods of DNA replication.     

Phloem loading in Vicia faba leaves: Effect of N-ethylmaleimide and parachloromercuribenzenesulfonic acid on H+ extrusion,K+ and sucrose uptake

The effects of a penetrating (NEM) and a non-penetrating (PCMBS) sulfhydryl-specific reagent on proton extrusion, ⁸⁶Rb and [U-¹⁴C]sucrose uptake by Vicia faba leaves have been studied. Proton extrusion was strongly or completely inhibited by 0.1 mM NEM. ⁸⁶Rb and [U-¹⁴C]sucrose uptake were markedly reduced by NEM concentrations equal to or higher than 0.5 mM. Under our experimental conditions, PCMBS (1 mM) exerted a strong inhibition on [¹⁴C]sucrose uptake but did not inhibit proton extrusion and ⁸⁶Rb uptake. The sensitivity of phloem loading to PCMBS is thought to be a consequence of sugar-carrier blockage and not of inhibition of the proton pump.Abbreviations CCCP carbonylcyanide-m-chlorophenylhydrazone - DES diethylstilbestrol - DCCD dicyclohexylcarbodiimide - FC Fusicoccin - NEM N-ethylmaleimide - PCMBS p-chloromercuribenzenesulfonic acid     

The effect of red and far-red light on proton secretion from mesophyll-cell protoplasts of Vicia faba L.

The influence of red and far-red irradiation on the transport of H⁺ and ⁸⁶Rb⁺ through the plasmalemma was studied using parenchymal protoplasts isolated from Vicia faba leaves. The results indicate that red light stimulates H⁺ secretion and the uptake of ⁸⁶Rb⁺. Moreover, it has been demonstrated that far-red irradiation acts antagonistically with respect to red light in both these processes.     

Sugar uptake by the dermal transfer cells of developing cotyledons of Vicia faba L.

The mechanism of carrier-mediated sucrose uptake by the dermal transfer cells of developing Vicia faba L. cotyledons was studied using excised cotyledons and isolated transfer cell protoplasts. Addition of sucrose resulted in a transitory alkalinization of the bathing solution whereas additions of glucose, fructose or raffinose had no effect. Dissipating the proton motive force by exposing cotyledons and isolated transfer cell protoplasts to an alkaline pH, carbonylcyanide m-chlorophenylhydrazone, weak acids (propionic acid and 5,5-dimethyl-oxazolidine-2,4-dione) or tetraphenylphos-phonium ion resulted in a significant reduction of sucrose uptake. The ATPase inhibitors, erythrosin B (EB), diethylstilbestrol (DES) and N,N-dicyclohexylcarbodiimide (DCCD) were found to abolish the sucrose-induced medium alkanization as well as reduce sucrose uptake. Cytochemical localization of the ATPase, based on lead precipitation, demonstrated that the highest activity was present in the plasma membranes located in wall ingrowth regions of the dermal transfer cells. The presence of a transplasma-membrane redox system was detected by the extracellular reduction of the electron acceptor, hexacyanoferrate III. The reduction of the ferric ion was coupled to a release of protons. The redox-induced proton extrusion was abolished by the ATPase inhibitors EB, DES and DCCD suggesting that proton extrusion was solely through the H⁺-ATPase. Based on these findings, it is postulated that cotyledonary dermal transfer cells take up sucrose by a proton symport mechanism with the proton motive force being generated by a H ⁺ -ATPase. Sucrose uptake by the storage parenchyma and inner epidermal cells of the cotyledons did not exhibit characteristics consistent with sucrose-proton symport.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - DCCD N,N-dicyclohexylcarbodiimide - DES diethylstilbestrol - EB erythrosin B - Em membrane potential - FC fusicoccin - HCF II hexacyanoferrate II - HCF III hexacyanoferrate III - Mes 2-(N-morpholino)ethanesulfonic acid - pmf proton motive force - TPP⁺ tetraphenylphosphonium ion The investigation was supported by funds from the Research Management Committee, The University of Newcastle and the Australian Research Council. One of us, R. McDonald, gratefully acknowledges the support of an Australian Postgraduate Research Award. We are indebted to Stella Savory for preparing the ultrathin sections for electron microscopy.     

Asymmetric distribution of the plasma-membrane H+-ATPase in embryos of Vicia faba L. with special reference to transfer cells

The ultrastructural localization of the plasma-membrane H⁺ -ATPase by immunocytochemistry was studied in Vicia faba embryos which absorb nutrients from the maternal organism through the transfer cells of their external epidermis. The samples were embedded in LR White resin and the specificity of immunolabelling was checked by inhibition in the presence of purified H⁺-ATPase. The following results were obtained: (i) The H⁺-ATPase density varied according to the cell type, being higher in transfer cells than in other cell types, especially the non-modified cells of the internal epidermis. (ii) There was a marked polarity in transfer cells as proton pumps were more numerous in the area of plasmalemma infoldings where active nutrient uptake is assumed to take place, (iii) No clear immunolabelling occurred on the plasma membrane of plasmodesmata. These results demonstrate that in transfer cells the area of plasmalemma infoldings is highly specialized for active solute transport; they also support the idea of specific structural properties of the plasmalemma in plasmodesmata.This work was supported by the Centre National de la Recherche Scientifique (URA CNRS 574). We express our gratitude to Dr M.G. Palmgren (Royal Veterinary and Agricultural University, Copenhagen, Denmark) for his gift of purified H⁺-ATPase. We wish to thank J.C. Fromont for his skillful technical assistance with the immunological procedures. We are grateful to J.M. Perault and C. Besse of the Electron Microscopy Service (Service Universitaire de Microscopie Electronique Appliquée à la Biologie Poitiers, France) for their contribution to the microscopical techniques.     

Studies on the movement and distribution of ethylene in Vicia faba L.

In Vicia faba ethylene does not appear to move between different parts of the plant in physiologically significant amounts. The resistance to longitudinal movement is such that lateral emanation effectively isolates different parts of the plant from each other. When emanation is prevented, ethylene can be channelled to any part of the plant. Exposure of one section of a plant to ¹⁴C-labelled ethylene (up to 200 l/l) increased the internal concentration in other parts with ethylene that did not originate from the feeding chamber. A basipetal gradient of endogenous ethylene concentration was found in the lacuna of intact plants, the source of ethylene being the stem tissue. The permeability of stem tissue to ethylene decreases with age. The concentration of ethylene in tissues surrounding the lacuna is always higher than that in the lacuna and it is argued that compartmentation of ethylene occurs within these tissues.     

Identification and biochemical characterization of the plasma-membrane H+-ATPase in guard cells of Vicia faba L.

The plasma-membrane H⁺-pump in guard cells generates the driving force for the rapid ion fluxes required for stomatal opening. Since our electrophysio-logical studies revealed a two fold higher pump-current density in guard cells than in mesophyll cells of Vicia faba L. we elucidated the biochemical properties of this proton-translocating ATPase in plasma-membrane vesicles isolated from both cell types. The capability of the H⁺ —ATPase to create an H⁺ gradient is maintained in plasma-membrane vesicles derived from purified guard cells via blender maceration, high-pressure homogenization and polymer separation. The H⁺-pumping activity of these vesicles coincides with the presence of two polypeptides of approx. 100 and 92 kDa which are recognized by a monoclonal antibody raised against the plasma-membrane H⁺-ATPase from Zea mays L. coleoptiles. Comparison of H⁺-pumping activities of isolated membranes revealed an approximately two fold higher activity in guard cells than in mesophyll cells with respect to the total membrane protein content. Furthermore, we demonstrated by western blotting that the difference in pump activities resulted from a higher abundance of the electroenzyme per unit membrane protein in guard-cell plasma membranes. We suggest that the high H⁺-pump capacity is necessary to enable guard cells to respond to sudden changes in the environment by a change in stomatal aperture.     

Measurement of bacterial growth rates on the rhizoplane using 3H-thymidine incorporation into DNA

Bacterial growth rates on the rhizoplane of rape seedlings grown in sand were determined using ³H-thymidine incorporation into DNA. Axenic roots incorporated thymidine into DNA, which had to be subtracted from values for roots with associated bacteria. Thymidine incorporation into rhizoplane bacterial DNA ranged between 0.6 and 1.4 pmol thymidine h^–1 root^–1 for 6 to 26-day-old plants. Using a conversion factor, the turnover time of bacteria was calculated to decrease from 9.2 h for 6-day-old plants to 160h for 26-day-old plants. A similar value was found for rhizosphere bacteria of plants grown for 26 days in natural soil.     

Effects of Eu3+ on the metabolism of amino acid and protein in xerophytic Lathyrus sativus L.

The contents of amino acids and proteins and the activity of Na⁺, K⁺-ATPase were determined in roots, stems, and leaves of Eu³⁺-treated Lathyrus sativus L. The results showed that the treatment of Eu³⁺ made the contents of amino acid and protein and the activity of Na⁺, K⁺-ATPase change. The first possible mechanism was that Eu³⁺ directly made the electric potential of −NH₂ or −COOH of amino acid change. The second possible mechanism was that Eu³⁺ played a role in metallic-activated factors of certain enzymes, which catalyze the catabolism and anabolism of protein. Then, the contents of amino acids and proteins were relatively changed. The third possible mechanism was that Eu³⁺ regulated the activity of ATPase through changing the Na⁺/K⁺ ratio. The energy released by ATPase was the driving force for the translocation of amino acids and proteins in the plant cell. Because of the changeability of its valence, Eu played an, important role in regulating certain physiological reactions to increase the adaptability of L. sativus in arid environment. These authors contributed equally to this work.     

Effects of sodium chloride and polyethylene glycol on root-hair infection and nodulation of Vicia faba L. plants by Rhizobium leguminosarum

The effects of sodium chloride and polyethylene glycol (PEG) on the interaction between Rhizobium leguminosarum strain 29d and root hairs of field bean (Vicia faba L. cv. Maris Bead) plants were investigated. Two levels each of NaCl (50 and 100 mol·m^–3) and PEG (100 and 200 mol·m^–3) were given at the time of root-hair formation. Scanning electron microscopy showed rhizobial attachment and colonization on root-hair tips. Adhesion of rhizobia in both lateral and polar orientation, sometimes associated with microfibrils, occurred mainly in crooks at the root-hair tips; most of the infections also occurred here. Bacterial colonization and root-hair curling were both reduced by stress treatments. Polyethylene glycol but not NaCl significantly reduced root-hair diameter. The proportion of root hairs containing infection threads was reduced by 30% under NaCl and by 52% under PEG. The structure of some of the root hairs, epidermal and hypodermal cells, as seen by light microscopy in ultrasections, was distorted as a result of NaCl and PEG treatments; cells showed plasmolysis and folded membranes. After three weeks of treatment, both NaCl and PEG inhibited nodule number by about 50% and nodule weight by more than 60%. It is concluded that the root-hair infection process in Vicia faba is impaired by NaCl and PEG treatments and this in turn results in fewer nodules being produced.Abbreviation PEG polyethylene glycol     

Screening of Rhizobacteria for Their Plant Growth Promotion Ability and Antagonism Against Damping off and Root Rot Diseases of Broad Bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.)

Development of microbial inoculants from rhizobacterial isolates with potential for plant growth promotion and root disease suppression require rigorous screening. Fifty-four (54) fluorescent pseudomonads, out of a large collection of rhizobacteria from broad bean fields of 20 different locations within Imphal valley of Manipur, were initially screened for antifungal activity against Macrophomina phaseolina and Rhizoctonia solani, of diseased roots of broad bean and also three other reference fungal pathogens of plant roots. Fifteen fluorescent pseudomonas isolates produced inhibition zone (8–29 mm) of the fungal growth in dual plate assay and IAA like substances (24.1–66.7 μg/ml) and soluble P (12.7–56.80 μg/ml) in broth culture. Among the isolates, RFP 36 caused a marked increase in seed germination, seedling biomass and control of the root borne pathogens of broad bean. PCR–RAPD analysis of these isolates along with five MTCC reference fluorescent pseudomonas strains indicated that the RFP-36 belonged to a distinct cluster and the PCR of its genomic DNA with antibiotic specific primers Phenazine-1-carboxylic acid and 2, 4-diacetyl phloroglucinol suggested possible occurrence of gene for the potent antibiotics. Overall, the result of the study indicated the potential of the isolate RFP 36 as a microbial inoculant with multiple functions for broad bean.     

Tritiated thymidine autoradiographic study on the origin and renewal of argentaffin cells in the pyloric gland of hamsters

Summary The origin and renewal of the argentaffin cells in the pyloric glands of hamsters were studied by flash, cumulative and pulse labelling autoradiography with ³H-thymidine. The argentaffin cells were identified by the Diazo Method using Fast Red B Salt.By flash labelling autoradiography, it was shown that the argentaffin cells located from the middle to the lower level of the pyloric mucosa were not labelled with ³H-thymidine, indicating that this cell type has no proliferative activity. On the 10th and the 20th day of cumulative labelling, 31% and 63% of the argentaffin cells in the gland were found to be labelled, respectively. The labelled argentaffin cells were concentrated in the upper part of the gland (around the region of the isthmus), and no label was found over nuclei of the cells at the lowermost level of the gland. These labelled cells were shown to undergo a downward migration in the days following pulse labelling. They were replaced by unlabelled (and weakly or very weakly labelled) cells which arose at the region of the isthmus. The argentaffin cells in the pyloric gland are thought to arise from epithelial precursor cells at the region of the isthmus.The labelled argentaffin cells in the gland were found to decrease in number almost exponentially after pulse labelling. This indicates that the life span of argentaffin cells is not fixed, but their renewal conforms to the random loss system. The half time of turnover of this cell population was 15 days on average.Supported by a Grant-in-Aid for Cancer Research from the Ministry of Education, Science and Culture, Japan     

Development of a Simple PCR-based Marker for the Determination of Growth Habit in Vicia faba L. using a Candidate Gene Approach

We have developed the first molecular marker suitable for the selection of determinate growth habit in faba bean using the candidate gene approach. We obtained the sequences of TFL1/CEN like genes from public databases and designed primers on conserved domains. We used three cultivars with determinate growth habit and four accessions (two cultivars and two lines) with indeterminate growth habit. All these genotypes are used in our faba bean breeding program. A single monomorphic PCR fragment was obtained. A set of restriction enzymes was assayed. The enzyme Hind1II produced a clear polymorphism between determinate and indeterminate genotypes. This new cleaved amplified polymorphism (CAP) marker was tested using an F₂ population contrasting for growth habit derived from the cross ‘Verde Bonita’ × 2N52. This marker showed 100% efficiency in discriminating both types of genotypes. Moreover, the codominancy of this marker allows the detection of heterozygous individuals facilitating the breeding process when pyramiding different genes. The perfect cosegregation of the marker with the trait indicates that an orthologue of TFL1/CEN controls the growth habit in faba bean. This marker has been tested in all the genotypes used in our faba bean breeding program as donors of the determinate growth habit. Therefore, it is expected to work well in all the crosses performed with these parental lines as happens in the F₂ tested. The CAPS marker developed in this work will be useful for Marker Assisted Selection programs. In addition, this marker is useful for quality control to determinate the percentage of outsider seeds in commercial seed lots. Moreover, it is a valuable tool to breeders when submitting new faba bean varieties for registration since the method allows guaranteeing that outsider plants remains under the requested limit for registration.     

Effects of Valeriana Officinalis Extracts on [3H]Flunitrazepam Binding,Synaptosomal [3H]GABA Uptake,and Hippocampal [3H]GABA Release

Extracts of Valeriana officinalis have been used in folkloric medicine for its sedative, hypnotic, tranquilizer and anticonvulsant effects, and may interact with -aminobutyric acid (GABA) and/or benzodiazepine sites. At low concentrations, valerian extracts enhance [³H]flunitrazepam binding (EC₅₀ 4.13 × 10^–10 mg/ml). However, this increased [³H]flunitrazepam binding is replaced by an inhibition at higher concentrations (IC₅₀ of 4.82 × 10^–1 mg/ml). These results are consistent with the presence of at least two different biological activities interacting with [³H]flunitrazepam binding sites. Valerian extracts also potentiate K⁺ or veratridine-stimulated release of radioactivity from hippocampal slices preloaded with [³H]GABA. Finally, inhibition of synaptosomal [³H]GABA uptake by valerian extracts also displays a biphasic interaction with guvacine. The results confirm that valerian extracts have effects on GABA_A receptors, but can also interact at other presynaptic components of GABAergic neurons.     

海马神经元ROS和Ca2+对极低频电磁场实时响应的方法研究

现有极低频电磁场(extremely low frequency-electromagnetic fields,ELF-EMFs)生物效应的研究方法是非实时组间对照,这种方法无法排除细胞对电磁场反应个体敏感性的差异,以及实验过程中条件变化的差异.本文提出一种对同一个细胞在同一条件下的前后对照实时观察ELF-EMFs反应的方法.采用稳定域鉴别ELF-EMFs暴露前海马神经元的稳定性,在电磁场加入时刻(t=60 s)分别用0、0.09、0.38、0.76、7.33、14.78 mT的ELF-EMFs作用于海马神经元,实时记录海马神经元活性氧(reactive oxygen species,ROS)和Ca²⁺荧光响应曲线,建立ELF-EMFs与ROS和Ca²⁺的自相关函数.结果表明:a.在ROS暴发时间和Ca²⁺暴发时间,ROS和Ca²⁺荧光响应具有阶跃性,这是判断实时响应的重要标志;b. ROS和Ca²⁺对ELF-EMFs响应时间具有延时性,并且不一致;c. ROS和Ca^...     

Effects of acute and subacute cocaine administration on the CNS dopaminergic system in Wistar-Kyoto and spontaneously hypertensive rats: III. Dopamine uptake

The characteristics of dopamine uptake after acute and subacute cocaine administration were determined in striata from WKY and SHR. In acutely-treated (40 mg/kg, s.c.) rats, significant increases in the V_max of dopamine uptake were observed 30 min after the cocaine injection in both strains, without changes in K_m values. The in vitro IC₅₀ for cocaine was significantly decreased at 30 min in WKY and at 2 h in SHR. However, the in vitro IC₅₀ for GBR-12909 was significantly increased at 30 min and at 2 h in both strains following cocaine administration. In both strains, the density (B_max) of the [³H]GBR-12935 binding site was significantly increased at 30 min and at 2 h with no charges in K_d. In subacutely-treated (20 mg/kg, twice daily for 3 or 7 days) rats, a significant increase in the K_m for dopamine uptake was observed in 7 day treated SHR. The in vitro IC₅₀ for GBR-12909 was significantly increased in 3 day treated WKY. The results suggest that cocaine administration alters dopamine uptake and characteristics of dopamine uptake sites in the rat brain.     

芽孢杆菌ZJM-P5与氮配施对红小豆幼苗生长和氮吸收的影响

以红小豆品种‘晋红小豆5号’和促生芽孢杆菌ZJM-P5为材料,设置不同氮素用量和菌液浓度处理,采用盆栽试验,测定各菌氮处理下红小豆幼苗农艺性状、生理特性及含氮量等指标,研究促生芽孢杆菌ZJM-P5与氮配施对红小豆幼苗生长发育的影响,并筛选出最佳促生菌液浓度水平,为提高红小豆幼苗的养分吸收利用效率及其高产优质栽培提供理论依据。结果表明:(1)在同一菌液浓度下,红小豆苗期株高、茎粗、叶面积、地上部干重、根系干重、根表面积、根系体积、根系平均直径均随施氮量的增加呈先升高后降低的趋势,而最大根长随着施氮量增加有所下降;根系可溶性糖及MDA含量随施氮量增加均呈先降后升的趋势;根系SOD、POD活性及根系活力均随施氮量增加先升高后降低;根系可溶性蛋白含量、NR活性和植株含氮量均随施氮量增加而增加。(2)在相同施氮量条件下,除根系可溶性糖和MDA含量随着菌浓度的增加先下降后升高外,其余各指标均呈先升后降的变化趋势;各指标除低氮(50mg·kg~(-1))水平下在10~8 cfu·mL~(-1)菌液浓度达到最大值外,其它氮水平均在10~7 cfu·mL~(-1)菌液浓度达到最大值。研究发现,芽孢杆菌ZJM-P5与氮合理配施有利于红小豆植株的氮素吸收和生长发育,并以100mg·kg~(-1)为最佳施氮(纯氮)水平,10~7 cfu·mL~(-1)为最佳菌浓度,且两者组合处理的幼苗农艺性状、根系活力及保护酶活性、氮素含量均达到较高水平;过量施氮、施菌或菌氮比例不合理则会导致幼苗干物质积累及氮素提升受限,进而抑制植株正常生长。     

Relevance of bacterioplankton abundance and production in the oligotrophic equatorial Indian Ocean

Bacterioplankton abundance and production, chlorophyll a (Chl a) concentrations and primary production (PP) were measured from the equatorial Indian Ocean (EIO) during northeast (NEM), southwest (SWM) and spring intermonsoon (SpIM) seasons from 1°N to 5°S along 83°E. The average bacterial abundance was 0.52 ± 0.29, 0.62 ± 0.33 and 0.46 ± 0.19 (× 10⁸ cells l⁻¹), respectively during NEM, SWM and SpIM in the top 100 m. In the deep waters (200 m and below), the bacterial counts averaged ∼0.35 ± 0.14 × 10⁸ cells l⁻¹ in SWM and 0.39 ± 0.16 × 10⁸ cells l⁻¹ in SpIM. The 0–120 m column integrated bacterial production (BP) ranged from 19 to 115 and from 10 to 51 mg C m⁻² d⁻¹ during NEM and SWM, respectively. Compared with many open ocean locations, bacterial abundance and production in this region are lower. The bacterial carbon production, however, is notably higher than that of phytoplankton PP (BP:PP ratio 102% in SWM and 188% in NEM). With perpetually low PP (NEM: 20, SWM: 18 and SpIM: 12 mg C m⁻² d⁻¹) and Chl a concentration (NEM: 16.5, SWM: 15.0 and SpIM: 20.9 mg m⁻²), the observed bacterial abundance and production are pivotal in the trophodynamics of the EIO. Efficient assimilation and mineralization of available organics by bacteria in the euphotic zone might serve a dual role in the ultra-oligotrophic regions including EIO. Thus, bacteria probably sustain microheterotrophs (micro- and meso-zooplankton) through microbial loop. Further, rapid mineralization by bacteria will make essential nutrients available to autotrophs.