Growth-limiting Nutrient Research Articles

Functional regimes define the response of the soil microbiome to environmental change.

The metabolic activity of soil microbiomes plays a central role in carbon and nitrogen cycling. Given the changing climate, it is important to understand how the metabolism of natural communities responds to environmental change. However, the ecological, spatial, and chemical complexity of soils makes understanding the mechanisms governing the response of these communities to perturbations challenging. Here, we overcome this complexity by using dynamic measurements of metabolism in microcosms and modeling to reveal regimes where a few key mechanisms govern the response of soils to environmental change. We sample soils along a natural pH gradient, construct >1500 microcosms to perturb the pH, and quantify the dynamics of respiratory nitrate utilization, a key process in the nitrogen cycle. Despite the complexity of the soil microbiome, a minimal mathematical model with two variables, the quantity of active biomass in the community and the availability of a growth-limiting nutrient, quantifies observed nitrate utilization dynamics across soils and pH perturbations. Across environmental perturbations, changes in these two variables give rise to three functional regimes each with qualitatively distinct dynamics of nitrate utilization over time: a regime where acidic perturbations induce cell death that limits metabolic activity, a nutrient-limiting regime where nitrate uptake is performed by dominant taxa that utilize nutrients released from the soil matrix, and a resurgent growth regime in basic conditions, where excess nutrients enable growth of initially rare taxa. The underlying mechanism of each regime is predicted by our interpretable model and tested via amendment experiments, nutrient measurements, and sequencing. Further, our data suggest that the long-term history of environmental variation in the wild influences the transitions between functional regimes. Therefore, quantitative measurements and a mathematical model reveal the existence of qualitative regimes that capture the mechanisms and dynamics of a community responding to environmental change.

Root phosphatase activity is coordinated with the root conservation gradient across a phosphorus gradient in a lowland tropical forest.

Soil phosphorus (P) is a growth-limiting nutrient in tropical ecosystems, driving diverse P-acquisition strategies among plants. Particularly, mining for inorganic P through phosphomonoesterase (PME) activity is essential, given the substantial proportion of organic P in soils. Yet, the relationship between PME activity and other nutrient-acquisition root traits remains unclear. We measured root PME activity and commonly measured root traits, including root diameter, specific root length (SRL), root tissue density (RTD), and nitrogen concentration ([N]) in 18 co-occurring species across soils with varying P availability to better understand trees response to P supply. Root [N] and RTD were inversely related, and that axis was not clearly related to soil P supply. Both traits, however, correlated positively and negatively with PME activity, which responded strongly to P supply. Conversely, root diameter was inversely related to SRL, but this axis was not related to P supply. This pattern suggests that limiting similarity influenced variation along the diameter-SRL axis, explaining local trait diversity. Meanwhile, variation along the root [N]-RTD axis might best reflect environmental filtering. Overall, P availability indicator traits such as PME activity and root hairs only tended to be associated with these axes, highlighting limitations of these axes in describing convergent adaptations at local sites.

Characterizing a panel of amino acid auxotrophs under auxotrophic starvation conditions.

Auxotrophic strains starving for their cognate nutrient, termed auxotrophic starvation, are characterized by a shorter lifespan, higher glucose wasting phenotype, and inability to accomplish cell cycle arrest when compared to a "natural starvation," where a cell is starving for natural environmental growth-limiting nutrients such as phosphate. Since evidence of this physiological response is limited to only a subset of auxotrophs, we evaluated a panel of auxotrophic mutants to determine whether these responses are characteristic of a broader range of amino acid auxotrophs. Based on the starvation survival kinetics, the panel of strains was grouped into three categories-short-lived strains, strains with survival similar to a prototrophic wild type strain, and long-lived strains. Among the short-lived strains, we observed that the tyrosine, asparagine, threonine, and aspartic acid auxotrophs rapidly decline in viability, with all strains unable to arrest cell cycle progression. The three basic amino acid auxotrophs had a survival similar to a prototrophic strain starving in minimal media. The leucine, tryptophan, methionine, and cysteine auxotrophs displayed the longest lifespan. We also demonstrate how the phenomenon of glucose wasting is limited to only a subset of the tested auxotrophs, namely the asparagine, leucine, and lysine auxotrophs. Furthermore, we observed pleiotropic phenotypes associated with a subgroup of auxotrophs, highlighting the importance of considering unintended phenotypic effects when using auxotrophic strains especially in chronological aging experiments.

Aeolian dust and hydro-biological characteristics: Decoding dust storm impacts on phytoplankton in the northern Arabian Gulf

Aeolian dust is an essential source of growth-limiting nutrients for marine phytoplankton. Despite being at the core of the Global Dust Belt, the response of the Arabian Gulf ecosystem to such atmospheric forcing is rarely documented. Here, the hydro-biological effect of mineral dust was studied in the northern Arabian Gulf (NAG) off Kuwait through monthly water sampling (December 2020 to December 2021), dust-storm follow-up sampling, and mineral dust and nutrient addition in-situ experiments. The multivariate analysis of oceanographic data revealed pronounced hydro-biological seasonality. The mineral dust deposition during two severe dust storm events in March and June 2021 showed a spatially varying effect of dust on coastal waters. The dust storms elevated the surface dissolved iron levels by several magnitudes, increased the dissolved inorganic nitrogen and phosphorous levels, changed their stoichiometry, and offset the hydrobiological seasonality. In the microcosms, dust input temporarily reduced phytoplankton phosphorous limitation in a dose-dependent manner when mesozooplankton (copepods) grazing was minimal. The microphytoplankton response to mineral dust inputs was comparable to that with nitrogen and phosphorous treatment. While Both treatments increased diatom size structure and biomass, the abundance of single-celled diatoms was comparatively higher in dust treatment. Multivariate analysis indicated that dust deposition alters the hydrographical properties of the surface ocean during dust storm events. The effects, though transient, were traceable for 3–16 days post-storm in coastal waters. The response of the summer phytoplankton to these changes, if delayed or muted, should be interpreted with caution given the summer water column stratification, the high nitrogen: phosphorous ratio and the low phosphorous solubility of aerosol dust, and the complex pelagic microbial food web interactions in the NAG. This study thus underlines the importance of a multivariate approach in documenting the ecological implications of Aeolian dust storms on marine environments closer to the dust source regions.

Product-inhibition feedbacks, not microbial population level tradeoffs or soil pH, regulate decomposition potential under nutrient eutrophication

Global eutrophication of growth-limiting nutrients for plants and microbes, including nitrogen (N) and phosphorus (P), can cause enzymatic feedbacks at different levels of biological organization that in turn drive soil organic matter (SOM) decomposition. Because land management practices, such as fire in mesic grasslands, also impact nutrient availability, it is important to understand how soil feedbacks may manifest under multifactorial change. Here, using a thirty-year field experiment that crosses N- and P-fertilization (alone and in combination) with contrasting fire treatments (annual burning and fire suppression) in a tallgrass prairie ecosystem, we evaluated evidence for three possible feedbacks (product-inhibition, enrichment of fast-growing copiotrophic taxa, and soil pH shift) by measuring soil extracellular enzyme activities (EEAs) and microbial community composition (16S rRNA genes) monthly for a year. Consistent with product-inhibition feedback predictions, both N-fertilization and fire suppression (which increases soil N availability) decreased N-acquiring relative to C-acquiring EEAs, suggesting reduced N limitation. P-fertilization did not lower acid phosphatase activity, thus the product-inhibition feedback hypothesis was not supported under P eutrophication conditions. Although N- and P-fertilization, alone and in combination, did decrease putatively oligotrophic populations in favor of putatively copiotrophic populations, the hypothesized tradeoff for faster growth, a concomitant decrease in all soil EEAs, was only observed in unburned soils fertilized with N. Finally, observed changes in EEAs were not driven by acidification, in contrast to other fertilization experiments. Overall, results suggest that higher N availability affects enzymatic feedbacks primarily through product-inhibition, but a threshold-like growth tradeoff (copiotrophic) response is possible under the combination of fire suppression and N-fertilization. Inherent soil properties may modulate site-specific responses to fertilization, as evidenced by weak pH and EEA responses to N- and P-fertilization, respectively. Finally, soil microbial community change following chronic fertilization did not reliably predict the “copiotrophic” feedback of reduced SOM decomposition potential.

Reconciling contrasting effects of nitrogen on host immunity and pathogen transmission using stoichiometric models.

Hosts rely on the availability of nutrients for growth, and for defense against pathogens. At the same time, changes in host nutrition can alter the dynamics of pathogens that rely on their host for reproduction. For primary producer hosts, enhanced nutrient loads may increase host biomass or pathogen reproduction, promoting faster density-dependent pathogen transmission. However, the effect of elevated nutrients may be reduced if hosts allocate a growth-limiting nutrient to pathogen defense. In canonical disease models, transmission is not a function of nutrient availability. Yet, including nutrient availability is necessary to mechanistically understand the response of infection to changes in the environment. Here, we explore the implications of nutrient-mediated pathogen infectivity and host immunity on infection outcomes. We developed a stoichiometric disease model that explicitly integrates the contrasting dependencies of pathogen infectivity and host immunity on nitrogen (N) and parameterized it for an algal-host system. Our findings reveal dynamic shifts in host biomass build-up, pathogen prevalence, and the force of infection along N supply gradients with N-mediated host infectivity and immunity, compared with a model in which the transmission rate was fixed. We show contrasting responses in pathogen performance with increasing N supply between N-mediated infectivity and N-mediated immunity, revealing an optimum for pathogen transmission at intermediate N supply. This was caused by N limitation of the pathogen at a low N supply and by pathogen suppression via enhanced host immunity at a high N supply. By integrating both nutrient-mediated pathogen infectivity and host immunity into a stoichiometric model, we provide a theoretical framework that is a first step in reconciling the contrasting role nutrients can have on host-pathogen dynamics.

OsIPK2, a rice inositol polyphosphate kinase gene, is involved in phosphate homeostasis and root development.

Phosphorus (P) is one kind of growth-limiting nutrient for plants, which is taken up by root tissue from the environment as inorganic phosphate (Pi). To maintain an appropriate status of cellular Pi, plants have developed sophisticated strategies to sense Pi level and modulate their root system architecture (RSA) under the ever-changing growth condition. However, the molecular basis underlying the mechanism remains elusive. Inositol polyphosphate kinase (IPK2) is a key enzyme in the inositol phosphate metabolism pathway, which catalyzes the phosphorylation of IP3 into IP5 by consuming ATP. In this study, the function of a rice inositol polyphosphate kinase gene (OsIPK2) in plant Pi homeostasis and thus physiological response to Pi signal were characterized. As a biosynthetic gene for phytic acid in rice, overexpression of OsIPK2 led to distinct changes of inositol polyphosphate profiles and an excessive accumulation of Pi level in transgenic rice under Pi-sufficient conditions. The inhibitory effects of OsIPK2 on root growth were alleviated by Pi-deficient treatment compared with wild-type plants, suggesting the involvement of OsIPK2 in the Pi-regulated reconstruction of RSA. In OsIPK2-overexpressing plants, the altered acid phosphatase (APases) activities and misregulation of Pi starvation induced (PSI) genes were observed in roots under different Pi supply conditions. Notably, the expression of OsIPK2 also altered the Pi homeostasis and root system architecture in transgenic Arabidopsis. Taken together, our findings demonstrated that OsIPK2 plays an important role in Pi homeostasis and root system architecture adjustment in response to different environmental Pi levels in plant.

A Rapid Method to Determine Growth‐Limiting Nutrient Concentrations for Yeast in a Microplate Spectrophotometer

Determining the minimal concentration of a substance-whether a compound used to inhibit cell growth or a growth-limiting nutrient-can be an arduous process. Carrying out the experiment in flasks or tubes and estimating cell density in a single-sample spectrophotometer has been a routine method of choice due to the ease of measurement and standardized conversions from optical density measurements to cell concentrations. However, when dealing with dozens or more samples, several challenges arise, including increased processing time, increased risk of contamination through repeated sampling, and an increased risk of confounding one sample for another. The protocol described here details a rapid method to estimate cell concentrations for such experiments using a microplate spectrophotometer. Using a microplate spectrophotometer to measure optical density of many cultures can be automated for high throughput and benefits from a reduced risk of contamination. Since one of the caveats of a microplate spectrophotometer is its low saturation limit, we further describe how to convert optical density readings of the microplate spectrophotometer into its single-sample spectrophotometric equivalent. To further illustrate the applicability of this protocol, we compare OD600 readings generated using a microplate spectrophotometer and a single-sample spectrophotometer for different nutrient starvations and show that the results are comparable. Overall, this method reduces the required resources, reduces the risk of contamination, and allows for faster processing of samples. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Generating an equation to convert microplate spectrophotometer readings to single-sample spectrophotometric values Basic Protocol 2: Evaluating growth-limiting nutrient concentrations using a microplate spectrophotometer.

EFFECT OF DIFFERENT RATES OF ORGANIC NITROGEN INPUTS ON PLANT TOTAL NUTRIENT, UPTAKE, AND GROWTH PERFORMANCE OF LEAFY VEGETABLE

In organic farming, nitrogen (N) is one of the most crucial factors. Other than the limited source of N for organic fertilizer as the key factor for growth-limiting nutrients in organic farming, the cost of fertilization is also affected by different N application rates in organic farming running. Therefore, in this study, emphasis was given to the effect of different rates of N in organic farming on the plant total nutrient, uptake, and growth performance of leafy vegetables. A study was conducted in a glasshouse at an integrated organic farming research area in the Malaysian Agricultural Research and Development Institute (MARDI) at Serdang, Selangor for two consecutive cycles. The experimental treatments were focused on the different rates of N inputs (12, 24, and 36 kg/ha of N from organic sources) with soil alone as a control. The treatments were applied 14 days after seeding (14-DAS) using green spinach as a test crop. The study result found that a 24-63 kg/ha rate of organic N input had improved N and P uptake in spinach (leafy vegetables), as well as their growth performances. Study findings have shown that the range of application rate of organic N between 24 to 36 kg/ha, promotes nutrient uptake in plants and can be scaled up to farm level to sustain organic system productivity.

Colloidal organic phosphorus in the South China Sea

Phosphorus (P) is a growth-limiting nutrient that controls primary producers in marine environments. Recent studies suggested that dissolved organic P (DOP) may promote microbial production in the euphotic zone of the oligotrophic ocean. The characteristics of dissolved organic P (DOP) are crucial because it affects microbial utilization of P in the organic pool. Here we provide evidence of colloidal organic P (high-molecular-weight [HMW]-DOP, 1 kDa – 0.45 μm) in the South China Sea (SCS) and highlight the importance of size fractionation in the marine P cycle. Both soluble reactive phosphorus and DOP exhibited geographic variations, indicating that P cycling is strongly affected by hydrography and water mixing. DOP accounted for more than 90% of the P in the surface water of the SCS. A positive correlation between DOP and chlorophyll-a concentrations suggested that primary producers played a crucial role in DOP production. HMW-DOP comprised 75% of the surface DOP. The enrichment of P in the colloidal fraction compared with its low-molecular-weight (LMW, <1 kDa) counterpart suggested that HMW-DOP is a significant inventory to promote microbial growth. HMW-DOP depth profiles indicated surface enrichment, whereas LMW-DOP exhibited low variation throughout the water column. The difference in the features of these depth profiles indicated that P has different recycling pathways for the two size fractions. Overall, the size distribution of DOP reveals information on DOP characteristics and reactivity, which further provide insight into microbial P utilization in the ocean.

Wastewater irrigation: An opportunity for improving soil phosphorus availability; PHREEQC modeling and adsorption studies

Wastewater, an alternative supply of water and nutrients, is being allocated as a priority for human population sustainability in arid and semi-arid regions. This work proposes phosphorus (P), a vital growth-limiting nutrient, adsorption behavior in wastewater irrigated agricultural soils in comparison to non-irrigated soils using laboratory batch experiments. The adsorption mechanism was assessed using different adsorption isotherm models. Saturation indices were modeled, using the hydro-geochemical transport code PHREEQC and MINTEQ geochemical software. Phosphorus buffering parameters were also calculated based on the standard equations. The equilibrium data were well fitted with the Freundlich isotherm model. The physical adsorption mechanism was found based on the calculated isotherm parameters. The maximum adsorption capacity was two times more in non-wastewater irrigated soils than irrigated. Results highlighted the effectiveness of wastewater irrigation in P availability in soil. Based on the PHREEQC modeling data, precipitation of Pb and Zn mineral phases was probable in soils by wastewater influence. Meanwhile, the precipitation of stable calcium phases, that affect the P sorption and/or co-precipitation, in non-wastewater irrigated soils was highlighted in the PHREEQC calculations. The standard buffer capacity (SBC) was 43 and 64 L kg−1 in wastewater irrigated soils and non-irrigated soils, respectively. Findings of the present study demonstrate the importance of wastewater reuse opportunities for agricultural application, especially soil P availability, and are helpful to minimize the environmental impacts of wastewater and solid waste.

Physical and physicochemical characteristics of tomato fruits grown under different irrigation water levels and phosphorus sources and rates

Phosphorus (P) is one of the most limiting nutrients for growth and development of crops, and is important for soil fertility managements, since it affects the quality of fruits. With the hypothesis that different levels of irrigation and phosphorus fertilization interfere in the physical and physicochemical properties of tomatoes, the objective of this experiment was to verify this hypothesis and analyze the behavior of the fruits’ responses. The experiment was conducted in a greenhouse at the Hydraulic and Irrigation Laboratory of the Federal Institute of Education, Science, and Technology of the State of Goiás, in Rio Verde, Goiás, Brazil. A randomized block design was used, with a 4×2×4 split-plot arrangement and three replications, totaling 96 experimental plots. The treatments consisted of four P2O5 rates: 25%, 50%, 100%, and 200% of the recommended rate; two P sources: monoammonium phosphate (MAP) and organo-mineral (OM); and four IWL: 50%, 75%, 100%, and 125% of the field capacity. The fruits were evaluated for longitudinal (FLD) and transversal (FTD) diameters, potential hydrogen (pH), titratable acidity (TA), total soluble solid (TSS) contents, and TSS to TA ratio (TSS/TA). The effect of the interactions between the factors IWL and P rate, IWL and P source, and P rate and source were significant for FLD and FTD, TSS, TA, and TSS/TA. The effects of the interactions between IWL and P rate, and IWL and P source were significant for pH. The MAP source combined with the rate of 25% and IWL of 125% resulted in tomato fruits with larger FLD and FTD. The rate of 25% combined with IWL of 77% and 100% resulted in tomato fruits with higher TSS and pH. The OM source resulted in tomato fruits with higher TA when combined with IWL of 50%, and larger FLD when combined with IWL of 105% and 125%.&#x0D; (This article belongs to the Special Issue: Agricultural Water Management in Arid and Semi-arid Regions)

Shifts in microbial stoichiometry upon nutrient addition do not capture growth-limiting nutrients for soil microorganisms in two subtropical soils

Microbial stoichiometry has become a key aspect in ecological research as shifts in microbial C:N, C:P and N:P ratios upon nutrient addition are presumed to give insight into relative nutrient limitations for soil microorganisms–with far-reaching implications for biogeochemical processes. However, this expectation has never been tested against direct methods of microbial growth responses to nutrient addition. We therefore manipulated a subtropical grassland and forest soil with multifactorial C-, N- and P-additions during 30 days to induce changes in limiting resources and evaluated the resulting soil microbial growth rates, microbial biomass stoichiometry, potential enzyme activities and microbial community composition. Our results show that microbial stoichiometric shifts upon nutrient addition ambiguously predict growth-limiting nutrients for soil microbes. For example, P- and NP-addition to the grassland soil significantly shifted the microbial N:P ratio, which suggests increased N- relative to P-limitation. Microbial growth responses however indicated that soil microbes remained C limited. The same applies for the forest soil, where P-, CN-, NP- and CNP-additions shifted the microbial N:P ratio, yet microbial growth remained C limited. This indicates that microorganisms can immobilize N and P for storage when C is the main limiting nutrient, and that intracellular storage of N and P is responsible for the observed shifts in microbial stoichiometry. Moreover, our data imply that shifts in microbial C:N ratios do not necessarily indicate shifts in microbial community composition and suggest that soil microorganisms–when subject to resource pulses–are stoichiometrically quite plastic.

Biome boundary maintained by intense belowground resource competition in world’s thinnest-rooted plant community

Recent findings point to plant root traits as potentially important for shaping the boundaries of biomes and for maintaining the plant communities within. We examined two hypotheses: 1) Thin-rooted plant strategies might be favored in biomes with low soil resources; and 2) these strategies may act, along with fire, to maintain the sharp boundary between the Fynbos and Afrotemperate Forest biomes in South Africa. These biomes differ in biodiversity, plant traits, and physiognomy, yet exist as alternative stable states on the same geological substrate and in the same climate conditions. We conducted a 4-y field experiment to examine the ability of Forest species to invade the Fynbos as a function of growth-limiting nutrients and belowground plant-plant competition. Our results support both hypotheses: First, we found marked biome differences in root traits, with Fynbos species exhibiting the thinnest roots reported from any biome worldwide. Second, our field manipulation demonstrated that intense belowground competition inhibits the ability of Forest species to invade Fynbos. Nitrogen was unexpectedly the resource that determined competitive outcome, despite the long-standing expectation that Fynbos is severely phosphorus constrained. These findings identify a trait-by-resource feedback mechanism, in which most species possess adaptive traits that modify soil resources in favor of their own survival while deterring invading species. Our findings challenge the long-held notion that biome boundaries depend primarily on external abiotic constraints and, instead, identify an internal biotic mechanism-a selective feedback among traits, plant-plant competition, and ecosystem conditions-that, along with contrasting fire regime, can act to maintain biome boundaries.

Nitrogen fertilization differentially affects the symbiotic capacity of two co-occurring ectomycorrhizal species.

Forest trees rely on ectomycorrhizal (ECM) fungi to obtain growth-limiting nutrients. While addition of nitrogen (N) has the potential to disrupt these critical relationships, there is conflicting evidence as to the mechanism by which ECM:host mutualism may be affected. We evaluated how N fertilization altered host interactions and gene transcription between Eucalyptus grandis and Pisolithus microcarpus or Pisolithus albus, two closely related ECM species that typically co-occur within the same ecosystem. Our investigation demonstrated species-specific responses to elevated N: P. microcarpus maintained its ability to transport microbially sourced N to its host but had a reduced ability to penetrate into root tissues, while P. albus maintained its colonization ability but reduced delivery of N to its host. Transcriptomic analysis suggests that regulation of different suites of N-transporters may be responsible for these species-specific differences. In addition to N-dependent responses, we were also able to define a conserved 'core' transcriptomic response of Eucalyptus grandis to mycorrhization that was independent of abiotic conditions. Our results demonstrate that even between closely related ECM species, responses to N fertilization can vary considerably, suggesting that a better understanding of the breadth and mechanisms of their responses is needed to support forest ecosystems into the future.

The Extended Effect of Top-Dressed Recovered Struvite Fertiliser on Residual Irish Grassland Soil Phosphorus Levels Compared to Commercial Phosphorus Fertiliser

Phosphorus (P) is an essential growth-limiting nutrient that cannot be synthetically produced. Mineral P fertilisers are typically applied to crops to sustain modern farming practice and food production. These fertilisers are generally derived from finite phosphate ore, for which there is much concern over long-term sustainability. To address these concerns, various technologies have been developed to recover P from municipal wastewater treatment plants. One product recovered from these processes is struvite, which is a precipitate formed of magnesium, ammonium, and phosphate. To analyse the fertilisation value of the recovered struvite, field trials were conducted on perennial ryegrass (Lolium perenne) over three growing seasons, analysing the dry matter yield of recovered struvite fertiliser. The trial was based on a three-crop silage system designed to mimic typical Irish agricultural practice. This research highlights that recovered struvite as a fertiliser provides additional benefits including increasing the soil P levels. The struvite test case produced a statistically significantly increased soil P level from the baseline of Morgan’s extractable P content of 6.4 mg/L to the optimum Morgan’s soil P level of 11.13 mg/L. The findings of this research provide insight into the added benefits of recovered struvite fertiliser as a sustainable renewable P fertiliser.

Comparing the bacterial growth potential of ultra-low nutrient drinking water assessed by growth tests based on flow cytometric intact cell count versus adenosine triphosphate

The bacterial growth potential (BGP) of drinking water is widely assessed either by flow cytometric intact cell count (BGPICC) or adenosine triphosphate (BGPATP) based methods. Combining BGPICC and BGPATP measurements has been previously applied for various types of drinking water having high to low growth potential. However, this has not been applied for water with ultra-low nutrient content, such as remineralised RO permeate. To conduct a sound comparison, conventionally treated drinking water was included in this study, which was also used as an inoculum source. BGPICC, BGPATP, intact cell-yield (YICC), and ATP-yield (YATP) were determined for conventionally treated drinking water (Tap-water) and remineralised RO permeate (RO-water). In addition, both BGPICC and BGPATP methods were used to identify the growth-limiting nutrient in each water type. The results showed that the BGPICC ratio between Tap-water/RO-water was ∼7.5, whereas the BGPATP ratio was only ∼4.5. Moreover, the YICC ratio between Tap-water/RO-water was ∼2 (9.8 ± 0.6 × 106 vs. 4.6 ± 0.8 × 106 cells/µg-C), whereas the YATP ratio was ∼1 (0.39 ± 0.12 vs. 0.42 ± 0.06 ng ATP/µg-C), resulting in a consistently higher ATP per cell in RO-water than that of Tap-water. Both BGPICC and BGPATP methods revealed that carbon was the growth-limiting nutrient in the two types of water. However, with the addition of extra carbon, phosphate limitation was detected only with the BGPICC method, whereas BGPATP was not affected, suggesting that a combination of carbon and phosphate is essential for biomass synthesis, whereas carbon is probably utilised for cellular activities other than cell synthesis when phosphate is limited. It was estimated that the intact cell-yield growing on phosphate would be 0.70 ± 0.05 × 109 cells/µg PO4-P.

Duration of forest fertilization effects on streamwater chemistry in a catchment in central Sweden

Demands for forest biomass production for energy, construction and carbon storage purposes are increasing, and therefore measures to increase tree growth are required. One potential measure is nitrogen (N) fertilization, as N is usually the most growth-limiting nutrient in boreal forests and partly due to decreasing atmospheric N deposition in northern Europe in recent decades. However, N fertilization can have adverse effects, such as soil acidification and N leaching, particularly nitrate leaching via streamwater flow. To mitigate the acidification risk, dolomite (CaMg(CO3)2) is added to N fertilizer boron (B) as increased tree growth hamper tree uptake of this essential micronutrient.This study examined the effects of forest fertilization on streamwater chemistry in the 45 ha Swedish catchment Risfallet (RF), around 80% of which was treated with fertilizer. That was rather exceptional, as most previous catchments studied have had <50% treated area, which may give weaker treatment signals. A paired catchment method combined with the control area and calibration period technique was applied to evaluate leaching effects from forest fertilization.Effects over 7.5 years were compared with previously reported initial effects in the first year, in order to assess the duration of fertilization effects on surface water. High excess outflow of N over five years was detected, with 20% of the applied amount leached and with nitrate dominating total nitrogen. Excess outflow of Ca and Mg was highest in the first year. Effects on pH were limited, with calculated untreated pH on 5.9 being on average 0.4 units lower during the first six months and then remaining at 0.2 units lower.Recommended could be to mainly fertilize well-drained soil, avoid wet areas and open streams. Consider the hydrological conditions while weather would be more hazardous to foresee.

A biophysical limit for quorum sensing in biofilms

Bacteria grow on surfaces in complex immobile communities known as biofilms, which are composed of cells embedded in an extracellular matrix. Within biofilms, bacteria often interact with members of their own species and cooperate or compete with members of other species via quorum sensing (QS). QS is a process by which microbes produce, secrete, and subsequently detect small molecules called autoinducers (AIs) to assess their local population density. We explore the competitive advantage of QS through agent-based simulations of a spatial model in which colony expansion via extracellular matrix production provides greater access to a limiting diffusible nutrient. We note a significant difference in results based on whether AI production is constitutive or limited by nutrient availability: If AI production is constitutive, simple QS-based matrix-production strategies can be far superior to any fixed strategy. However, if AI production is limited by nutrient availability, QS-based strategies fail to provide a significant advantage over fixed strategies. To explain this dichotomy, we derive a biophysical limit for the dynamic range of nutrient-limited AI concentrations in biofilms. This range is remarkably small (less than 10-fold) for the realistic case in which a growth-limiting diffusible nutrient is taken up within a narrow active growth layer. This biophysical limit implies that for QS to be most effective in biofilms AI production should be a protected function not directly tied to metabolism.

Characterizing bioavailable phosphorus concentrations in an agricultural stream during hydrologic and streambed disturbances.

In freshwater ecosystems, phosphorus (P) is often considered a growth-limiting nutrient. The use of fertilizers on agricultural fields has led to runoff-driven increases in P availability in streams, and the subsequent eutrophication of downstream ecosystems. Isolated storms and periodic streambed dredging are examples of two common disturbances that contribute dissolved and particulate P to agricultural streams, which can be quantified as soluble reactive P (SRP) using the molybdate-blue method on filtered water samples, or total P (TP) measured using digestions on unfiltered water reflecting all forms of P. While SRP is often considered an approximation of bioavailable P (BAP), research has shown that this is not always the case. Current methods used to estimate BAP do not account for the role of biology (e.g., NaOH extractions) or require specialized platforms (e.g., algal bioassays). Here, in addition to routine analysis of SRP and TP, we used a novel yeast-based bioassay with unfiltered sample water to estimate BAP concentrations during two storms (top 80% and > 95% flow quantiles), and downstream of a reach where management-associated dredging disturbed the streambed. We found that the BAP concentrations were often greater than SRP, suggesting that SRP is not fully representative of P bioavailability. The SRP concentrations were similarly elevated during the two storms, but remained consistently low during streambed disturbance. In contrast, turbidity and TP were elevated during all events. The BAP concentrations were significantly related to turbidity during all disturbance events, but with TP only during storms. The novel yeast assay suggests that BAP export can exceed SRP, particularly when streams are not in equilibrium, such as the rising limb of storms or during active dredging.