P-limited Conditions Research Articles

Response of Long-Term Water and Phosphorus of Wheat to Soil Microorganisms

Phosphorus deficiency critically constrains crop growth. Soil microbial diversity, which is crucial for maintaining terrestrial ecosystem integrity, plays a key role in promoting soil P cycling. Therefore, it is imperative to understand the survival strategies of microorganisms under P-limited conditions and explore their roles in community regulation. We initiated a comprehensive, long-term, in situ wheat field experiment to measure soil physicochemical properties, focusing on the different forms of soil inorganic P. Subsequently, 16S rRNA and ITS marker sequencing was employed to study changes in soil microbial abundance and community structure and predict functional alterations. The results showed that soil water and P deficiencies significantly affected wheat growth and development, soil physicochemical properties, and microbial diversity and function. Prolonged P deficiency lowered soil pH, significantly increasing phosphatase content (58%) under W1 (normal irrigation) conditions. Divalent calcium phosphate decreased significantly under W0 (lack of irrigation) and W1 conditions, and the most stable ten-valent calcium phosphate began to transform under W0 conditions. Soil microbial diversity increased (e.g., Proteobacteria and Vicinamibacterales) and enhanced the transport capacity of bacteria. P deficiency affected the coexistence networks between bacteria and fungi, and SEM (structural equation modeling) analysis revealed a stronger correlation in bacteria (r2 = 0.234) than in fungi (r2 = 0.172). In soils deprived of P for 7 years, the soil P content and forms were coupled with microbial changes. Microorganisms exhibited community and functional changes in response to low-phosphorus soil, concurrently influencing soil P status. This study enhances our understanding of rhizospheric processes in soil P cycling under microbial feedback, particularly the impact of microbial interactions on changes in soil P forms under P-limited conditions.

Multiomics dissection of Brassica napus L. lateral roots and endophytes interactions under phosphorus starvation

Many plants associate with endophytic microbes that improve root phosphorus (P) uptake. Understanding the interactions between roots and endophytes can enable efforts to improve P utilization. Here, we characterize the interactions between lateral roots of endophytes in a core collection of 50 rapeseed (Brassica napus L.) genotypes with differing sensitivities to low P conditions. With the correlation analysis result between bacterial abundance and plant physiological indices of rapeseeds, and inoculation experiments on plates and soil, we identify one Flavobacterium strain (C2) that significantly alleviates the P deficiency phenotype of rapeseeds. The underlying mechanisms are explored by performing the weighted gene coexpression network analysis (WGCNA), and conducting genome-wide association studies (GWAS) using Flavobacterium abundance as a quantitative trait. Under P-limited conditions, C2 regulates fatty acid and lipid metabolic pathways. For example, C2 improves metabolism of linoleic acid, which mediates root suberin biosynthesis, and enhances P uptake efficiency. In addition, C2 suppresses root jasmonic acid biosynthesis, which depends on α-linolenic acid metabolism, improving C2 colonization and activating P uptake. This study demonstrates that adjusting the endophyte composition can modulate P uptake in B. napus plants, providing a basis for developing agricultural microbial agents.

Root exudation patterns of contrasting rice (Oryza sativa L.) lines in response to P limitation

Main conclusionRice exudation patterns changed in response to P deficiency. Higher exudation rates were associated with lower biomass production. Total carboxylate exudation rates mostly decreased under P-limiting conditions.Within the rhizosphere, root exudates are believed to play an important role in plant phosphorus (P) acquisition. This could be particularly beneficial in upland rice production where P is often limited. However, knowledge gaps remain on how P deficiency shapes quality and quantity of root exudation in upland rice genotypes. We therefore investigated growth, plant P uptake, and root exudation patterns of two rice genotypes differing in P efficiency in semi-hydroponics at two P levels (low P = 1 µM, adequate P = 100 µM). Root exudates were collected hydroponically 28 and 40 days after germination to analyze total carbon (C), carbohydrates, amino acids, phenolic compounds spectrophotometrically and carboxylates using a targeted LC–MS approach. Despite their reported role in P solubilization, we observed that carboxylate exudation rates per unit root surface area were not increased under P deficiency. In contrast, exudation rates of total C, carbohydrates, amino acids and phenolics were mostly enhanced in response to low P supply. Overall, higher exudation rates were associated with lower biomass production in the P-inefficient genotype Nerica4, whereas the larger root system with lower C investment (per unit root surface area) in root exudates of the P-efficient DJ123 allowed for better plant growth under P deficiency. Our results reveal new insights into genotype-specific resource allocation in rice under P-limiting conditions that warrant follow-up research including more genotypes.

Different ectomycorrhizal fungal species impact poplar growth but not phosphorus utilization under low P supply.

Tree growth is often limited by phosphorus (P) availability. The trade-off between P homeostasis and growth is unknown. Ectomycorrhizal fungi (EMF) facilitate P availability but this trait varies among different fungal species and isolates. Here, we tested the hypotheses that (i) colonization with EMF boosts plant growth under P-limited conditions and that (ii) the poplars show P homeostasis because increased P uptake is used for growth and not for P accumulation in the tissues. We used two P treatments (high phosphate [HP]: 64μM Pi, low phosphate [LP]: 0.64μM Pi in the nutrient solution) and four fungal treatments (Paxillus involutus MAJ, Paxillus involutus NAU, Laccaria bicolor dikaryon LBD, Laccaria bicolor monokaryon LBM) in addition to non-inoculated poplar plants (NI) to measure growth, biomass, gas exchange and P contents. High phosphate (HP) stimulated growth compared with LP conditions. Poplars colonized with MAJ, NAU and NI showed higher growth and biomass production than those with LBD or LBM. Photosynthesis rates of poplars with lower biomass production were similar to or higher than those of plants with higher growth rates. The tissue concentrations of P were higher under HP than LP conditions and rarely affected by ectomycorrhizal colonization. Under LP, the plants produced 44% greater biomass per unit of P than under HP. At a given P supply, the tissue concentration was stable irrespective of the growth rate indicating P homeostasis. Laccaria bicolor caused growth inhibition, irrespective of P availability. These results suggest that in young poplars distinct species-specific ectomycorrhizal traits overshadowed potential growth benefits.

Species interactions and bacterial inoculation enhance plant growth and shape rhizosphere bacterial community structure in faba bean – wheat intercropping under water and P limitations

Grain legumes / cereals intercropping systems with microbial inoculants, hold promise for improving crop productivity under stressful conditions. However, the tripartite interaction involving intercropping system, associated microbiota and stress combining water deficit and low phosphorus (P) availability remains understudied. This study evaluated the impact of three bacterial consortia (C4, C6, and Cref containing Rhizobium and PSB strains) including single Rhizobium (Rhizobium laguerreae) on the agro-physiological performance of wheat (Triticum durum) and faba bean (Vicia faba) grown as intercrops or sole-crops under P and water deficient conditions. Inoculation, especially with C6, significantly improved shoot and root biomasses of both wheat (up to 66 and 81 %) and faba bean (up to 54 and 266 %) intercrops compared to single Rhizobium inoculation and control treatments. Intercropping generally outperformed sole-cropping in above-ground physiology, root morphological traits, shoot and root P content, with a notable effect in response to C6 exhibiting low microbial biomass P. Changes in bacterial community structure were primarily driven by cropping pattern and water regime rather than bacterial inoculation. Intercropping maintained bacterial diversity but shifted community structure, favoring Proteobacteria. Overall, inoculating intercropped wheat and faba bean with Rhizobium-containing consortia induced beneficial below-ground interspecies interactions under water and P-limiting conditions.

Overexpression of GmPAP4 Enhances Symbiotic Nitrogen Fixation and Seed Yield in Soybean under Phosphorus-Deficient Condition.

Legume crops establish symbiosis with nitrogen-fixing rhizobia for biological nitrogen fixation (BNF), a process that provides a prominent natural nitrogen source in agroecosystems; and efficient nodulation and nitrogen fixation processes require a large amount of phosphorus (P). Here, a role of GmPAP4, a nodule-localized purple acid phosphatase, in BNF and seed yield was functionally characterized in whole transgenic soybean (Glycine max) plants under a P-limited condition. GmPAP4 was specifically expressed in the infection zones of soybean nodules and its expression was greatly induced in low P stress. Altered expression of GmPAP4 significantly affected soybean nodulation, BNF, and yield under the P-deficient condition. Nodule number, nodule fresh weight, nodule nitrogenase, APase activities, and nodule total P content were significantly increased in GmPAP4 overexpression (OE) lines. Structural characteristics revealed by toluidine blue staining showed that overexpression of GmPAP4 resulted in a larger infection area than wild-type (WT) control. Moreover, the plant biomass and N and P content of shoot and root in GmPAP4 OE lines were also greatly improved, resulting in increased soybean yield in the P-deficient condition. Taken together, our results demonstrated that GmPAP4, a purple acid phosphatase, increased P utilization efficiency in nodules under a P-deficient condition and, subsequently, enhanced symbiotic BNF and seed yield of soybean.

Phosphorus limitation combined with aluminum triggers synergistic responses on the freshwater microalgae Raphidocelis subcapitata (Chlorophyceae)

In the environment, algae are exposed to several stressors such as limitation of essential nutrients and excess of toxic substances. It is well known the importance of phosphorus (P) supply for healthy metabolism of algae and impacts at this level can affect the whole aquatic trophic chain. Aluminum (Al) is the most abundant metal on Earth and it is toxic to different trophic levels. Processes related to P and Al assimilation still need to be clarified and little is known about the responses of microalgae exposed to the two stressors simultaneously. We evaluated the effects of environmental concentrations of Al and P limitation, isolated and in combination, on growth, pigment production and photosynthesis of the freshwater microalga Raphidocelis subcapitata. Both stressors affected cell density, chlorophyll a, carotenoids, and maximum quantum yield. Al did not affect any other evaluated parameter, while P limitation affected parameters related to the dissipation of heat by algae and the maximum electron transport rate, decreasing the saturation irradiance. In the combination of both stressors, all parameters evaluated were affected in a synergistic way, i.e., the results were more harmful than expected considering the responses to isolated stressors. Our results indicate that photoprotection mechanisms of algae were efficient in the presence of both stressors, avoiding damages to the photosynthetic apparatus. In addition, our data highlight the higher susceptibility of R. subcapitata to Al in P-limited conditions.

On the presence and detectability of polyphosphates in soil microbial biomass

Polyphosphates (PolyP), i.e. phosphate polymers, are commonly found in pure cultures of various microorganisms. Although they have been the subject of intensive microbiological research in the past, they have never been directly studied in species-rich soil microbial communities. So far, there are only few studies indirectly suggesting that soil microorganisms build up PolyP as a storage for phosphorus (P), and use them when soil P availability decreases. We attempted to provide direct evidence for PolyP presence in soil microorganisms, and test if the PolyP can be detected in the soil microbial biomass P pool applying the standard chloroform-fumigation extraction method. Twelve different soil samples were collected along the gradient of forest recovery after the bark beetle outbreak in the catchments of two adjacent glacier lakes (Plešné and Čertovo, Bohemian forest, Czech Republic). The presence of PolyP in the samples was assessed by staining in a manipulative experiment designed to deplete any PolyP present. Carbon (C), nitrogen (N), and P in the microbial biomass were estimated by the chloroform-fumigation extraction method and soil slurries of fresh samples stained by the Neisser method. The soils were then mixed with sterile sand and supplemented with growth medium without P. The rate of growth of microbial biomass was estimated from oxygen consumption during one week incubation at dark. After one week, the microbial biomass C, N, and the P were estimated again and samples stained. The combination of the incubation experiment and staining proved that the soil microorganisms in the collected samples contained PolyP and that PolyP were used to achieve maximum growth rate under P-limited conditions. The C to N to P ratio increased significantly over one week of incubation reflecting the changing PolyP content. To further confirm that the fumigation extraction method is sensitive to PolyP content, manufactured PolyP was added to all soils at different steps of the fumigation extraction method, and its recovery was estimated. Recovery ranged from 80 to 100%. Abiotic depolymerisation at acidic conditions required for the correct quantification of P-PO4 using molybdenum-blue method was very likely responsible for half of the recovery, the remaining being enzymatic depolymerisation. We conclude that PolyP are ubiquitous in soils and affect microbial biomass P estimation. The high recovery rate of PolyP around 90% implies that presence of PolyP can cause a significant overestimation of the microbial biomass P when typical correction factor 0.4 is used.

Periphyton enzymatic activities in the water column along internal low-phosphorus nutrient gradients in the Everglades Stormwater Treatment Areas

The Everglades Stormwater Treatment Areas (STAs), important components in the Everglades Restoration efforts, were built to remove excess phosphorus (P) from stormwater runoff before it enters the Everglades Protection Area. The STAs operate within the low phosphorus domain of constructed wetlands and are mandated to achieve ultra-low P outflow concentrations to protect the receiving water bodies. Microbial processing by periphyton enzymatic activity under these low-P conditions may be critical for achieving nutrient reduction, particularly in the lower reaches of the STA treatment cells where nutrients are primarily in the dissolved organic and particulate forms. Enzymes are produced by periphyton to liberate required nitrogen (N), carbon (C), and P from dissolved organic matter under nutrient limiting conditions. Enzyme activity had not been measured within the STA treatment cells and this study was conducted to evaluate if enzyme activity occurs along the nutrient transect from inflow to outflow in well-performing treatment cells and if this activity differs within the dominant vegetation communities (i.e., emergent or submerged aquatic vegetation, EAV and SAV, respectively). The STAs receive variable hydraulic and nutrient loadings and the potential influence of these conditions on enzyme activity was evaluated under a range of loadings. The periphyton response in three different well-performing STA treatment cells (STA-2 Cells 1 and 3 and STA-3/4 Cell 3B) was evaluated using periphyton that accrued in the water column over a 7-d period. Because nutrient cycling is interrelated, a suite of enzyme activity associated with nutrient acquisition from dissolved organic material was measured. Enzymes associate with P-acquisition (phosphatase; alkaline phosphatase and bis-phosphodiesterase), C-acquisition (β-glucosidase), and N-acquisition (leucine aminopeptidase) were measured along internal transects from inflow to outflow.We found that enzyme activity occurs in the STA treatment cells and the most reactive enzymes were the phosphatase and N-acquisition enzymes; there was little activity and variability measured in the C-acquisition enzyme. Phosphatase activity increased along the nutrient gradient and the rates became more variable at the midflow and outflow transect locations. When dissolved organic P (DOP) in the surface water was above 10 μg/L, the phosphatase rates were reduced, an expected response since enzyme production occurs only during nutrient limiting conditions. When DOP concentrations were <10 μg/L, greatly increased rates were observed but there were still also times when the rates were reduced that did not appear to be influenced by surface water organic P or N concentrations or water depths. The activity for N-acquisition enzyme was also found to be variable, but this variability was between each treatment cell rather than along the transect as observed for phosphatase activity. The proportional change of each enzyme activity to the other was used to evaluate nutrient limiting conditions in the periphyton. Along the transect, both N- and P-limiting conditions were found in the periphyton at the inflow and midflow sites and solely P-limiting conditions at the outflows in all treatment cells. This implies that the mineralization processes can be disrupted under higher nutrient conditions, impacting cycling of the organic material. There were also indications that nutrient cycling may differ between periphyton from the EAV and SAV communities under low-P conditions, suggesting there may be a complementary enzymatic response within a mixed marsh configuration that may enhance nutrient cycling.

Transcriptomic Analysis of the Response of the Toxic Dinoflagellate Prorocentrum lima to Phosphorous Limitation.

Some dinoflagellates cause harmful algal blooms, releasing toxic secondary metabolites, to the detriment of marine ecosystems and human health. Phosphorus (P) is a limiting macronutrient for dinoflagellate growth in the ocean. Previous studies have been focused on the physiological response of dinoflagellates to ambient P changes. However, the whole-genome's molecular mechanisms are poorly understood. In this study, RNA-Seq was utilized to compare the global gene expression patterns of a marine diarrheic shellfish poisoning (DSP) toxin-producing dinoflagellate, Prorocentrum lima, grown in inorganic P-replete and P-deficient conditions. A total of 148 unigenes were significantly up-regulated, and 30 unigenes were down-regulated under 1/4 P-limited conditions, while 2708 unigenes were significantly up-regulated, and 284 unigenes were down-regulated under 1/16 P-limited conditions. KEGG enrichment analysis of the differentially expressed genes shows that genes related to ribosomal proteins, glycolysis, fatty acid biosynthesis, phagosome formation, and ubiquitin-mediated proteolysis are found to be up-regulated, while most of the genes related to photosynthesis are down-regulated. Further analysis shows that genes encoding P transporters, organic P utilization, and endocytosis are significantly up-regulated in the P-limited cells, indicating a strong ability of P. lima to utilize dissolved inorganic P as well as intracellular organic P. These transcriptomic data are further corroborated by biochemical and physiological analyses, which reveals that under P deficiency, cellular contents of starch, lipid, and toxin increase, while photosynthetic efficiency declines. Our results indicate that has P. lima evolved diverse strategies to acclimatize to low P environments. The accumulation of carbon sources and DSP toxins could provide protection for P. lima to cope with adverse environmental conditions.

Argonaute7 (AGO7) optimizes arbuscular mycorrhizal fungal associations and enhances competitive growth in Nicotiana attenuata.

Plants interact with arbuscular mycorrhizal fungi (AMF) and in doing so, change transcript levels of many miRNAs and their targets. However, the identity of an Argonaute (AGO) that modulates this interaction remains unknown, including in Nicotiana attenuata. We examined how the silencing of NaAGO1/2/4/7/and 10 by RNAi influenced plant-competitive ability under low-P conditions when they interact with AMF. Furthermore, the roles of seven miRNAs, predicted to regulate signaling and phosphate homeostasis, were evaluated by transient overexpression. Only NaAGO7 silencing by RNAi (irAGO7) significantly reduced the competitive ability under P-limited conditions, without changes in leaf or root development, or juvenile-to-adult phase transitions. In plants growing competitively in the glasshouse, irAGO7 roots were over-colonized with AMF, but they accumulated significantly less phosphate and the expression of their AMF-specific transporters was deregulated. Furthermore, the AMF-induced miRNA levels were inversely regulated with the abundance of their target transcripts. miRNA overexpression consistently decreased plant fitness, with four of seven-tested miRNAs reducing mycorrhization rates, and two increasing mycorrhization rates. Overexpression of Na-miR473 and Na-miRNA-PN59 downregulated targets in GA, ethylene, and fatty acid metabolism pathways. We infer that AGO7 optimizes competitive ability and colonization by regulating miRNA levels and signaling pathways during a plant's interaction with AMF.

Incorporating biotic phosphorus-acquisition strategies into soil phosphorus transformation under long-term salinization in a tidal wetland

Tidal wetland plants can maintain high primary productivity under salinization, even with low phosphorus (P) availability. However, little is known about how they adapt to salinization-induced ecosystem P limitation over time. We established mesocosms using tidal freshwater wetland soils and the salt-tolerant plant Cyperus malaccensis and subjected them to short-term (6 months) and long-term (3.5 years) salinization. Overall, short-term salinization did not change plant or microbial biomass nitrogen/P ratios, whereas long-term salinization increased both, indicating that short-term salinization did not alter ecosystem P limitation, but long-term salinization exacerbated it. Concurrently, short-term salinization reduced the moderately labile inorganic P (Pi) pool, whereas long-term salinization also reduced the hydrolyzable organic P (Po) and primary mineral P pools. During both short- and long-term salinization, moderately labile Pi mobilization exhibited a negative correlation with Fe sulfide concentration and a positive correlation with Fe(III) concentrations. These results suggested that both short- and long-term salinization obtained P through abiotic P-acquisition strategies. Mobilization of the primary mineral P pool and hydrolyzable Po pools was negatively linked to root arbuscular mycorrhizal fungi (AMF) biomass and soil alkaline phosphomonoesterase (ALP) activity, respectively. This indicated that long-term salinization acquired P via biotic P-acquisition strategies. Specifically, soil microorganisms increased fungi predominance and thereby increased ALP activity to convert hydrolyzable Po, while tidal wetland plants enhanced root AMF associations to release carboxylate to transform primary mineral P. Overall, our results highlight that abiotic P-acquisition strategies could offset the ecosystem P limitation during short-term salinization, whereas biotic P-acquisition strategies play a more important role in soil P transformation under long-term salinization. Through biotic P-acquisition, tidal wetland ecosystems can maintain high plant primary productivity through biotic P-acquisition even under P-limited conditions, exhibiting increased resilience to sea-level rise.

Long-Term Variations of Biogenic Elements and Nutritional Status in Daya Bay, Northern South China Sea

This study explored the variations in the characteristics of the trophic structure of Daya Bay island waters over the last four decades based on the survey findings and research data on biogenic elements (dissolved inorganic nitrogen (DIN), NO2−, NO3−, NH4+, PO43−, and SiO32−) in Daya Bay during 1985–2021. At this time, the DIN concentration increased from 21.14 µg·L−1 to 558.42 µg·L−1 (26.41-fold increase), whereas the SiO32− concentration increased by only 3.6-fold. The PO43− concentrations attained a peak in 2004 and experienced a steady decline over the rest of the survey period. The fractions of NH4+, NO3−, and NO2− in DIN changed from 0.45, 0.40, and 0.15 in 1986 to 0.26, 0.74, and 0.003 in 2021, respectively. Overall, the mean values of NH4+, NO3−, and NO2− accounted for 45.2%, 42.5%, and 12.3%, respectively. The N/P(DIN/PO43−) ratio in Daya Bay increased from 28.08 in the 1980s to 51.63 in the 2010s. Meanwhile, the nutrient limitation conditions showed a gradual shift from N-limited to P-limited conditions. According to the nutrient quality index (NQI) analysis, the trophic state level of Daya Bay waters fell into the oligotrophic category 30 years ago (1985–2002, NQI &lt; 2), whereas it increased from the mesotrophic level in 2005 (NQI = 2.03) to the eutrophic level in 2019 (NQI = 3.33) over the last 20 years. The results based on the eutrophication index (EI) of Daya Bay waters were generally consistent with those based on the NQI, displaying that the trophic level of Daya Bay waters indicated an increasing trend from 2005 to 2019. Moreover, the assessment data in 2021 indicated a decrease in the NQI to 0.90, thereby attaining the oligotrophic level again. This may be related to the decrease in aquacultural area in the bay over the last two years. The correlation analysis among the DIN, PO43−, and nutrient levels of Daya Bay waters indicated that the input of nitrogen and phosphorus was the primary reason for the higher nutrient levels in the water bodies; among them, municipal sewage discharge, aquaculture, and atmospheric deposition from industry are the main factors for the over importation. This indicates that the changes in the biogenic element concentrations led to variations in the trophic structure and level of Daya Bay and may be attributed to population growth and the development of the seaside industry and agriculture in the region.

Groundwater as a source of phosphorus and silicate in an estuarine zone: results from continuous monitoring of nutrients and 222Rn

The concentrations of 222Rn and dissolved inorganic nutrients in river water at a fixed station of the Nakdong River estuary which has an artificial barrage were continuously measured from October 2014 to May 2015. Monthly benthic 222Rn flux from the river bottom was estimated using a simple mass balance model, taking into account 222Rn sources and sinks. The estimated benthic 222Rn flux shows a significant correlation with groundwater level, suggesting that groundwater level can be used as a representative of the groundwater input. Based on correlation analyses, the concentration of dissolved inorganic nitrogen (DIN) was found to be dependent primarily on river water input. In contrast, the concentrations of dissolved inorganic phosphorus (DIP) and dissolved inorganic silicate (DSi) were predominantly controlled by groundwater input. Our results suggest that groundwater input may be an important source of DIP, especially under P-limited condition, which can affect marine primary production and ecological problems such as eutrophication and algal blooms in the coastal zones.

Domoic acid production by Pseudo-nitzschia australis: Re-evaluating the role of macronutrient limitation on toxigenicity

The toxigenic diatom Pseudo-nitzschia australis (Frenguelli), isolated from the California Current System (CCS), was examined in unialgal laboratory cultures to evaluate domoic acid (DA) production and cellular growth as a response to macronutrient limitation. Toxic blooms of P. australis are common in the coastal waters of eastern boundary upwelling systems (EBUS), including those of the CCS off the west coast of the United States where limitation by macronutrients, specifically silicon as silicic acid [Si(OH)4], or phosphorus as phosphate [PO43-], has been suggested to increase the production of DA by these diatoms. This study used batch cultures grown under conditions of macronutrient sufficiency and limitation, expected during and after a natural upwelling event, to determine whether PO43- or Si(OH)4 deficiency enhances the production of DA and the expected risk of DA toxicity in natural coastal ecosystems. These controlled lab studies demonstrate that despite increases in cell-specific DA concentrations found during the nutrient-limited stationary phase, DA production rates did not increase due to either PO43- or Si(OH)4 limitation, and total DA production rates were statistically greater during the nutrient-replete, exponential growth phase compared to the nutrient-limited, stationary phase. In addition, the relative contribution of particulate DA (pDA) and dissolved DA (dDA) varied markedly with growth phase, where the contribution of pDA to total DA (pDA + dDA) declined from an average of 70% under P- and Si-replete conditions to 49% under P-limited conditions and 39% under Si-limited conditions. These laboratory results demonstrate that macronutrient sufficiency does not regulate the biosynthetic production of DA by this strain of P. australis. This finding, together with a comparative analysis of the various equations employed to estimate DA production, suggests that the current paradigm of increased toxigenicity due to macronutrient limitation be carefully re-examined, particularly when attempting to forecast the toxic threat of DA to coastal ecosystems as a function of macronutrient availability.

Effects of Phosphorus Limitation on the Bioavailability of DOM Released by Marine Heterotrophic Prokaryotes.

Heterotrophic prokaryotes (HP) contribute largely to dissolved organic matter (DOM) processing in the ocean, but they also release diverse organic substances. The bioavailability of DOM released by HP under varying environmental conditions has not been fully elucidated. In this study, we investigated the bioavailability of DOM released by a single bacterial strain (Sphingopyxis alaskensis) and 2 natural HP communities grown under P-replete and P-limited conditions. The released DOM (HP-DOM) was used as a substrate for natural HP communities at a coastal site in the Northwestern Mediterranean Sea. We followed changes in HP growth, enzymatic activity, diversity, and community composition together with the consumption of HP-DOM fluorescence (FDOM). HP-DOM produced under P-replete and P-limited conditions promoted significant growth in all incubations. No clear differences in HP-DOM lability released under P-repletion and P-limitation were evidenced based on the HP growth, and P-limitation was not demonstrated to decrease HP-DOM lability. However, HP-DOM supported the growth of diverse HP communities, and P-driven differences in HP-DOM quality were selected for different indicator taxa in the degrading communities. The humic-like fluorescence, commonly considered recalcitrant, was consumed during the incubations when this peak was initially dominating the FDOM pool, and this consumption coincided with higher alkaline phosphatase activity. Taken together, our findings emphasize that HP-DOM lability is dependent on both DOM quality, which is shaped by P availability, and the composition of the consumer community.

Vegetation restoration of abandoned cropland improves soil ecosystem multifunctionality through alleviating nitrogen-limitation in the China Danxia.

The microbial requirement for nutrient resources can be estimated by soil extracellular enzyme stoichiometry (EES) and their stoichiometries. Implementing the Grain for Green Program has significantly impacted land use and soil nutrient management in the China Danxia. However, drivers of soil microbial nutrient limitation changes in abandoned cropland (AC) remained unclear after vegetation restoration. Here, according to vector analysis, we evaluated microbial nutrient limitation by studying soil EES across vegetation restoration types (naturally restored secondary forests (NF) and artificially planted forests (AF)) with AC as a control. Results showed both NF and AF soils averaged higher C- and P- acquiring enzyme, indicating rapid C and P turnover rates after vegetation restoration. However, vegetation restoration resulted in higher C requirement for microorganisms with higher enzyme C:N and vector length. In addition, microorganisms shifted from N- (< 45°) to P-limited (> 45°) conditions with enzyme N:P less than 1 after vegetation restoration, and NF exacerbated microbial P limitation compared to AF. Decreased N limitation following vegetation restoration could be contributed to improving soil ecosystem multifunctionality. The greater variation of EES was explained by the interaction of pH, soil nutrient, and microbial biomass than by any one of these factors alone, suggesting that both abiotic and biotic factors regulate microbial nutrient limitation and microbial process. Overall, our results revealed vegetation restoration could alleviate N limitation in the China Danxia, and thus enhance soil ecosystem by regulating lower microbial N limitation, which provide insight into nutrient management strategies under ecological restoration of degraded areas.

Carbohydrate and lipid balances in the positive plant phenotypic response to arbuscular mycorrhiza: increase in sink strength.

The exchange of phosphorus (P) and carbon (C) between plants and arbuscular mycorrhizal fungi (AMF) is a major determinant of their mutualistic symbiosis. We explored the C dynamics in tomato (Solanum lycorpersicum) inoculated or not with Rhizophagus irregularis to study their growth response under different NaH2 PO4 concentrations (Null P, 0 mM; Low P, 0.065 mM; High P, 1.3 mM). The percentage of AMF colonization was similar in plants under Null and Low P, but severely reduced under High P. However, the AMF mass biomarker 16:1ω5 revealed higher fungal accumulation in inoculated roots under Low P, while more AMF spores were produced in the Null P. Under High P, AMF biomass and spores were strongly reduced. Plant growth response to mycorrhiza was negative under Null P, showing reduction in height, biovolume index, and source leaf (SL) area. Under Low P, inoculated plants showed a positive response (e.g., increased SL area), while inoculated plants under High P were similar to non-inoculated plants. AMF promoted the accumulation of soluble sugars in the SL under all fertilization levels, whereas the soluble sugar level decreased in roots under Low P in inoculated plants. Transcriptional upregulation of SlLIN6 and SlSUS1, genes related to carbohydrate metabolism, was observed in inoculated roots under Null P and Low P, respectively. We conclude that P-limiting conditions that increase AMF colonization stimulate plant growth due to an increase in the source and sink strength. Our results suggest that C partitioning and allocation to different catabolic pathways in the host are influenced by AMF performance.

Phosphorus availability drives mycorrhiza induced resistance in tomato.

Arbuscular mycorrhizal (AM) symbiosis can provide multiple benefits to the host plant, including improved nutrition and protection against biotic stress. Mycorrhiza induced resistance (MIR) against pathogens and insect herbivores has been reported in different plant systems, but nutrient availability may influence the outcome of the interaction. Phosphorus (P) is a key nutrient for plants and insects, but also a regulatory factor for AM establishment and functioning. However, little is known about how AM symbiosis and P interact to regulate plant resistance to pests. Here, using the tomato-Funneliformis mosseae mycorrhizal system, we analyzed the effect of moderate differences in P fertilization on plant and pest performance, and on MIR against biotic stressors including the fungal pathogen Botrytis cinerea and the insect herbivore Spodoperta exigua. P fertilization impacted plant nutritional value, plant defenses, disease development and caterpillar survival, but these effects were modulated by the mycorrhizal status of the plant. Enhanced resistance of F. mosseae-inoculated plants against B. cinerea and S. exigua depended on P availability, as no protection was observed under the most P-limiting conditions. MIR was not directly explained by changes in the plant nutritional status nor to basal differences in defense-related phytohormones. Analysis of early plant defense responses to the damage associated molecules oligogalacturonides showed primed transcriptional activation of plant defenses occurring at intermediate P levels, but not under severe P limitation. The results show that P influences mycorrhizal priming of plant defenses and the resulting induced-resistance is dependent on P availability, and suggest that mycorrhiza fine-tunes the plant growth vs defense prioritization depending on P availability. Our results highlight how MIR is context dependent, thus unravel molecular mechanism based on plant defence in will contribute to improve the efficacy of mycorrhizal inoculants in crop protection.

Effects of nutrient limitation on cell growth, exopolysaccharide secretion and TEP production of Phaeocystis globosa

Phaeocystis globosa (P. globosa) often colonizes and produces mucus, which may cause massive blooms in coastal areas. To understand mechanism of the growth and the impact factors for better control of the bloom, we conducted a laboratory experiment on the effect of nitrogen (N) or phosphorus (P) limitation on the cell growth, production of exopolysaccharide (EPS), and transparent exopolymeric particles (TEP) of P. globosa. Results show no obvious differences in the N- and/or P-limitation in TEP production, polysaccharide secretion, and colony growth of P. globosa. Particularly in the death phase of the algae growth, the TEP production level in the experiment differed significantly, and was higher in the P-limitation group than that in the N-limitation group; additionally, the P-limitation group produced a relatively higher amount of EPS than N-limitation group, with greater cellular chlorophyll-a content, and in greater photosynthetic reaction rate of P. globosa cells, than those of the N-limitation group. However, under N-limited conditions, the algae colony survived longer. Under P-limited condition, P. globosa cells spend the photosynthesis-produced substances and energy for the secretion of extracellular substances but for cell reproduction, which was indicated by P. globosa cell growth and carbon content ratio between TEP and biomass.