Response of soil extracellular enzyme activity and stoichiometry to short-term warming and phosphorus addition in desert steppe

Background

Phosphorus (P) is regarded as one of the major limiting factors in grassland ecosystems. Soil available phosphorus deficiency could affect soil extracellular enzyme activity, which is essential for microbial metabolism. Yet it is still unclear how soil available phosphorus affects soil extracellular enzyme activity and microbial nutrient limitation of desert steppe in the context of climate warming.

Methods

This study carried out a short-term open-top chambers (OTCs) experiment in a desert steppe to examine the effects of warming, P addition, and their interaction on soil properties, the activities of soil extracellular enzymes, and stoichiometries.

Results

The findings demonstrated that soil acquisition enzyme stoichiometry of C: N: P was 1.2:1:1.5 in this experiment region, which deviated from the global mean scale (1:1:1). Warming increased soil AN (ammonium nitrogen and nitrate nitrogen) contents and decreased microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN). Phosphorus addition raised soil available phosphorus and microbial biomass phosphorus (MBP) contents. Soil extracellular enzyme activities and stoichiometries in desert steppe are largely impacted by soil AN, MBC: MBP, and MBN: MBP. These results revealed that the changes of soil available nutrients and stoichiometries induced by short-term warming and P addition could influence soil microbial activities and alleviate soil microbial carbon and phosphorus limitation. Our findings highlight that soil available phosphorus played a critical role in regulating soil extracellular enzyme activity and microbial nutrient limitation of desert steppe. Further research on soil microbial communities should explore the microbiological mechanisms underlying these findings.

Monsoon-induced response of algal chlorophyll to trophic state, light availability, and morphometry in 293 temperate reservoirs

Eutrophication management is one of the greatest environmental challenges for lacustrine systems worldwide. The empirically predicted models between algal chlorophyll (CHL-a) and total phosphorus (TP) provide a basis for managing eutrophication in lakes and reservoirs, but other environmental factors influencing the empirical relations must be considered. Here, we tested the impacts of morphological and chemical variables, as well as the effect of the Asian monsoon, on the functional response of CHL-a to TP using two-year data of 293 agricultural reservoirs. This study was based on the approaches of empirical models (linear and sigmoidal), CHL-a:TP ratio, and trophic state index deviation (TSID). Algal CHL-a exhibited a strong log-linear relation with TP on the basis of 2-year average data (R² = 0.69, p < 0.001), whereas it had a more sigmoidal relation in terms of monsoon-seasonal averages (R² = 0.52, p < 0.001). The linear segment of the CHL-a-TP relation aligned with the gradient of TP (10 mg/L < TP < 100 mg/L) from mesotrophic to eutrophic conditions. The transfer efficiency of TP to CHL-a based on the 2-year mean CHL-a:TP was high (0.6 <) across all assessed agricultural systems. CHL-a:TP showed insignificant correlations with reservoir morphological variations, but it decreased (<0.5) in eutrophic and hypereutrophic systems during the monsoon season (July-August). Because TP and total suspended solids (TSS) have become increasingly abundant, light conditions become insufficient for algal growth during and after the monsoon season. Light-limited conditions become more prevalent in hypereutrophic systems with shallow depth and high dynamic sediment ratio (DSR) because of the intense rainfall inputs and wind-induced sediment resuspension of the post-monsoon season. TSID reflected the degree of phosphorus limitation and the reduction in underwater light corresponding to changes in reservoir water chemistry (ionic content, TSS, and TN:TP ratio), trophic state gradient, and morphological metrics (mainly mean depth and DSR). Our findings suggest that monsoon-induced changes in water chemistry and light attenuation, which are also associated with anthropogenic pollutant runoffs and reservoir morphology, are critical factors that influence the functional response of algal CHL-a to TP in temperate reservoirs. Modeling and assessing eutrophication should therefore take into account monsoon seasonality along with individual morphological features further.

Nitrogen removal in improved subsurface wastewater infiltration system: Mechanism, microbial indicators and the limitation of phosphorus

To enhance the nitrogen removal capacity, scrap iron filings and Si-Al porous clay mineral material (PCMW) was used to improve a subsurface wastewater infiltration system (SWIS). The results showed TN and NH₄⁺-N removal efficiencies of improved SWIS were 20.72% and 5.49% higher than those of the control SWIS, respectively. Based on the response of the removal performance, microbial community and function analysis of 16s rRNA amplicon sequencing results, the amending soil matrix substantially enriched the nitrogen removal bacteria (Rhizobiales_Incertae_Sedis and Gemmatimonadaceae), and significantly improved the activities of key enzymes (Hao, NasAB, NarGHI, NirK, NorBC, NirA and NirBD), particularly at co-occurrence zone of nitrification and denitrification (70-130 cm depth). The amending soil matrix not only extended the growth space of microbes, but also provided additional electrons and carbon sources for denitrifying bacteria by regulating the structure and function of the microbial community. In addition, amending soil matrix could enhance phosphate metabolism genes and phosphate solubilizing microbes in the denitrification zone by increasing the phosphorus source, thus strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis and Gemmatimonadaceae related to nitrogen removal and Bacillaceae with phosphate-solubilizing ability could be used as microbial indicators of nitrogen removal in SWISs. The reciprocal action of environmental on microbial characteristics exhibited microbial functional were related to DO, Fe²⁺, TOC, TP, TN, NH₄⁺-N and NO₃^--N. Those could be used as physicochemical and biological indicators for application and monitoring of SWIS. In conclusion, this study provided a low-cost and efficient enhancement approach for the application of SWIS in decentralized domestic sewage treatment, and furnished theoretical support for subsequent applications.

Impact of phosphorus limitation on medium-chain-length polyhydroxyalkanoate production by activated sludge

For a sustainable economy, biodegradable biopolymers polyhydroxyalkanoates (PHA) are desirable substitutes to petroleum-based plastics that contaminate our environment. Medium-chain-length (MCL) PHA bioplastics are particularly interesting due to their thermoplastic properties. To hamper the high cost associated to PHA production, the use of bacterial mixed cultures cultivated in open systems and using cheap resources is a promising strategy. Here, we studied the operating conditions favouring direct MCL accumulation by activated sludge, using oleic acid as a model substrate and phosphorus limitation in fed-batch bioreactors. Our results confirm the presence of PHA-accumulating organisms (PHAAO) in activated sludge able to accumulate MCL from oleic acid. A positive correlation between phosphorus (P) limitation and PHA accumulation was demonstrated, allowing up to 26% PHA/total biomass accumulation, and highlighted its negative impact on the MCL/PHA fraction in the polymer. Diversity analysis through 16S rRNA amplicon sequencing revealed a differential selection of PHAAO according to the P-limitation level. A differential behaviour for the orders Pseudomonadales and Burkholderiales at increasing P-limitation levels was revealed, with a higher abundance of the latter at high levels of P-limitation. The PHA accumulation observed in activated sludge open new perspectives for MCL-PHA production system based on P-limitation strategy applied to mixed microbial communities. KEY POINTS: • Direct accumulation of MCL-PHA in activated sludge was demonstrated. • MCL-PHA content is negatively correlated with P-limitation. • Burkholderiales members discriminate the highest P-limitation levels.

Phosphorus - The main limiting factor in riverine ecosystems in China

River receive substantial nutrient inputs, and serve as the main channel for nitrogen and phosphorus to enter the lake, their nutrient control is of great significance to the alleviation of lake eutrophication. While nutrient limitation affects the primary productivity of water ecosystems and the biodiversity of aquatic communities, identifying the limiting factors in riverine ecosystems across China remains elusive. Here, we explore which nutrients have a stronger effect on nutritional balance and aquatic ecosystems in China's rivers based on the total nitrogen (TN) and total phosphorus (TP) observations from 1412 sampling sites in 2018. This study supports the following three main conclusions. Though the percentages of the sites with TN or TP exceeding the limits varied as per different mesotrophic targets, and TP (53.7 %) contributed more to nutrient enrichment than TN (46.3 %). In addition, the spatial distribution characteristics of river nutrients were high in the north (arid zone) and low in the south (humid zone) in China. According to four classification criteria of N:P ratio, 70.8 % of the sampling sites were attributed to phosphorus limiting, much higher than the sites with nitrogen limiting (4.1 %). TN and TP have a synergistic effect on river nutrients, while TP has a stronger regulation framework. Our results reveal that the nutrients in China's rivers are mainly phosphorus limiting, which implies that phosphorus-oriented best management practices are more likely to maintain the nutrient balance of rivers towards healthy aquatic ecosystems. Synopsis: Phosphorus is the key factor that affecting the stability and nutrient balance of riverine ecosystem.

The Symbiodinium Proteome Response to Thermal and Nutrient Stresses

Coral bleaching is primarily caused by high sea surface temperatures, and nutrient enrichment of reefs is associated with lower resilience to thermal stress and ecological degradation. Excess inorganic nitrogen relative to phosphate has been proposed to sensitize corals to thermal bleaching. We assessed the physiological and proteomic responses of cultures of the dinoflagellate coral symbiont Symbiodinium microadriaticum to elevated temperature under low-nutrient, high-nutrient and phosphate-limited conditions. Elevated temperature induced reductions of many chloroplast proteins, particularly the light-harvesting complexes, and simultaneously increased the abundance of many chaperone proteins. Proteomes were similar when the N:P ratio was near the Redfield ratio, regardless of absolute N and P concentrations, but were strongly affected by phosphate limitation. Very high N:P inhibited Symbiodinium cell division while increasing the abundance of chloroplast proteins. The proteome response to phosphate limitation was greater than that to elevated temperature, as measured by the number of differentially abundant proteins. Increased physiological sensitivity to high temperatures under high nutrients or imbalanced N:P ratios was not apparent; however, oxidative stress response proteins were enriched among proteins responding to thermal stress under imbalanced N:P ratios. These data provide a detailed catalog of the effects of high temperatures and nutrients on a coral symbiont proteome.

Long-term phosphorus addition alleviates CO2 and N2O emissions via altering soil microbial functions in secondary rather primary tropical forests

Tropical forests, where the soils are nitrogen (N) rich but phosphorus (P) poor, have a disproportionate influence on global carbon (C) and N cycling. While N deposition substantially alters soil C and N retention in tropical forests, whether P input can alleviate these N-induced effects by regulating soil microbial functions remains unclear. We investigated soil microbial taxonomy and functional traits in response to 10-year independent and interactive effects of N and P additions in a primary and a secondary tropical forest in Hainan Island. In the primary forest, N addition boosted oligotrophic bacteria and phosphatase and enriched genes responsible for C-, P-mineralization, nitrification and denitrification, suggesting aggravated P limitation while N excess. This might stimulate P excavation via organic matter mineralization, and enhance N losses, thereby increasing soil CO₂ and N₂O emissions by 86% and 110%, respectively. Phosphorus and NP additions elevated C-mining enzymes activity mainly due to intensified C limitation, causing 82% increase in CO₂ emission. In secondary forest, P and NP additions reduced phosphatase activity, enriched fungal copiotrophs and increased microbial biomass, suggesting removal of nutrient deficiencies and stimulation of fungal growth. Meanwhile, soil CO₂ emission decreased by 25% and N₂O emission declined by 52-82% due to alleviated P acquisition from organic matter decomposition and increased microbial C and N immobilization. Overall, N addition accelerates most microbial processes for C and N release in tropical forests. Long-term P addition increases C and N retention via reducing soil CO₂ and N₂O emissions in the secondary but not primary forest because of strong C limitation to microbial N immobilization. Further, the seasonal and annual variations in CO₂ and N₂O emissions should be considered in future studies to test the generalization of these findings and predict and model dynamics in greenhouse gas emissions and C and N cycling.

Long-term alfalfa (Medicago sativa L.) establishment could alleviate phosphorus limitation induced by nitrogen deposition in the carbonate soil

Phosphorus (P) limitation is a widespread problem of primary production in dryland submitted to persistent nitrogen (N) deposition. The legume alfalfa (Medicago sativa L.), which can fix N₂, might potentially strengthen P limitation in dryland ecosystems and is widely distributed as forage. However, there is still unclear how alfalfa grassland mobilizes the soil P to meet its demand. In this experiment, alfalfa introduction was used for long-term revegetation to evaluate the P uptake of plants from deep soil and assess the P limitation induced by N deposition compared with fallow. Our results showed that alfalfa introduction increased the soil P storage significantly at 0-2.4 m soil depth (+0.74 Mg ha^-1), whereas it decreased at 2.4-4.8 m soil depth (-0.21 Mg ha^-1) after 15-year establishment. Alfalfa establishment increased soil organic P concentration (180.9 mg kg^-1 vs. 67.2 mg kg^-1) and its relative contribution to total P (19.64% vs. 8.08%) at 0-4.8 m. Alfalfa establishment also increased the concentration and proportion of labile and intermediate P fractions at 0-4.8 m (9.12 mg kg^-1 vs. 6.87 mg kg^-1, 1.12% vs. 0.98%; 16.06 mg kg^-1 vs. 8.39 mg kg^-1, 1.69% vs. 1.17%). Alfalfa introduction decreased the concentrated HCl-Pi (250.66 mg kg^-1 vs. 229.32 mg kg^-1, 36.81% vs. 28.91%) in 2.4-4.8 m soil depth. These results indicated that the deep root system of alfalfa grassland could promote the P mobilization from deep to shallow soil. The concentrated HCl-Pi may be the main potential P source of alfalfa from 2.4-4.8 m to 0-2.4 m of soil depth, and long-term establishment of alfalfa can alleviate P limitation caused by N deposition in carbonate soil. Our results suggested that species with deep roots (such as alfalfa) could be selected as an economical way to mitigate nitrogen deposition in drylands.

Differential aboveground-belowground adaptive strategies to alleviate N addition-induced P deficiency in two alpine coniferous forests

Increasing atmospheric nitrogen (N) deposition has resulted in phosphorus (P) limitation in multiple terrestrial ecosystems, yet how plants coordinate aboveground and belowground strategies to adapt to such P deficiency remains unclear. In this study, we conducted a field N fertilization experiment in two alpine coniferous plantations (Picea asperata Mast. and Pinus armandii Franch.) with different soil N availability on the eastern Tibetan Plateau of China, to examine N addition effects on plant nutrient limiting status and plant adaptive strategies corresponding to aboveground P conservation and belowground P acquisition. The results showed that N addition aggravated P deficiency in both plantations, as indicated by decreased needle P concentrations and increased N:P ratios, and that plant strategies for addressing such P deficiency differed in the two plantations with different initial soil N availabilities. In the P. asperata plantation with relatively high N availability, significantly enhanced needle phosphatase activity and shifts in P fraction allocation (downregulation of the structural P fraction and increased allocation to the residual P fraction) co-occurred with increased rhizosphere effects on phosphatase activity under N addition, indicating a synergistic strategy of aboveground P conservation and belowground P mining to alleviate P deficiency. In the P. armandii plantation with relatively low N availability, however, N addition only enhanced phosphatase activity and increased allocation to residual P fraction in the aboveground but had little effect on belowground P acquisition-associated traits, suggesting a decoupling relationship between aboveground P conservation and belowground P acquisition. This study highlights the vital significance of initial soil nutrient availability in regulating the coordination of aboveground and belowground strategic alternatives, emphasizing the need to integrate soil nutrient conditions for a holistic understanding of forest adaptation to anthropogenic N enrichment.

Foliar phosphorus allocation and photosynthesis reveal plants' adaptative strategies to phosphorus limitation in tropical forests at different successional stages

High atmospheric nitrogen (N) deposition and low soil phosphorus (P) availability occur simultaneously in tropical areas, and thus tropical plants need to adapt nutrient-use strategies to maintain growth and survival. Therefore, identifying the adaptative strategies of tropical plants at different successional stages under low soil P availability is indispensable. Here, we separately investigated foliar traits, photosynthetic characteristics, and P fractions of 8 species in the primary and secondary tropical forests after 10 years of N and P fertilization. P addition increased foliar P concentrations and deceased N:P ratio in the primary forest and secondary forest. The foliar photosynthetic rates did not significantly respond to nutrient additions, and the foliar photosynthetic P-use efficiency (PPUE) reduced under the P addition in the primary forest. In contrast, the foliar photosynthetic rates and photosynthetic nitrogen (N)-use efficiency (PNUE) were enhanced with nutrient additions in the secondary forest. The allocations of foliar nucleic acid P and residual P were reduced by P addition in the primary forest, whereas the allocation of metabolic P was enhanced and the allocation of residual P was reduced by P addition in the secondary forest. Additionally, a higher proportion of structural P was found in the primary forest, and a higher proportion of metabolic P was observed in the secondary forest. Interesting, structural equation model analysis revealed that the plants decreased the allocation of foliar nucleic acid P and increased the allocation of structural P in the primary forest, thereby reducing photosynthetic rates. Whereas the plants enhanced photosynthetic rates by promoting PPUE and the allocation of foliar metabolic P in the secondary forest. Our findings highlighted tropical plants at different successional stages can reasonably allocate foliar P to regulate photosynthetic rates and acclimate to low P environments.

Reversing impact of phytoplankton phosphorus limitation on coastal hypoxia due to interacting changes in surface production and shoreward bottom oxygen influx

Phosphorus (P) limitation of phytoplankton growth is increasingly common in estuarine and coastal waters due to rising anthropogenic nitrogen input faster than that of phosphorus. However, the impact of P limitation on coastal hypoxia remains inconclusive and is challenging to observe. By combining observations with results from a three-dimensional physical-biogeochemical model off the Pearl River Estuary, we illustrate that during the summer upwelling period, the impact of P limitation reverses from suppressing hypoxia to amplifying hypoxia as P-limitation severity decreases. When P limitation is severe in the ecosystem (i.e., P limitation extensively covers the stratified waters where hypoxia tends to develop), the surface primary production and the coupled bottom oxygen consumption are diluted along the upstream-downstream axis because of the P limitation. In addition, the increased downstream bottom oxygen level enhances the shoreward bottom oxygen influx. These effects, together, reduce coastal hypoxia. In contrast, when P-limitation severity is low (i.e., P limitation is spatially constrained), the downstream relocated surface production reduces upstream hypoxia but increases downstream hypoxia, which subsequently weakens the shoreward bottom oxygen influx and hence lowers its capacity to relieve upstream hypoxia. The net effect can amplify the coastal hypoxic extent. Our results emphasize how different P-limitation severity can reverse its impact on coastal hypoxia due to the interacting changes in surface production and bottom oxygen influx. We propose the potential of using the spatial extent of P limitation as a proxy to predict its impact on coastal hypoxia and support ecosystem nutrient management.

Recycling spent water from microalgae harvesting by fungal pellets to re-cultivate Chlorella vulgaris under different nutrient loads for biodiesel production

Fungal pellet is an emerging material to collect oleaginous microalgae, but rare studies have noticed that harvested water is available resource for the next round of cultivation. To systematically optimize regrowth performances of microalgae Chlorella vulgaris, separated water after harvesting by fungi Aspergillus oryzae was prepared under different N/P ratios. The results showed that chlorophylls and enzymes were significantly affected by the proportion of N and P. Although nutrient deficiency was functioned as a stress factor to restrict carbohydrate and protein synthesis, lipid content was obviously increased by 12.69%. The percentage of saturated fatty acids associated with oxidation stability increased, while this part in fresh wastewater accounted for only 36.96%. The favorable biomass concentration (1.37 g/L) with the highest lipid yield (0.42 g/L) appeared in N/P of 6:1. More strikingly, suitable conditions could save 52.4% of cultivation costs. These experiments confirmed that reusing bioflocculated water could be effectively utilized for biodiesel production.

Warming reduces the production of a major annual forage crop on the Tibetan Plateau

Climate warming has been proposed to increase primary production of natural grasslands in cold regions. However, how climate warming affects the production of artificial pastures in cold regions remains unknown. To address this question, we used open-top chambers to simulate warming in a major artificial pasture (forage oat) on the cold Tibetan Plateau for three consecutive years. Surprisingly, climate warming decreased aboveground and belowground biomass production by 23.1%-44.8% and 35.0%-46.5%, respectively, without a significant impact on their ratio. The adverse effects on biomass production could be attributed to the adverse effects of high-temperatures on leaf photosynthesis through increases in water vapor pressure deficit (by 0.05-0.10 kPa), damages to the leaf oxidant system, as indicated by a 46.6% increase in leaf malondialdehyde content, as well as reductions in growth duration (by 4.7-6.7 days). The adverse effects were also related to exacerbated phosphorus limitation, as indicated by decreases in soil available phosphorus and plant phosphorus concentrations by 31.9%-40.7% and 14.3%-49.4%, respectively, and increases in the plant nitrogen: phosphorus ratio by 19.2%-108.3%. The decrease in soil available phosphorus concentration could be attributed to reductions in soil phosphatase activities (by 9.6%-18.5%). The findings of this study suggest an urgent need to advance agronomic techniques and cultivate more resilient forage genotypes to meet the increasing demand of forage for feeding livestock and to reduce grazing damage to natural grasslands on the warming-sensitive Tibetan Plateau.

Negative effects of long-term phosphorus additions on understory plants in a primary tropical forest

Human activities have disturbed global phosphorus (P) cycling by introducing substantial amounts of P to natural ecosystems. Although natural P gradients and fertilization studies have found that plant community traits are closely related to P availability, it remains unclear how increased P supply affects plant growth and diversity in P-deficient tropical forests. We used a decadal P-addition experiment (2007-2017) to study the effects of increased P input on plant growth and diversity in understory layer in tropical forests. We monitored the dynamics of seedling growth, survival rate, and diversity of understory plants throughout the fertilization period under control and P addition at 15 g P m^-2 yr^-1. To identify the drivers of responses, P concentration, photosynthesis rate and nonstructural carbon were analyzed. Results showed that long-term P addition significantly increased P concentrations both in soil pools and plant tissues. However, P addition did not increase the light-saturated photosynthesis rate or growth rate of the understory plants. Furthermore, P addition significantly decreased the survival rate of seedlings and reduced the species richness and density of understory plants. The negative effects of P addition may be attributed to an increased carbon cost due to the tissue maintenance of plants with higher P concentrations. These findings indicate that increased P supply alone is not necessary to benefit the growth of plants in ecosystems with low P availability, and P inputs can inhibit understory plants and may alter community composition. Therefore, we appeal to a need for caution when inputting P to tropical forests ecosystems.

Spatially divergent trends of nitrogen versus phosphorus limitation across European forests

Nitrogen (N) and phosphorus (P) are essential nutrients that widely limit plant growth in global terrestrial ecosystems. Rising atmospheric CO₂ concentration generally stimulates terrestrial net primary productivity and consequently may cause or aggravate N and P limitation due to a dilution effect, but the spatial variation of temporal trends in N versus P limitation and its key regulating factors is poorly understood. Using the leaf N:P ratio of 15 dominant tree species as an indicator, we analysed the spatial variation of plot-level shift towards N or P limitation across 163 European forest plots during 1995-2017. Phosphorus limitation increased from 25% to 33% of the studied plots between 1995-1997 and 2015-2017, while N limitation occurred in a negligible number of plots. A major proportion (56%) of the plots showed no significant trend in leaf N:P ratio, implying no shifts in N versus P limitation status. In the remaining plots, 38% of the plots showed a significant increase of leaf N:P ratio and only 6% of the plots showed a significant decrease of leaf N:P ratio. The spatial variation in the rate of decrease in leaf N:P ratio was associated with a significant decrease in leaf N concentration and mainly explained by the rate of decrease in N deposition. In contrast, the spatial variation in the rate of increase in leaf N:P ratio was associated with a significant decrease in leaf P concentration and mainly explained by forest category (broadleaf vs. conifer), mean annual temperature and soil C:N ratio. Our findings highlight a remarkable spatial divergence in temporal trends of nutrient limitation status across European forests over the past two decades, but overall, P is becoming more limiting versus N, especially in broadleaved forests.

Exergy analysis for docosahexaenoic acid production by fermentation and strain improvement by adaptive laboratory evolution for Schizochytrium sp

Exergy analysis is powerful tool for process optimization and mechanism analysis. In this study, exergy analysis was performed for docosahexaenoic acid (DHA) fermentation process. More than 86% of input exergy was contributed by glucose. The exergy of biomass was about 64.66% of the total output exergy when the phosphate concentration was 4 g L^-1. The exergy efficiencies of DHA (η_DHA) for the starting strains and the evolved strains under high oxygen concentration, low temperature, and two-factor conditions were also investigated. The η_DHA in the collected experimental data was not more than 20.9%. It was proved that there was a positive correlation between η_DHA and the biomass yield. It was indicated that adaptive laboratory evolution (ALE) improved biomass yield which had the most important effect on enhancing η_DHA and DHA yield (or DHA productivity). It is necessary to improve η_DHA through process optimization and ALE in future.

Alkaline phosphatase activity in the phosphorus-limited southern Chinese coastal waters

Three fractions of alkaline phosphatase activity (APA), including phytoplankton APA (phyto-APA), bacterial APA (bact-APA), and free-APA, were examined in the sea surface microlayer (SML) and the subsurface water (SSW) from Daya Bay, Guishan Island, and Guanghai Bay of southern China. Relationships between APA and environmental parameters were analyzed. The growth of phytoplankton was significantly limited by dissolved inorganic phosphorus (DIP) in the three sea areas, especially in Daya Bay. Total-APA ranged between 1.41 and 35.26 nmol/L/hr, and the highest value was found in Daya Bay. The increased APA in Daya Bay was the result of the increase of phytoplankton biomass and the response of phytoplankton to P limitation. Phyto-APA was the main contributor in Daya Bay, while phyto- and free-APA co-dominated in Guishan Island and Guanghai Bay. Bact-, phyto-, and total-APA showed a significant inverse power function relationship with DIP, and 0.2 μmol/L was the threshold for DIP on particulates and total-APA. Pearson correlation analysis suggested that DIP limitation together with high N levels enhanced APA. High water temperature and freshwater input accelerated APA as well. Principal component analysis clearly separated samples from the three sea areas, as well as from the SML and the SSW, which indicated the differences in environmental parameters and APA levels. Our results highlight the influence of phosphorus limitation and environmental parameters on APA.

Effects of Excess and Limited Phosphate on Biomass, Lipid and Fatty Acid Contents and the Expression of Four Fatty Acid Desaturase Genes in the Tropical Selenastraceaen Messastrum gracile SE-MC4

In this study, the effects of limited and excess phosphate on biomass content, oil content, fatty acid profile and the expression of three fatty acid desaturases in Messastrum gracile SE-MC4 were determined. It was found that total biomass (0.67-0.83 g L^-1), oil content (30.99-38.08%) and the duration for cells to reach stationary phase (25-27 days) were not considerably affected by phosphate limitation. However, excess phosphate slightly reduced total biomass and oil content to 0.50 g L^-1 and 25.36% respectively. The dominant fatty acids in M. gracile, pamitic acid (C16:0) and oleic acid (C18:1) which constitute more than 81% of the total fatty acids remained relatively high and constant across all phosphate concentrations. Reduction of phosphate concentration to 25% and below significantly increased total MUFA, whereas increasing phosphate concentration to ≥ 50% and ≥ 100% significantly increased total SFA and PUFA content respectively. The expression of omega-3 fatty acid desaturase (ω-3 FADi1, ω-3 FADi2) and omega-6 fatty acid desaturase (ω-6 FAD) was increased under phosphate limitation, especially at ≤ 12.5% phosphate, whereas levels of streoyl-ACP desaturase (SAD) transcripts were relatively unchanged across all phosphate concentrations. The first isoform of ω-3 FAD (ω-3 FADi) displayed a binary upregulation under limited (≤ 12.5%) and excess (200%) phosphate. The expression of ω-6 FAD, ω-3 FAD and SAD were inconsistent with the accumulation of oleic acid (C18:1), linoleic acid (C18:2) and alpha-linolenic acid (C18:3), suggesting that these genes may be regulated indirectly by phosphate availability via post-transcriptional or post-translational mechanisms.

Polyhydroxyalkanoates production from methane emissions in Sphagnum mosses: Assessing the effect of temperature and phosphorus limitation

The isolation of highly efficient methanotrophic communities is crucial for the optimization of methane bioconversion into products with a high market value such as polyhydroxyalkanoates (PHA). The research here presented aimed at enriching a methanotrophic consortium from two different inocula (Sphagnum peat moss (Sp) and Sphagnum and activated sludge (M)) able to accumulate PHA while efficiently oxidizing CH₄. Moreover, the effect of the temperature and phosphorus limitation on the biodegradation rate of CH₄ and the PHA accumulation potential was investigated. Higher CH₄ degradation rates were obtained under P availability at increasing temperature (25, 30 and 37 °C). The biomass enriched from the mixed inoculum always exhibited a superior biodegradation performance regardless of the temperature (a maximum value of 84.3 ± 8.4 mg CH₄ h^-1 g biomass^-1 was recorded at 37 °C). The results of the PHB production showed that phosphorus limitation is required to promote PHB accumulation, the highest PHB content being observed with the Sphagnum inoculum at 25 °C (13.6 ± 5.6%). The differential specialization of the microbial communities depending on the enrichment temperature supported the key role of this parameter on the results obtained. In all cases after the completion of the enrichment process and of the P limitation tests, Methylocystis, a type II methanotroph known for its ability to accumulate PHA, was the genus that became dominant (reaching percentages from 16 to 46% depending on the enrichment temperature). Thus, the results here obtained demonstrated for the first time the relevance of the temperature used for the enrichment of the methanotrophic bacteria to boost PHA production yields under P limiting condition, highlighting the importance of optimizing culture conditions to improve the cost-efficiency of bioprocesses based on using methane as the primary feedstock for the PHA industrial market.

Polyhydroxyalkanoates (PHA) production from fermented thermal-hydrolyzed sludge by mixed microbial cultures: The link between phosphorus and PHA yields

Production of polyhydroxyalkanoates (PHA) from wastes has gained increasing attention for the related low costs and high environmental benefits. Phosphorus limitation is a potential strategy used to facilitate PHA production, yet excessive limitation was previously reported to cause negative effects. This study was the first to investigate the optimum phosphorus limitation for PHA accumulation from thermal-hydrolyzed sludge. The results showed that the maximum PHA content increased from 23 wt% to 51 wt% when phosphorus concentration was limited from 127.60 to 1.35 mg/L, indicating that a lower phosphorus concentration would promote maximum PHA accumulation. Batch tests performed with synthetic substrates (containing one specific VFA for each batch) confirmed that the effect of phosphorus content on PHA production was mainly devoted by the efficiency of the conversion of acetate to PHA. The PHA yields on acetate (Y_PHA/ac) were 0.68 and 0.05 Cmol/Cmol under phosphorus-limited (1 mg/L) and -excess (100 mg/L) conditions, respectively. A mathematical model was developed to describe the correlation between phosphorus concentration and Y_PHA/ac, which can fit the experimental data and predict the results properly. Finally, further (ammonium-) nitrogen restriction did not efficiently cause the additional improvement of PHA production under the conditions of phosphorus limitation.

Phosphorus strategy in bloom-forming cyanobacteria (Dolichospermum and Microcystis) and its role in their succession

Dolichospermum (formerly Anabaena) and Microcystis cause harmful cyanobacterial blooms in freshwater ecosystems worldwide. Input reduction of both nitrogen (N) and phosphorus (P) are commonly recognized as basic ways of controlling blooms, but little is known about the roles of nutrients and their using strategy among cyanobacteria in triggering the succession of diazotrophic to non-diazotrophic cyanobacteria. In this study, we investigated in situ responses of cyanobactria to ambient P status during the transition from Dolichospermum flos-aquae to Microcystis spp. in Lake Taihu and Lake Chaohu. While dominant in phytoplankton community, D. flos-aquae experienced P deficiency as evidenced by qualitative detection of extracellular phosphatase via enzyme labeled fluorescence (ELF). The percentage of ELF-labelled D. flos-aquae cells was 33% when it dominated the phytoplankton community, and was 78% when it co-dominated with Microcystis spp., indicating an increase in P deficiency. Meanwhile, no ELF-labelled Microcystis cells were observed while polyphosphate body (PPB) were present, suggesting that Microcystis spp. were not P deficient. Additionally, the percentages of Microcystis cells containing PPB showed an inverted "U-shaped" relationship with concentrations on soluble reactive phosphorus (SRP). To validate the field observation, a laboratory study of the monocultures of the dominant cyanobacteria was conducted. Extracellular alkaline phosphatase activity (APA) and PPB accumulation were regulated by P availability in monocultures of D. flos-aquae. Interestingly, no cell bound extracellular phosphatase was found on Microcystis aeruginasa even in the culture without P supply. Consistently, the expressions of phosphatase encoding gene phoX showed no differences among the treatments. The way in which PPB accumulation occurred in Microcystis spp. in response to P availability in the cultures was similar to that observed in the field, demonstrating a strategy of energy conservation over P accumulation. The competitive advantage of Microcystis spp. was displayed at low P concentrations: where it could rapidly uptake and store inorganic P, which also increased the P deficiency of the coexisting phytoplankton species. Responses of P-transport gene pstS confirmed this hypothesis. The physiological and molecular mechanisms mentioned above enable Microcystis to survive and proliferate in environment with low available P supply more efficiently. In conclusion, different cyanobacterial species have distinct ways of responding to P availability, suggesting that the control of cyanobacterial blooms by targeted nutrient reduction is largely dependent upon the dominant species. P reduction is more effective in controlling diazotrophic cyanobacteria than non-diazotrophic cyanobacteria.

Phosphate removal combined with acetate supplementation enhances lipid production from water hyacinth by Cutaneotrichosporon oleaginosum

BACKGROUND

Microbial lipids derived from various lignocellulosic feedstocks have emerged as a promising candidate for the biodiesel industry and a potential substitute for high value-added fats. However, lignocellulosic biomass, especially herbaceous biomass, such as water hyacinth, contains high concentrations of nitrogenous components. These compounds impede microbial lipid production, as lipid biosynthesis is commonly induced by imposing a nutrient deficiency, especially nitrogen starvation. Novel strategies and bioprocesses are pivotal for promoting lipid production from nitrogen-rich biomass.

RESULTS

Here a combined strategy of phosphate removal and acetate supplementation was described for enhanced microbial lipid production on water hyacinth hydrolysates by Cutaneotrichosporon oleaginosum (formerly Cryptococcus curvatus). Lipid production was significantly improved, when the phosphorus limitation and sugars/acetate co-utilization strategies were used separately. In this case, acetate and glucose were consumed simultaneously. Lipid production was observed by the combination of phosphate removal with acetate supplementation. Lipid titer, content, and yield were determined to be 7.3 g/L, 59.7% and 10.1 g/100 g raw water hyacinth, respectively. These data were increased by 4.2, 4.6, and 4.3 times, respectively, compared to those from the unprocessed hydrolysates. The fatty acid compositions of the resulting lipids bear a marked resemblance to those of rapeseed oil, indicating their applicability to the biodiesel industry.

CONCLUSIONS

The combination of phosphate removal and acetate supplementation was successful in significantly enhancing microbial lipid production. This strategy offers a valuable solution for nitrogen-rich lignocellulosic feedstocks utilization, which should foster more economical nitrogen-rich biomass-to-lipid bioprocesses.

Enhancement of lipid accumulation by oleaginous yeast through phosphorus limitation under high content of ammonia

Low concentrations of acetic acid were used as carbon source to cultivate Cryptococcus curvatus MUCL 29819 for lipid production under high content of ammonia. Phosphorus limitation combined with initial pH regulation (pH = 6) weakened inhibition of free ammonia and promoted lipid accumulation. In batch cultivation, the produced lipid content and yield was 30.3% and 0.92 g/L, higher than those under unlimited condition (18.3% and 0.64 g/L). The content of monounsaturated fatty acid also increased from 37.3% (unlimited condition) to 45.8% (phosphorus-limited condition). During sequencing batch cultivation (SBC), the lipid content reached up to 51.02% under phosphorus-limited condition while only 31.88% under unlimited condition, which can be explained by the higher conversion efficiency of the carbon source to lipid. The total energy consumption including lipid extraction, transesterification and purification was 7.47 and 8.33 GJ under phosphorus-limited and unlimited condition, respectively.

Comparison of heterotrophic and autotrophic denitrification processes for nitrate removal from phosphorus-limited surface water

Phosphorus (P) limitation has been demonstrated for micro-polluted surface water denitrification treatment in previous study. In this paper, a lab-scale comparative study of autotrophic denitrification (ADN) and heterotrophic denitrification (HDN) in phosphorus-limited surface water was investigated, aiming to find out the optimal nitrogen/phosphorus (N/P) ratio and the mechanism of the effect of P limitation on ADN and HDN. Furthermore, the optimal denitrification process was applied to the West Lake denitrification project, aiming to improve the water quality of the West Lake from worse than grade V to grade IV (GB3838-2006). The lab-scale study showed that the lack of P indeed inhibited HDN more greatly than ADN. The optimal N/P ratio for ADN and HDN was 25 and a 0.15 mg PO₄^3--P L^-1 of microbial available phosphorus (MAP) was observed. P additions could greatly enhance the resistance of ADN and HDN to hydraulic loading shock. Besides, The P addition could effectively stimulate the HDN performance via enriching the heterotrophic denitrifiers and the denitrifying phosphate-accumulating organisms (DNPAOs). Additionally, HDN was more effective and cost-effective than ADN for treating P-limited surface water. The study of the full-scale HDBF (heterotrophic denitrification biofilter) indicated that the denitrification performance was periodically impacted by P limitation, particularly at low water temperatures.

Phosphorus availability in the source population influences response to dietary phosphorus quantity in a New Zealand freshwater snail

We investigated whether previously documented variation among populations in availability of dietary phosphorus (P) is linked to heterogeneity in growth rate of the New Zealand freshwater snail Potamopyrgus antipodarum on a P-limited diet. We chose this system because P. antipodarum inhabits water bodies that vary in P availability and because P. antipodarum growth rate varies considerably in response to low P. We quantified specific growth rate and alkaline phosphatase (AP) expression in a diverse array of juvenile P. antipodarum fed high vs. low-P diets. We found strong associations between P content of epilithon in the source lake and P. antipodarum growth rate on high vs. low-P diets, with snails collected from lakes with relatively low-P epilithon showing the greatest increase in growth rate on the high-P relative to low-P diet. We also found substantial intraspecific variation in growth response to P limitation. Expression of AP also varied among lineages and was negatively associated with C: P of lake epilithon but did not explain the relationship between C: P in the lake of origin and sensitivity to P limitation. Together, our results demonstrate a strong signature of the P environment in the lake of origin on how this snail responds to P limitation as well as preliminary evidence for intraspecific variation of AP expression in animals.

Phosphatase activities of a microepiphytic community during a bloom of Ostreopsis cf. ovata in the northern Adriatic Sea

It is becoming more apparent that increased organic nutrient loads deriving from anthropogenic activities and natural processes frequently cause the eutrophication of coastal waters. Concurrently, an increasing number of phototrophs have been shown to make use of organic nutrients, mainly through indirect studies of surface enzyme activities or through direct studies of growth in media containing organic-only nutrients. The potential utilization of dissolved organic phosphorus (DOP) by microepiphytic-mats associated with frequently problematic, toxic dinoflagellate Ostreopsis cf. ovata, was investigated throughout a full cycle of a bloom that occurs annually (over the last few decades) during summer along several stretches of the Mediterranean coast. Measurements of phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities of the epiphytic mats (including cells and exopolymeric substances) and a range of chemico-physical parameters were made from late summer to early autumn at a eutrophic site. Analyses of ambient nutrient fractions revealed very high aquatic N:P values (TN:TP = 178 ± 50), very low filterable reactive P (FRP) concentrations (13 out 19 under the limits of detection) and DOP concentrations that were on average 85% of the total dissolved P. We recorded a rapid increase in PMEase and PDEase activities in the epiphytic microalgal community that coincided with the onset of a proliferation of the Ostreopsis population. Chromogenic staining of samples showed that activity was closely associated with the Ostreopsis cells, located both extracellularly (cell surface and within the EPS) and intracellularly (ventral cytoplasm). The increase in both phosphatases indicates that Ostreopsis can utilise a wide range of DOP types. The intense activity in the EPS was suggested to aid in the efficient entrapment and processing of high concentration nutrient pulses, for extracellular processing of larger organic materials and to prevent loss of substrates and products to the surrounding water. Based on the present findings, Ostreopsis seems to have adaptations that allow it to thrive in P-limited environments where organic P is the main source of P.

Long-term nitrogen and phosphorus enrichment alters vegetation species composition and reduces carbon storage in upland soil

Reactive nitrogen (N) deposition can affect ecosystem processes, particularly in oligotrophic upland habitats. Phosphorus (P) addition has been proposed to reduce the effects of N enrichment on N leaching and acidification, since P limitation can reduce biomass production and consequent sequestration of reactive N. However, biodiversity is often reduced in more productive ecosystems and P limitation may protect against this effect. Responses to P availability in instances of high N deposition are poorly understood. This study investigated the ecosystem response to alleviation of P limitation, using a long-term nutrient addition experiment (1996-2012) three years after ceasing N inputs and 15years after a single P application. Substantial differences were observed in the structure and composition of vegetation species and above-ground vegetation biomass. Vegetation height was greater in the N+P addition treatments (+38% cf. control), with increased cryptogam cover (+47%), whereas N addition increased graminoid species cover (+68%). Vegetation diversity was significantly reduced by the addition of P (-21%), indicating that P limitation is likely to be an important mechanism that limits biodiversity loss in upland habitats exposed to chronic N deposition. Significant differences in soil C and N contents were also observed between treatments. Relative to control, the addition of N increased soil C (+11%) and N (+11%) pool sizes, whereas the addition of N and P reduced soil C (-12%) and N (-13%) pool sizes. This demonstrated the importance of P availability for upland ecosystem processes, and highlights the long-term effects of P addition on vegetation species composition and C storage. Thus, the addition of P cannot be endorsed as a method for reducing impacts of N deposition. Capsule: Phosphorus limitation is a major mechanism governing ecosystem processes in situations of high atmospheric nitrogen deposition.

Lipid production and molecular dynamics simulation for regulation of accD gene in cyanobacteria under different N and P regimes

BACKGROUND

Microalgae grown under different nutrient deficient conditions present a good source of natural lipids with applications for several types of biofuels. The expression of acetyl-CoA carboxylase gene can further provide an insight to the mechanisms leading to enhanced lipid production under such stresses. In this study, two nutrients viz. nitrogen and phosphorus were modulated to see its effect on lipid productivity in selected cyanobacteria and its correlation with Accase followed by molecular dynamics simulation.

RESULTS

Selected cyanobacteria viz. Oscillatoria sp. (SP8), Anabaena sp. (SP12), Anabaena sp. (SP13), Microcoleus sp. (SP18), and Nostoc sp. (SP20) varied in their ability to accumulate lipids which ranged from a lowest of 0.13% in Anabaena sp. (SP13) to the maximum of 7.24% in Microcoleus sp. (SP18). Microcoleus sp. (SP18) also recorded highest lipid accumulation at both N (6 mM NaNO₃) and P (0.20 mM K₂HPO₄) limiting conditions. The overall expression of accD was found to be upregulated in both Oscillatoria sp. (SP8) and Microcoleus sp. (SP18) for all nitrogen concentrations but was differentially regulated with both positive and negative induction under phosphorus stress conditions. Maximum induction was observed in Microcoleus sp. (SP18) at 0.20 mM K₂HPO₄. The obtained 3D structure of SP8 protein (21.8 kDa) showed six alpha helices, while SP18 protein (16.7 kDa) exhibited four alpha helices and four beta sheets. The phi (ϕ)/psi(ψ) angles of the amino acid residues observed in Ramachandran plot analysis showed that both SP8 and SP18 proteins were highly stable with more than 90% amino acids in allowed regions. The molecular dynamics simulation results also indicated the stability of ligand-bound protein complexes.

CONCLUSION

It has been demonstrated that cyanobacterial isolates are affected differently by nutrient limitation leading to variation in their lipid productivity. The same has been revealed by the behavior of accD gene expression which was regulated more by nutrients concentrations rather than the organism. However, the ligand-bound protein complexes were stable throughout MD simulations.

Contrasting effects of exogenous phosphorus application on N2O emissions from two tropical forest soils with contrasting phosphorus availability

An incubation study was conducted to test the effects of phosphorus (P) addition on nitrous oxide (N₂O) emissions from the soils taken from two tropical rain forests established on different parent materials [meta-sedimentary (MS) and ultrabasic (UB) rock] on Mt. Kinabalu, Borneo. Earlier studies suggest that the forest on UB soils is more strongly limited by P than that on MS soils is. In MS soils, P addition significantly reduced N₂O emissions. Since neither ammonium (NH₄⁺) nor nitrate (NO₃⁻) contents were reduced by P addition, we assumed that the decrease in N₂O emissions were not due to the previously-reported mechanism: P addition stimulated microbial nitrogen (N) immobilization and collateral inorganic N consumption, reducing resources for producing N₂O. Since P addition enhanced the ratios of microbial biomass to CO₂ and N₂O emissions (indicators of nitrifying and/or denitrifying respiratory efficiency), it was suggested that the N required for the respiration of nitrifying and/or denitrifying bacteria was reduced, leading to reduced N₂O emissions. On the other hand, P addition had no effects on N₂O emissions in UB soils. The respiratory efficiency did not change significantly by P addition, possibly because the microbial community in the highly-P-depleted UB soils shifted by P addition, with which the enhancement of respiration efficiency did not co-vary. We concluded that (1) P addition may control N₂O emissions through increasing respiratory efficiency, and (2) the effects may be different depending on the differences in P availability.

ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating shoot branching

ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase that may mediate strigolactone biosynthesis highly responsive to phosphorus deficiency and undergoes negative selection over domestication from Zea ssp. parviglumis to Zea mays. Carotenoid cleavage dioxygenase 7 (CCD7) functions to suppress shoot branching by controlling strigolactone biosynthesis. However, little is known about CCD7 and its functions in maize and its ancestor (Zea ssp. parviglumis) with numerous shoot branches. We found that ZmCCD7 and ZpCCD7 had the same coding sequence, indicating negative selection of the CCD7 gene over domestication from Zea ssp. parviglumis to Zea mays. CCD7 expression was highly responsive to phosphorus deficiency in both species, especially in the meristematic zone and the pericycle of the elongation zone of maize roots. Notably, the crown root had the strongest ZmCCD7 expression in the meristematic zone under phosphorus limitation. Transient expression of GFP tagged ZmCCD7/ZpCCD7 in maize protoplasts indicated their localization in the plastid. Further, ZmCCD7/ZpCCD7 efficiently catalyzed metabolism of six different linear and cyclic carotenoids in E. coli, and generated β-ionone by cleaving β-carotene at the 9,10 (9',10') position. Together with suppression of shoot branching in the max3 mutant by transformation of ZmCCD7/ZpCCD7, our work suggested that ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating strigolactone biosynthesis in maize and its ancestor.

ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating shoot branching

ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase that may mediate strigolactone biosynthesis highly responsive to phosphorus deficiency and undergoes negative selection over domestication from Zea ssp. parviglumis to Zea mays. Carotenoid cleavage dioxygenase 7 (CCD7) functions to suppress shoot branching by controlling strigolactone biosynthesis. However, little is known about CCD7 and its functions in maize and its ancestor (Zea ssp. parviglumis) with numerous shoot branches. We found that ZmCCD7 and ZpCCD7 had the same coding sequence, indicating negative selection of the CCD7 gene over domestication from Zea ssp. parviglumis to Zea mays. CCD7 expression was highly responsive to phosphorus deficiency in both species, especially in the meristematic zone and the pericycle of the elongation zone of maize roots. Notably, the crown root had the strongest ZmCCD7 expression in the meristematic zone under phosphorus limitation. Transient expression of GFP tagged ZmCCD7/ZpCCD7 in maize protoplasts indicated their localization in the plastid. Further, ZmCCD7/ZpCCD7 efficiently catalyzed metabolism of six different linear and cyclic carotenoids in E. coli, and generated β-ionone by cleaving β-carotene at the 9,10 (9',10') position. Together with suppression of shoot branching in the max3 mutant by transformation of ZmCCD7/ZpCCD7, our work suggested that ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating strigolactone biosynthesis in maize and its ancestor.

Physiological and molecular alterations in plants exposed to high [CO2] under phosphorus stress

Atmospheric [CO2] has increased substantially in recent decades and will continue to do so, whereas the availability of phosphorus (P) is limited and unlikely to increase in the future. P is a non-renewable resource, and it is essential to every form of life. P is a key plant nutrient controlling the responsiveness of photosynthesis to [CO2]. Increases in [CO2] typically results in increased biomass through stimulation of net photosynthesis, and hence enhance the demand for P uptake. However, most soils contain low concentrations of available P. Therefore, low P is one of the major growth-limiting factors for plants in many agricultural and natural ecosystems. The adaptive responses of plants to [CO2] and P availability encompass alterations at morphological, physiological, biochemical and molecular levels. In general low P reduces growth, whereas high [CO2] enhances it particularly in C3 plants. Photosynthetic capacity is often enhanced under high [CO2] with sufficient P supply through modulation of enzyme activities involved in carbon fixation such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, high [CO2] with low P availability results in enhanced dry matter partitioning towards roots. Alterations in below-ground processes including root morphology, exudation and mycorrhizal association are influenced by [CO2] and P availability. Under high P availability, elevated [CO2] improves the uptake of P from soil. In contrast, under low P availability, high [CO2] mainly improves the efficiency with which plants produce biomass per unit P. At molecular level, the spatio-temporal regulation of genes involved in plant adaptation to low P and high [CO2] has been studied individually in various plant species. Genome-wide expression profiling of high [CO2] grown plants revealed hormonal regulation of biomass accumulation through complex transcriptional networks. Similarly, differential transcriptional regulatory networks are involved in P-limitation responses in plants. Analysis of expression patterns of some typical P-limitation induced genes under high [CO2] suggests that long-term exposure of plants to high [CO2] would have a tendency to stimulate similar transcriptional responses as observed under P-limitation. However, studies on the combined effect of high [CO2] and low P on gene expression are scarce. Such studies would provide insights into the development of P efficient crops in the context of anticipated increases in atmospheric [CO2].

Phosphorus limitation strategy to increase propionic acid flux towards 3-hydroxyvaleric acid monomers in Cupriavidus necator

Properties of polyhydroxybutyrate-co-hydroxyvalerate (P(3HB-co-3HV)) depend on their 3HV content. 3HV can be produced by Cupriavidus necator from propionic acid. Few studies explored carbon distribution and dynamics of 3HV and 3HB monomers production, and none of them have been done with phosphorus as limiting nutrient. In this study, fed-batch cultures of C. necator with propionic acid, as sole carbon source or mixed with butyric acid, were performed. Phosphorus deficiency allowed sustaining 3HV production rate and decreasing 3HB production rate, leading to an instant production of up to 100% of 3HV. When a residual growth is sustained by a phosphorus feeding, the maximum 3HV percentage produced from propionic acid is limited to 33% (Mole.Mole(-1)). The association of a second carbon source like butyric acid lead to higher conversion of propionic acid into 3HV. This study showed the importance of the limiting nutrient and of the culture strategy to get the appropriate product.

Grazing resistance in nutrient-stressed phytoplankton

Grazing experiments were performed with the zooplankters Daphnia pulex and Daphnia magna, feeding on phosphorus-saturated and phosphorus-limited cells of two green algae (Scenedesmus subspicatus and Selenastrum capricornutum). P-limited algal cells passed largely intact through the gut and were thus spared from heavy grazing pressure. P-saturated algal cells, in contrast, were efficiently assimilated. Structural and morphological changes in the P-limited cells most probably reduced their digestibility. This phenomenon may be an important factor in zooplankton production and competition, and may serve as an example of a highly efficient strategy of P-limited algae to resist heavy grazing pressure.