Plant Growth-promoting Bacterium Azospirillum Brasilense Research Articles

Thermal and salt stress effects on the survival of plant growth-promoting bacteria Azospirillum brasilense in inoculants for maize cultivation.

The plant growth-promoting bacteria (PGPB) Azospirillum brasilense is widely used as an inoculant for important grass crops, providing numerous benefits to the plants. However, one limitation to develop viable commercial inoculants is the control of PGPB survival, requiring strategies that guarantee their survival during handling and field application. The application of sublethal stress appears to be a promising strategy to increase bacterial cells tolerance to adverse environmental conditions since previous stress induces the activation of physiological protection in bacterial cell. In this work, we evaluated the effects of thermal and salt stresses on the survival of inoculant containing A. brasilense Ab-V5 and Ab-V6 strains and we monitored A. brasilense viability in inoculated maize roots after stress treatment of inoculant. Thermal stress application (> 35 °C) in isolated cultures for both strains, as well as salt stress [sodium chloride (NaCl) concentrations > 0.3 mol L-1], resulted in growth rate decline. The A. brasilense enumeration in maize roots obtained by propidium monoazide quantitative polymerase chain reaction (PMA-qPCR), for inoculated maize seedlings grown in vitro for 7 days, showed that there is an increased number of viable cells after the salt stress treatment, indicating that A. brasilense Ab-V5 and Ab-V6 strains are able to adapt to salt stress (0.3 mol L-1 NaCl) growth conditions. Azospirillum brasilense Ab-V5 and Ab-V6 strains had potential for osmoadaptation and salt stress, resulting in increased cell survival after inoculation in maize plants. © 2024 Society of Chemical Industry.

The Metabolic Response of Brachypodium Roots to the Interaction with Beneficial Bacteria Is Affected by the Plant Nutritional Status.

The potential of plant growth promoting (PGP) bacteria in improving the performance of plants in suboptimal environments is increasingly acknowledged, but little information is available on the mechanisms underlying this interaction, particularly when plants are subjected to a combination of stresses. In this study, we investigated the effects of the inoculation with the PGP bacteria Azospirillum brasilense (Azospirillum) on the metabolism of the model cereal Brachypodium distachyon (Brachypodium) grown at low temperatures and supplied with insufficient phosphorus. Investigating polar metabolite and lipid fluctuations during early plant development, we found that the bacteria initially elicited a defense response in Brachypodium roots, while at later stages Azospirillum reduced the stress caused by phosphorus deficiency and improved root development of inoculated plants, particularly by stimulating the growth of branch roots. We propose that the interaction of the plant with Azospirillum was influenced by its nutritional status: bacteria were sensed as pathogens while plants were still phosphorus sufficient, but the interaction became increasingly beneficial for the plants as their phosphorus levels decreased. Our results provide new insights on the dynamics of the cereal-PGP bacteria interaction, and contribute to our understanding of the role of beneficial microorganisms in the growth of cereal crops in suboptimal environments.

Natural occurrence of Azospirillum brasilense in petunia with capacity to improve plant growth and flowering.

To evaluate the natural occurrence of the plant growth-promoting bacterium Azospirillum brasilense and petunia plants, local strains were isolated and characterized by biochemical and molecular methods. Three strains were assessed in greenhouse conditions using Petunia × hybrida Ultra™. Treatments: Plants without bacterial inoculation or chemical fertilization; fertilized with NPK and KNO3 ; and independently inoculated with the strains 2A1, 2A2, and 2E1 by submerging their roots in a bacterial suspension (~106 CFU·ml-1 ). Root length, dry weight of roots and shoots, leaf area, leaf greenness, and nutrient content were evaluated. The number of days from transplanting to the opening of the first flower and the number of flowers per plant were also determined. As a result, five isolates were characterized as A. brasilense, showing the capacity to produce indoles and siderophores, to solubilize phosphate, nitrogenase activity, and nifH-PCR amplification. In general, all the parameters of the plant assay were improved in plants inoculated with A. brasilense, with variations among the strains, as well as the onset of flowering and the number of flowers per plant, compared with uninoculated or fertilized plants. This is the first report on the natural occurrence of A. brasilense in petunia with the capacity to improve plant growth and flowering.

Expression and function of the cdgD gene, encoding a CHASE\u2013PAS-DGC-EAL domain protein, in Azospirillum brasilense

The plant growth-promoting bacterium Azospirillum brasilense contains several genes encoding proteins involved in the biosynthesis and degradation of the second messenger cyclic-di-GMP, which may control key bacterial functions, such as biofilm formation and motility. Here, we analysed the function and expression of the cdgD gene, encoding a multidomain protein that includes GGDEF-EAL domains and CHASE and PAS domains. An insertional cdgD gene mutant was constructed, and analysis of biofilm and extracellular polymeric substance production, as well as the motility phenotype indicated that cdgD encoded a functional diguanylate protein. These results were correlated with a reduced overall cellular concentration of cyclic-di-GMP in the mutant over 48 h compared with that observed in the wild-type strain, which was recovered in the complemented strain. In addition, cdgD gene expression was measured in cells growing under planktonic or biofilm conditions, and differential expression was observed when KNO3 or NH4Cl was added to the minimal medium as a nitrogen source. The transcriptional fusion of the cdgD promoter with the gene encoding the autofluorescent mCherry protein indicated that the cdgD gene was expressed both under abiotic conditions and in association with wheat roots. Reduced colonization of wheat roots was observed for the mutant compared with the wild-type strain grown in the same soil conditions. The Azospirillum-plant association begins with the motility of the bacterium towards the plant rhizosphere followed by the adsorption and adherence of these bacteria to plant roots. Therefore, it is important to study the genes that contribute to this initial interaction of the bacterium with its host plant.

The Protein-Protein Interaction Network Reveals a Novel Role of the Signal Transduction Protein PII in the Control of c-di-GMP Homeostasis in Azospirillum brasilense.

The PII family comprises a group of widely distributed signal transduction proteins ubiquitous in prokaryotes and in the chloroplasts of plants. PII proteins sense the levels of key metabolites ATP, ADP, and 2-oxoglutarate, which affect the PII protein structure and thereby the ability of PII to interact with a range of target proteins. Here, we performed multiple ligand fishing assays with the PII protein orthologue GlnZ from the plant growth-promoting nitrogen-fixing bacterium Azospirillum brasilense to identify 37 proteins that are likely to be part of the PII protein-protein interaction network. Among the PII targets identified were enzymes related to nitrogen and fatty acid metabolism, signaling, coenzyme synthesis, RNA catabolism, and transcription. Direct binary PII-target complex was confirmed for 15 protein complexes using pulldown assays with recombinant proteins. Untargeted metabolome analysis showed that PII is required for proper homeostasis of important metabolites. Two enzymes involved in c-di-GMP metabolism were among the identified PII targets. A PII-deficient strain showed reduced c-di-GMP levels and altered aerotaxis and flocculation behavior. These data support that PII acts as a major metabolic hub controlling important enzymes and the homeostasis of key metabolites such as c-di-GMP in response to the prevailing nutritional status.IMPORTANCE The PII proteins sense and integrate important metabolic signals which reflect the cellular nutrition and energy status. Such extraordinary ability was capitalized by nature in such a way that the various PII proteins regulate different facets of metabolism by controlling the activity of a range of target proteins by protein-protein interactions. Here, we determined the PII protein interaction network in the plant growth-promoting nitrogen-fixing bacterium Azospirillum brasilense The interactome data along with metabolome analysis suggest that PII functions as a master metabolic regulator hub. We provide evidence that PII proteins act to regulate c-di-GMP levels in vivo and cell motility and adherence behaviors.

Biofertilization with Macrocystis pyrifera algae extracts combined with PGPR-enhanced growth in Lactuca sativa seedlings

To improve crop yields the application of fertilizers to provide nutrients to soils is recommended. An alternative option to substitute nutritional supplements of chemical origin is the use of biofertilizers. This study evaluated the effectiveness of a biofertilizer of natural origin, Macrocystis pyrifera algal extracts and its combination with the plant growth–promoting bacterium Azospirillum brasilense, on the germination and establishment of lettuce seedlings (Lactuca sativa) under two different water irrigation treatments. Results demonstrated that germination energy and germination power improved upon inoculation of lettuce seeds with the seaweed extract. After 7 days of culture, seedlings showed increased root growth which would help to ensure their establishment. The application of bacteria and algae individually or in combination exerted a positive effect on lettuce plant growth compared with controls (without applications). In addition, treatments with the brown seaweed extract favoured seedlings adaptation to an unfavourable environment (water deficit) by increasing their root mass and protecting them from the negative effects imposed by stress. These results suggest that the use of M. pyrifera extract to improve plant growth and to provide stress protection may be considered an interesting tool for agricultural purposes. Nevertheless, when A. brasilense (whose plant growth–promoting properties are largely known) is combined with the brown seaweed extract, a new biofertilizing formula is obtained and proposed for future seed inoculation technology, based on the promising effects observed in this work.

Compatibility of Azospirillum brasilense with Pesticides Used for Treatment of Maize Seeds

Seed treatment with chemical pesticides is commonly used as an initial plant protection procedure against pests and diseases. However, the use of such chemicals may impair the survival and performance of beneficial microorganisms introduced via inoculants, such as the plant growth-promoting bacterium Azospirillum brasilense. We assessed the compatibility between the most common pesticide used in Brazil for the treatment of maize seeds, composed of two fungicides, and one insecticide, with the commercial strains Ab-V5 and Ab-V6 of A. brasilense, and evaluated the impacts on initial plant development. The toxicity of the pesticide to A. brasilense was confirmed, with an increase in cell mortality after only 24 hours of exposure in vitro. Seed germination and seedling growth were not affected neither by the A. brasilense nor by the pesticide. However, under greenhouse conditions, the pesticide affected root volume and dry weight and root-hair incidence, but the toxicity was alleviated by the inoculation with A. brasilense for the root volume and root-hair incidence parameters. In maize seeds inoculated with A. brasilense, the pesticide negatively affected the number of branches, root-hair incidence, and root-hair length. Therefore, new inoculant formulations with cell protectors and the development of compatible pesticides should be searched to guarantee the benefits of inoculation with plant growth-promoting bacteria.

Auxin-dependent alleviation of oxidative stress and growth promotion of Scenedesmus obliquus C1S by Azospirillum brasilense

There is currently an increasing interest in the use of microalgae for wastewater treatment and the use of its biomass as a feedstock for biofuels. Both of these applications are often performed more efficiently by microalgal-bacteria consortia. However, the mechanisms that account for the stability and robustness of this kind of interactions are poorly understood. In this study, we confirmed the growth promotion activity of the plant growth-promoting bacterium Azospirillum brasilense Sp245 on the microalgae Scenedesmus obliquus C1S. We show that this activity is critically dependent on bacterial indole-3 acetic acid (IAA) production, which results in a decrease in algal reactive oxygen species levels, higher cell densities and ameliorates algal cells bleaching after nitrogen deprivation. We also show a close inter-species interaction between both partners and an active expression of the bacterial Indole-3-pyruvate decarboxylase (ipdC) gene involved in production of IAA when co-cultivated.This study extends the current knowledge of the mechanisms underlying bacteria-microalgae consortia to improve their technological applications and to better understand ecological relationships in the environment.

Morphological Changes in Sugarcane Crop Induced by the Plant Growth-Promoting Bacterium Azospirillum brasilense

N fertilizers make the total cost of production of biofuels, sugar and bioelectricity expensive in the sugar-energy industry. Microbes could replace N fertilizers in the nutritional management of sugarcane crop by capturing N2 from the atmosphere. We accordingly conducted this scientific study to investigate the response of sugarcane crop to Azospirillum brasilense. We tested the strain Az-V5 at the doses of 0, 50, 100, 200 and 400 mL ha−1, by spraying it into the planting furrow, 7, 14 and 21 days after crop planting into 20-l pots filled with sieved Rhodic Hapludox, with a loamy texture. In order to examine biologically modulated morphological changes in the host plant material, namely sugarcane variety ‘RB86-7515,’ we harvested and then assessed 90-day-old plants for plant height, stalk diameter, number of tillers, number of leaves, leaf area, tiller length, dry matter of shoots and root dry matter. Sugarcane plants treated with the bioagent 14 and 21 days after crop planting had lower mean values of plant height, number of leaves, leaf area, dry matter of shoots and root dry matter. Therefore, applying A. brasilense into the planting furrow, instead of spraying it on sugarcane leaves, could protect it for a longer period from biotic and abiotic factors influencing cell viability, root colonization and ultimately symbiosis. The conclusion is, therefore, application of A. brasilense into planting furrow is the best environmentally friendly and low-cost strategy to promote growth and development in sugarcane plants at the beginning of their life cycle.

Early Changes in Nutritional Conditions Affect Formation of Synthetic Mutualism Between Chlorella sorokiniana and the Bacterium Azospirillum brasilense.

The effect of three different nutritional conditions during the initial 12h of interaction between the microalgae Chlorella sorokiniana UTEX 2714 and the plant growth-promoting bacterium Azospirillum brasilense Cd on formation of synthetic mutualism was assessed by changes in population growth, production of signal molecules tryptophan and indole-3-acetic acid, starch accumulation, and patterns of cell aggregation. When the interaction was supported by a nutrient-rich medium, production of both signal molecules was detected, but not when this interaction began with nitrogen-free (N-free) or carbon-free (C-free) media. Overall, populations of bacteria and microalgae were larger when co-immobilized. However, the highest starch production was measured in C. sorokiniana immobilized alone and growing continuously in a C-free mineral medium. In this interaction, the initial nutritional condition influenced the time at which the highest accumulation of starch occurred in Chlorella, where the N-free medium induced faster starch production and the richer medium delayed its accumulation. Formation of aggregates made of microalgae and bacteria occurred in all nutritional conditions, with maximum at 83h in mineral medium, and coincided with declining starch content. This study demonstrates that synthetic mutualism between C. sorokiniana and A. brasilense can be modulated by the initial nutritional condition, mainly by the presence or absence of nitrogen and carbon in the medium in which they are interacting.

Dry micro-polymeric inoculant of Azospirillum brasilense is useful for producing mesquite transplants for reforestation of degraded arid zones

Massive clear-cutting of wild stands of mesquite trees in the Mexican part of the Sonoran Desert result from high demand for this wood by the charcoal industry. Consequently, there is a need to develop techniques for reforestation of this tree in the desert and maintain its natural diversity at the same time. An outdoor nursery procedure to produce mesquite transplants from diversely originated seeds for reforestation of arid zones was developed. This procedure involved: 1) inoculation of the seedlings in the nursery with the plant growth-promoting bacteria (PGPB) Azospirillum brasilense immobilized in dry microbeads of alginate, and 2) developing a reliable way to monitor plant development and aerial volume in the nursery for the entire growth period of seven months before transplantation. Dry microbeads containing the PGPB and maintained at room temperature were tested for survival of bacteria for up to seven months. These dry microbeads maintained sufficient population levels of A. brasilense to inoculate the plant for the entire period. Inoculation with the PGPB enhanced all growth parameters of the plants, including biomass, aerial volume, root system, and chlorophyll pigments, but not the auxiliary photosynthetic pigments. The PGPB was specifically identified colonizing the roots of the transplants by fluorescent in situ hybridization for the entire growth period. Measuring a few simple parameters allowed development of a workable model for plant growth. This model was confirmed by data obtained from sacrificed plants whose parameters were measured directly. This study shows that outdoor nursery cultivation of inoculated mesquite transplants is feasible.

Role of ethylene and related gene expression in the interaction between strawberry plants and the plant growth-promoting bacterium Azospirillum brasilense.

Induced systemic resistance (ISR) is one of the indirect mechanisms of growth promotion exerted by plant growth-promoting bacteria, and can be mediated by ethylene (ET). We assessed ET production and the expression of related genes in the Azospirillum-strawberry plant interaction. Ethylene production was evaluated by gas chromatography in plants inoculated or not with A.brasilense REC3. Also, plants were treated with AgNO3 , an inhibitor of ET biosynthesis; with 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor of ET biosynthesis; and with indole acetic acid (IAA). Plant dry biomass and the growth index were determined to assess the growth-promoting effect of A.brasilense REC3 in strawberry plants. Quantitative real time PCR (qRT-PCR) was performed to analyse relative expression of the genes Faetr1, Faers1 and Faein4, which encode ET receptors; Factr1 and Faein2, involved in the ET signalling pathway; Faacs1 encoding ACC synthase; Faaco1 encoding ACC oxidase; and Faaux1 and Faami1 for IAA synthesis enzymes. Results showed that ET acts as a rapid and transient signal in the first 12h post-treatment. A.brasilense REC3-inoculated plants had a significantly higher growth index compared to control plants. Modulation of the genes Faetr1, Faers1, Faein4, Factr1, Faein2 and Faaco1 indicated activation of ET synthesis and signalling pathways. The up-regulation of Faaux1 and Faami1 involved in IAA synthesis suggested that inoculation with A.brasilense REC3 induces production of this auxin, modulating ET signalling. Ethylene production and up-regulation of genes associated with ET signalling in strawberry plants inoculated with A.brasilense REC3 support the priming activation characteristic of ISR. This type of resistance and the activation of systemic acquired resistance previously observed in this interaction indicate that both are present in strawberry plants, could act synergistically and increase protection against pathogens.

Enhanced performance of the microalga Chlorella sorokiniana remotely induced by the plant growth-promoting bacteria Azospirillum brasilense and Bacillus pumilus

Remote effects (occurring without physical contact) of two plant growth-promoting bacteria (PGPB) Azospirillum brasilense Cd and Bacilus pumilus ES4 on growth of the green microalga Chlorella sorokiniana UTEX 2714 were studied. The two PGPB remotely enhanced the growth of the microalga, up to six-fold, and its cell volume by about three-fold. In addition to phenotypic changes, both bacteria remotely induced increases in the amounts of total lipids, total carbohydrates, and chlorophyll a in the cells of the microalga, indicating an alteration of the microalga’s physiology. The two bacteria produced large amounts of volatile compounds, including CO2, and the known plant growth-promoting volatile 2,3-butanediol and acetoin. Several other volatiles having biological functions in other organisms, as well as numerous volatile compounds with undefined biological roles, were detected. Together, these bacteria-derived volatiles can positively affect growth and metabolic parameters in green microalgae without physical attachment of the bacteria to the microalgae. This is a new paradigm on how PGPB promote growth of microalgae which may serve to improve performance of Chlorella spp. for biotechnological applications.

Tryptophan, thiamine and indole-3-acetic acid exchange between Chlorella sorokiniana and the plant growth-promoting bacterium Azospirillum brasilense.

During synthetic mutualistic interactions between the microalga Chlorella sorokiniana and the plant growth-promoting bacterium (PGPB) Azospirillum brasilense, mutual exchange of resources involved in producing and releasing the phytohormone indole-3-acetic acid (IAA) by the bacterium, using tryptophan and thiamine released by the microalga, were measured. Although increased activities of tryptophan synthase in C. sorokiniana and indole pyruvate decarboxylase (IPDC) in A. brasilense were observed, we could not detect tryptophan or IAA in the culture medium when both organisms were co-immobilized. This indicates that no extra tryptophan or IAA is produced, apart from the quantities required to sustain the interaction. Over-expression of the ipdC gene occurs at different incubation times: after 48 h, when A. brasilense was immobilized alone and grown in exudates of C. sorokiniana and at 96 h, when A. brasilense was co-immobilized with the microalga. When A. brasilense was cultured in exudates of C. sorokiniana, increased expression of the ipdC gene, corresponding increase in activity of IPDC encoded by the ipdC gene, and increase in IAA production were measured during the first 48 h of incubation. IAA production and release by A. brasilense was found only when tryptophan and thiamine were present in a synthetic growth medium (SGM). The absence of thiamine in SGM yielded no detectable IAA. In summary, this study demonstrates that C. sorokiniana can exude sufficient tryptophan and thiamine to allow IAA production by a PGPB during their interaction. Thiamine is essential for IAA production by A. brasilense and these three metabolites are part of a communication between the two microorganisms.

Establishment of stable synthetic mutualism without co-evolution between microalgae and bacteria demonstrated by mutual transfer of metabolites (NanoSIMS isotopic imaging) and persistent physical association (Fluorescent in situ hybridization)

The demonstration of a mutualistic interaction requires evidence of benefits for both partners as well as stability of the association over multiple generations. A synthetic mutualism between the freshwater microalga Chlorella sorokiniana and the soil-derived plant growth-promoting bacterium (PGPB) Azospirillum brasilense was created when both microorganisms were co-immobilized in alginate beads. Using stable isotope enrichment experiments followed by high-resolution secondary ion mass spectrometry (SIMS) imaging of single cells, we demonstrated transfer of carbon and nitrogen compounds between the two partners. Further, using fluorescent in situ hybridization (FISH), mechanical disruption and scanning electron microscopy, we demonstrated the stability of their physical association for a period of 10days after the aggregated cells were released from the beads. The bacteria significantly enhanced the growth of the microalgae while the microalgae supported growth of the bacteria in a medium where it could not otherwise grow. We propose that this microalga-bacterium association is a true synthetic mutualism independent of co-evolution.

Inoculation of Brachiaria spp. with the plant growth-promoting bacterium Azospirillum brasilense: An environment-friendly component in the reclamation of degraded pastures in the tropics

Estimates are that in Brazil there are about 180 million hectares of pasturelands, 70% with some degree of degradation. Reclamation of such areas demands re-establishment of soil fertility, plant growth and forage production, and microbial inoculants might help in these processes. We evaluated the ability of two strains of Azospirillum brasilense to promote the growth of two genotypes of Brachiaria spp. (=Urochloa spp.). The experiments were set up at three different sites in Brazil, and forage production estimated for 26 cuts in two years. On average, increases of 5.4% and 22.1% in response to N-fertilizer alone and to N-fertilizer in combination with Azospirillum, respectively, were observed over the non-inoculated and non-N-fertilized control treatment. Increase in N accumulation in the biomass in response to Azospirillum was equivalent to a second application of 40kg of N-fertilizer ha−1. Estimates attributed to the inoculation were of gains of 0.103MgCha−1, corresponding to 0.309Mg CO2-eq ha−1. Inoculation with Azospirillum may represent a key component of programs to reclaim degraded pastures and help sequestration of CO2 from the atmosphere.

A need for disclosure of the identity of microorganisms, constituents, and application methods when reporting tests with microbe-based or pesticide-based products

Plant growth-promoting microorganisms, biological control agents, pesticides, and a variety of stimulants to enhance plant performance are commonly tested and commercially applied worldwide on increasing scales each year (Bashan et al. 2014; Calvo et al. 2014; Lugtenberg and Kamilova 2009). In recent years, inoculation of plants with microorganisms to enhance productivity has become widespread, with more sophisticated and complex formulations and application methods. These formulations use a variety of application techniques, such as using natural and synthetic polymers or add more than one microorganism to the inoculants. These microbial consortia in inoculants may have over ten different microorganisms. Inoculants may have synthetic vitamins, plant hormones, and humic and fulvic acids. Other inoculants add different plant or algae or seaweed extracts, protein hydrolytes and amino acids as stimulants, as well as bulking materials, such as clays, minerals (vermiculite and perlite), talcum, or undisclosed soil fractions. Numerous adhesives, surfactants, stabilizing and dispersing materials, and preservatives to enhance survival and dispersal of the microorganisms are commonly added. The identity of these components in microbial formulations is often not disclosed in the product label. Specific details of application are usually explained by the manufacturer on the container label, but rarely in a formal publication. As product formulations change, performance of bacterial strains in the product may change. Hence, it might not be possible to objectively compare the effects of an inoculated microorganism in laboratory experiments with the effects of the same microbe in a commercial formulation. This results from some components in the formulations having synergistic bioactivities that are greater than a single strain of microorganism. As an example, the plant growth-promoting bacterium Azospirillum brasilense strain AZ39 is one of the most common Azospirillum strains used in Latin America. Yet, its performance significantly varies, depending on the specific formulation and the method of inoculation. Similar phenomenon occurs with rhizobia. Pesticides are commonly used in agriculture and are classified, according to the pest organism, in insecticides, fungicides, herbicides (weeds killers), nematocides, and rodenticides. The first three pesticides are those most used. Pesticides differ greatly for their chemical structures, and this result in a great variety of toxicological properties, most of them contain aromatic rings and some are chlorinated, just to mention a few chemical characteristics. Field rates depend on several factors including soil type (Bremner and Mulvaney 1976; Cervelli et al. 1976). Usually, pesticides are applied in formulations containing several other constituents often not disclosed in the product label, as mentioned above for the microbial inocula. These additives may include solvents, by-products of the target compounds, salts, chemical vehicles of the target compound, etc. Obviously toxicological studies of the target pesticide should also consider the effect of all other constituents; these effects may be synergic or antagonist with those by the target pesticide (Cervelli et al. 1976). * Yoav Bashan ybb0001@auburn.edu

Growth and phosphorus removal by Synechococcus elongatus co-immobilized in alginate beads with Azospirillum brasilense

This study examined the co-immobilization of the cyanobacterium Synechococcus elongatus with the plant growth-promoting bacterium Azospirillum brasilense in alginate beads and its potential application for the removal of phosphorus from aquaculture wastewater. Co-immobilization of both microorganisms significantly increased the cell density of S. elongatus (2852.5 × 104 cells mL−1) compared with that of immobilization of cyanobacteria alone (1325.2 × 104 cells mL−1). Chlorophyll a content was similar in co-immobilized (11.1 ± 3.5 pg cell−1) and immobilized S. elongatus (14.5 ± 4.9 pg cell−1). Azospirillum brasilense showed continuous growth until day 2, after which its cell concentration declined until the end of the assay. Co-immobilized S. elongatus removed more phosphorus (44.8 %) than immobilized cyanobacteria cells alone (32.0 %). In conclusion, phosphate removal was greater with free cells of S. elongatus but overlapped with the values that were obtained with the treatment of co-immobilization of cells. Our results demonstrate that A. brasilense enhances the growth of S. elongatus and improves its removal of phosphorus when they are co-immobilized in alginate beads compared with only immobilization of cyanobacteria cells alone.

Photodegradation of the anthraquinonic dye Acid Green 25 by TiO2 immobilized on carbonized avocado kernels: Intermediates and toxicity

The aim of this research was to develop a TiO2 composite from a waste material in order to study the photocatalytic decomposition of an important residual anthraquinonic dye. Carbon-TiO2 composites were obtained from carbonized avocado kernels and sols of TiO2 prepared from NH4OH and TiCl4 using glycerol as a binder and thermally treating the composites at 500°C. XRD studies confirmed the anatase phase of TiO2. SEM images showed isolated TiO2 agglomerates attached on the carbon surface with a Ti:C ratio of 1:3.3 established by EDS semi-quantitative analysis. The photocatalytic activities of the prepared composites were tested in a decomposition assay of Acid Green 25. Titanium dioxide and the composite eliminated almost 100% of the dye under UV irradiation and the composite demonstrated degradation efficiency under solar light. The reaction was monitored by UV–vis spectrophotometry and by LC-ESI-(Qq)-TOF-MS-DAD, enabling the identification of some intermediate species. No evidence of toxic effects was found in the dye AG25 and its photodegradation products, which were evaluated on the plant growth-promoting bacteria Azospirillum brasilense.

Accumulation of fatty acids in Chlorella vulgaris under heterotrophic conditions in relation to activity of acetyl-CoAcarboxylase, temperature, and co-immobilization with Azospirillum brasilense [corrected

The relation between fatty acid accumulation, activity of acetyl-CoA carboxylase (ACC), and consequently lipid accumulation was studied in the microalgae Chlorella vulgaris co-immobilized with the plant growth-promoting bacterium Azospirillum brasilense under dark heterotrophic conditions with Na acetate as a carbon source. In C. vulgaris immobilized alone, cultivation experiments for 6days showed that ACC activity is directly related to fatty acid accumulation, especially in the last 3days. In co-immobilization experiments, A. brasilense exerted a significant positive effect over ACC activity, increased the quantity in all nine main fatty acids, increased total lipid accumulation in C. vulgaris, and mitigated negative effects of nonoptimal temperature for growth. No correlation between ACC activity and lipid accumulation in the cells was established for three different temperatures. This study demonstrated that the interaction between A. brasilense and C. vulgaris has a significant effect on fatty acid and lipid accumulation in the microalgae.