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- Short-term fertilizer application alters phenotypic traits of symbiotic nitrogen fixing bacteria
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Generally, these observations suggest that fertilizer addition could be an important selective force on natural rhizobia populations and may cause phenotypic changes in traits that are relevant to the free-living persistence and legume symbiosis of rhizobia.
Variation in traits is a ubiquitous property among rhizobia strains and has important consequences for understanding the impacts of ecological and evolutionary processes of altered soil environments on indigenous rhizobia populations.
Nutrient addition could act directly on trait variation related to free-living vigour or growth by selecting traits that can tolerate higher ranges of nutrient input.
Nitrogen Fixation | Definition of Nitrogen Fixation by Merriam-Webster
Nutrient addition could also select traits related to mutualistic association as a result of altered host feedback responses. For example, long term nitrogen addition has been shown to reduce the mutualistic benefit of rhizobia isolates towards their legume hosts Weese et al.
Phenotypic changes in traits related to fertilizer tolerance or mutualism benefit could also occur as a result of selection on genetically correlated traits. Conversely, if selection alters mutualism benefit traits, a genetic correlation would indirectly alter traits related to fertilizer tolerance during free-living growth. Therefore, genetic correlations between traits related to free-living growth and mutualism benefit are important in identifying additional evolutionary pathways that result in phenotypic changes in either trait.
In this study, we use a variety of approaches to investigate the effects of plant fertilizer containing all macro and micro-nutrients on phenotypic traits relevant for fitness of nitrogen-fixing rhizobia symbionts using a legume common to agriculturally disturbed systems, Medicago lupulina. We first applied fertilizer over a single growing season and then tested for community-level differences between fertilized and unfertilized soils on M.
Next we disentangled the effects of rhizobia populations from those of the rest of the soil community by culturing rhizobia isolates and comparing and correlating in vitro free-living growth and mutualism benefit i. We further evaluated if isolates from fertilized or unfertilized field soil differed in their plastic responses to in vitro growth assays containing variable fertilizer concentrations.
Methods Natural history of study system The field experiment was conducted in a recently disturbed old field habitat with dense populations of the legume Medicago lupulina growing in the Koffler Scientific Reserve www. Interactions between Medicago and Ensifer occur in the early spring during plant germination when symbiotic bacteria infect plant roots and induce nodule formation.
When plant seed set occurs in August and September, nodule plant tissue begins to senesce, releasing rhizobia cells into the soil—the undifferentiated fraction of the cells survive in a free-living state until the following growing season Hirsch, Testing for field fertilizer treatment on host performance using whole-soil inoculations We randomly positioned 16 plots 0.
Within each population, half of the plots were randomly selected for fertilizer application. We applied 1 tbsp.
K ratio of We initially tested for differential mutualistic effects of fertilized and unfertilized field soil on host plants by inoculating potted plants with whole soils in the glasshouse. P1 i-SA, P1 i, W6 i and a slow-release fertilizer application same as field treatment to determine whether the effects of field soil treatments were consistent across host genotype and fertilizer environment.
In total, our design included field fertilizer treatment, glasshouse fertilizer treatment and host genotype in a full factorial design i. Each pot contained steam sanitized low nutrient soil 1: To reduce effects on host performance as a result of chemical differences between fertilized and unfertilized field soil as opposed to differences driven by microbial communitieswe added autoclaved soil from the opposing field treatment to each pot in equal proportion.
The opposing soil was prepared by autoclaving a soil mixture containing a subset of soil from all plots that received the same fertilizer treatment. For example, pots assigned with the unfertilized field soil treatment received 15 ml of unfertilized field soil from a given plot and 15 ml of autoclaved soil from all other fertilized field plots.
We also added 10 control pots which received 15 ml of autoclaved soil sample from each field treatment. Prior to planting, seeds were scarified, sterilized in commercial bleach, stratified in the dark on 1. Plants were grown for 53 days and each pot was carefully top watered to minimize cross-contamination. Control plants showed significantly lower amounts of nodulation Our model included field fertilizer treatment, greenhouse fertilizer treatment, host genotype, block, final plant density in each pot since some mortality occurred during the experimentfield site and harvest date as fixed effects, and pot and plot as random effects.
Field soil treatments that received the additional glasshouse fertilizer application were excluded from the rhizobia isolation procedure.
A total of isolates were successfully cultured from unfertilized and 90 from fertilized field plots. Seeds were germinated as described above and planted in autoclaved turface: Preliminary culturing indicated that most isolates neared stationary phase of growth after 36 h. We measured host performance as the sum of total fruit and flower production per individual plant at harvest. Field fertilizer treatment, genotype of origin host where preserved nodule was obtainedorigin field site, greenhouse block and harvest date were included as fixed effects, while field sample plot of original field soil sample was included as a random factor.
We tested for isolate effects using a log-likelihood ratio test between the full mixed model containing isolate and reduced model excluding the isolate term.
Isolates were grown in TY media containing three different plant fertilizer concentrations: All media was prepared using sterile filtered stock fertilizer solutions containing the same slow release fertilizer brand that was applied in the field plots 5 tbsp.
Initial cell density measurements were taken immediately following inoculation and at 36 h. Isolates were randomly assigned over 6 trials and cell density was estimated by measuring optical density OD We found no indication of contamination in un-inoculated controls as indicated by unchanging optical density measurements during growth assays and a lack of cell growth when subsequently cultured on TY agar plates. Optical density at initial inoculation was included as a covariate to account for any absorbance differences caused by fertilizer media treatment and initial inoculation.
Isolate, trial, plate and field sample plot were included as random effects. We tested for isolate effects as above. Testing for associations between rhizobia growth and mutualism benefit traits We calculated mean growth and host performance traits for each isolate using fixed- effect lsmeans from a mixed model output.
We tested for associations between in vitro growth assays and mutualistic benefit using a general linear model, with host performance as the response and cell density, field fertilizer treatment and host genotype as predictors.
We repeated the model for each type of growth assay cell density count in control, low and high nutrient and the growth plasticity index. Results Hosts performance in field soil inoculations Plant biomass was larger when hosts were grown in unfertilized field soil compared to fertilized field soil Fig. Host genotype also explained variation in plant size Fig. Nodule number and mean nodule size was 3. Generally, these results indicate that the higher host performance in unfertilized field soil was consistent across host genotypes and greenhouse fertilizer treatments.
Host performance in whole-soil inoculations that have been fertilized or unfertilized in the field. Aboveground biomass g in Medicago lupulina when three plant genotypes FR, CA and US were inoculated with whole soil from field plots that were either fertilized with nutrients containing all conventional macro and micronutrients or remained unfertilized.
Mean values for each field soil treatment combine glasshouse fertilized and glasshouse unfertilized treatments. Error bars represent standard errors. In contrast to whole-soil inoculation effects, plants had higher performance measured by fruit and flower production when inoculated with isolates originating from fertilized field soil Fig. Biomass was non-significant, but trended in the same direction as fruit and flower production, being higher when inoculated with isolates from fertilized field soil not shown.
These results indicate that isolates isolated from unfertilized field soil provided lower mutualism benefits to their host compared to isolates from fertilized field soil. Media fertilizer had a consistent and positive effect on rhizobia growth, causing intermediate cell density at low fertilizer concentrations, and high cell density counts at high media fertilizer concentrations Fig.
Host genotype origin also affected growth reaction norms, with isolates from FR hosts exhibiting the highest increase in growth across media fertilizer concentrations Fig. These results indicate that field fertilizer treatment, rhizobia nodule isolates, and the genotype of the origin host all affect the degree of rhizobia plasticity in response to plant fertilizer in the liquid growth media.
However, the low fertilizer growth plasticity index the ratio of growth responses in low fertilizer vs. The high fertilizer growth plasticity index the ratio of growth responses in high fertilizer vs.
Discussion We investigated how a single season of field fertilizer application in natural field soil altered phenotypic properties of symbiotic rhizobia that associate with M. Our study shows that plant fertilizer changes host partner quality as defined by host biomass, flower or fruit production and free-living growth responses as well as environmentally dependent associations between these traits, demonstrating that long-term nutrient application across multiple years is not required to observe shifts in ecologically relevant phenotypic traits of symbiotic rhizobia populations.
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Growth responses of rhizobia to plant fertilizer Our data show that, even after a single season of fertilizer application, isolates from fertilized field soil grew faster than isolates from unfertilized field soil when the growing media contained the original plant fertilizer used on field soil.
These results suggests that field fertilizer application caused a community shift or within-species evolutionary change, favouring lineages or genotypes or even alleles at specific genes that are more capable of utilizing higher dosages of plant fertilizer for growth in the free-living state in the soil or in culture in more nutrient-rich conditions.
Given that rhizobia isolates from both field soil treatments responded positively to fertilizer in agar media, indicating that plant fertilizer provide nutrients that ordinarily limit free-living growth, our data support the hypothesis that fertilizer affected rhizobia fitness components related to free-living persistence in the soil.
However, we cannot exclude the possibility that fertilizer application affected symbiosis fitness components, favouring rhizobia isolates that are competitive for Medicago nodulation, thus gaining higher fitness through host feedbacks. We found no preliminary evidence of a fitness trade-off for growth in higher fertilizer—rhizobia from fertilized field soil did not have lower growth rates than rhizobia from unfertilized field on control media containing no plant nutrients supplement Fig.
However, previous empirical studies have found fitness costs affecting free-living persistence to adaptation to salt Thrall et al. It is possible that the fitness costs observed for higher metal and salt concentrations occurs because these factors are generally detrimental to rhizobia growth and may require physiological trade-offs for survival i. A lack of a fitness cost observed in our experiment may be because plant fertilizer stimulates growth and thus does not require physiological trade-offs to survive in low or high nutrient conditions.
To our knowledge, this is the first study to provide evidence that plant fertilizer containing all macro and micro nutrients is directly beneficial for rhizobia growth and favours isolates that have higher growth response to fertilizer. Mutualism benefit responses of rhizobia to plant fertilizer Initial whole field soil inoculations showed that Medicago had lower biomass in fertilized field soil.
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Since we expected higher biomass in fertilized field soil due simply to the presence of additional nutrients, these results suggest that plant fertilizer addition altered the microbial community composition in ways that are relevant to the aboveground productivity of Medicago.
However, legume hosts were larger when inoculated directly with the rhizobia isolates cultured from fertilized soil, which suggests that the rhizobia community is unlikely to be responsible for the differences in plant performance observed in the initial whole-soil inoculations. But rapid industrialization and immobilization have resulted in elevated emissions of toxic chemicals into the biosphere.
The release of heavy metals in biologically available forms by human activity, may damage or alter both natural and man-made ecosystem. Metals discharged from various sources severely pollute aquatic as well as terrestrial habitats. Metals being conservative tend to be bio-accumulated and biomagnified to alarming proportions in organism higher in the tropic chain. Recently there is a considerable in developing cost effective and environment friendly technology for the remediation of soil and wastewater polluted with toxic trace elements.
Cyanobacteria have the ability to accumulate heavy metals such as nickel and chromium and this ability could be harnessed to remove pollutant metals from the environment.
The cyanobacteria based bioremediation technologies have received recent attention as strategies to clean up contaminated soil and water. Aulosira fertilissima, which is present as a natural flora in the rice fields in the form of thick mats covering the entire paddy field soils, is particularly useful in the abatement and monitoring of heavy metals.
Various methods are available for immobilizing cells like entrapment, adsorption, covalent linkage, cross linkage or flocculation and encapsulation. Immobilization by entrapment is probably the most extensively used method.Mineral Nutrition in Plants - Biological Nitrogen Fixation - Nodule Formation