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18.10: The Phosphorus Cycle - Biology

18.10: The Phosphorus Cycle - Biology



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Learning Outcomes

  • Discuss the phosphorus cycle and phosphorus’s role on Earth

Phosphorus, a major component of nucleic acid (along with nitrogen), is an essential nutrient for living processes; it is also a major component of phospholipids, and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is often the limiting nutrient (necessary for growth) in aquatic ecosystems (Figure 1).

Phosphorus occurs in nature as the phosphate ion (PO43−). In addition to phosphate runoff as a result of human activity, natural surface runoff occurs when it is leached from phosphate-containing rock by weathering, thus sending phosphates into rivers, lakes, and the ocean. This rock has its origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of ocean organisms and from their excretions. However, in remote regions, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources. This sediment then is moved to land over geologic time by the uplifting of areas of the Earth’s surface.

Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine ecosystems. The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average phosphate ion having an oceanic residence time between 20,000 and 100,000 years.

Phosphorus is one of the main ingredients in artificial fertilizers used in agriculture and their associated environmental impacts on our surface water. Excess phosphorus and nitrogen that enters these ecosystems from fertilizer runoff and from sewage causes excessive growth of microorganisms and depletes the dissolved oxygen, which leads to the death of many ecosystem fauna, such as shellfish and finfish. This process is responsible for dead zones in lakes and at the mouths of many major rivers (Figure 2).

A dead zone is an area within a freshwater or marine ecosystem where large areas are depleted of their normal flora and fauna; these zones can be caused by eutrophication, oil spills, dumping of toxic chemicals, and other human activities. The number of dead zones has been increasing for several years, and more than 400 of these zones were present as of 2008. One of the worst dead zones is off the coast of the United States in the Gulf of Mexico, where fertilizer runoff from the Mississippi River basin has created a dead zone of over 8463 square miles. Phosphate and nitrate runoff from fertilizers also negatively affect several lake and bay ecosystems including the Chesapeake Bay in the eastern United States.

Chesapeake Bay

The Chesapeake Bay has long been valued as one of the most scenic areas on Earth; it is now in distress and is recognized as a declining ecosystem. In the 1970s, the Chesapeake Bay was one of the first ecosystems to have identified dead zones, which continue to kill many fish and bottom-dwelling species, such as clams, oysters, and worms. Several species have declined in the Chesapeake Bay due to surface water runoff containing excess nutrients from artificial fertilizer used on land. The source of the fertilizers (with high nitrogen and phosphate content) is not limited to agricultural practices. There are many nearby urban areas and more than 150 rivers and streams empty into the bay that are carrying fertilizer runoff from lawns and gardens. Thus, the decline of the Chesapeake Bay is a complex issue and requires the cooperation of industry, agriculture, and everyday homeowners.

Of particular interest to conservationists is the oyster population; it is estimated that more than 200,000 acres of oyster reefs existed in the bay in the 1700s, but that number has now declined to only 36,000 acres. Oyster harvesting was once a major industry for Chesapeake Bay, but it declined 88 percent between 1982 and 2007. This decline was due not only to fertilizer runoff and dead zones but also to overharvesting. Oysters require a certain minimum population density because they must be in close proximity to reproduce. Human activity has altered the oyster population and locations, greatly disrupting the ecosystem.

The restoration of the oyster population in the Chesapeake Bay has been ongoing for several years with mixed success. Not only do many people find oysters good to eat, but they also clean up the bay. Oysters are filter feeders, and as they eat, they clean the water around them. In the 1700s, it was estimated that it took only a few days for the oyster population to filter the entire volume of the bay. Today, with changed water conditions, it is estimated that the present population would take nearly a year to do the same job.

Restoration efforts have been ongoing for several years by non-profit organizations, such as the Chesapeake Bay Foundation. The restoration goal is to find a way to increase population density so the oysters can reproduce more efficiently. Many disease-resistant varieties (developed at the Virginia Institute of Marine Science for the College of William and Mary) are now available and have been used in the construction of experimental oyster reefs. Efforts to clean and restore the bay by Virginia and Delaware have been hampered because much of the pollution entering the bay comes from other states, which stresses the need for inter-state cooperation to gain successful restoration.

The new, hearty oyster strains have also spawned a new and economically viable industry—oyster aquaculture—which not only supplies oysters for food and profit, but also has the added benefit of cleaning the bay.

Video Review

Many organisms require nitrogen and phosphorus. This video explains just how they go about getting them via the nitrogen and phosphorus cycles.

A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/fob1/?p=578


What’s Limiting?

The concept of a limiting nutrient has a long history in both the agricultural and aquatic sciences. The limiting nutrient is the first of the essential nutrients to disappear from the environment, usually due to plant use of that nutrient, thus limiting plant growth. Without the limiting nutrient, plants cannot use other nutrients even when those others are abundant. The plants of interest are either agricultural crops or the algae in aquatic environments. These algae can live either in the water or on the bottom of the water body.

For estuary and water body managers, the question of which nutrient is limiting to growth is critical. The answer will inform decisions about how to expend resources to limit nutrients in the environment.

Early studies, where entire Canadian lakes were doused with phosphorus in the 1960s and 1970s, suggested phosphorus was the limiting nutrient. Reductions in phosphorus loading in Lakes Erie and Washington (near Seattle) during the same period also greatly improved water quality. Studies that added nitrogen to samples of estuarine water on the South Shore of Long Island and elsewhere suggested that nitrogen was the limiting nutrient in marine waters. Therefore, phosphorus is generally seen as limiting in lakes and streams, and nitrogen in estuaries and coastal waters, though there are important exceptions.

A common metric used in evaluating nutrient limitation is the Redfield ratio, which is the ratio of the molar concentration of nitrogen to phosphorus in the water. (A mole of nitrogen and phosphorus each has the same number of molecules, regardless of the differences in molecular weight.) When the ratio exceeds 16 to 1, which is the typical balanced ratio of nitrogen to phosphorus in many plants, the system may be phosphorus-limited, depending on the overall nutrient concentrations and other factors. The opposite situation, potential nitrogen limitation, may occur when the ratio is less than 16 to 1.

While there are still academic debates about nitrogen versus phosphorus limitation in estuaries and coastal waters, nitrogen is by far the most commonly limiting nutrient there. This includes estuaries in the Northeast. Coastal waters are naturally enriched in phosphorus relative to nitrogen, with a Redfield ratio of less than 16. Furthermore, in most experimental estuarine and coastal studies where water samples containing algae were incubated with added nutrients, the samples showed greater biological responses to nitrogen additions than phosphorus additions. As we succeed in reducing the nitrogen loading to many estuaries, the Redfield ratio in these estuaries declines further and nitrogen limitation becomes even stronger.

A macroalgae bloom, in which nitrogen is the limiting nutrient, at Centerport Harbor in Suffolk County. Photo credit: James Ammerman

Some unfamiliar with the mechanism of a limiting nutrient express concerns that phosphorus will become more important as nitrogen loading is reduced, since phosphorus concentrations may remain high. In fact, the opposite is true: as nitrogen becomes more limiting, the likelihood of phosphorus limitation declines even more.


Introduction

Phosphorus (P), a necessary macronutrient, is required for growth and development of all living organism. P is highly reactive with other elements, for example, hydrogen, and oxygen, and can therefore not exist alone (Jez et al., 2016). Furthermore, phosphate rock is the only source of phosphorus, and will be depleted in 50� years worldwide (Kochian, 2012). The bioavailable form of phosphorus that plants can directly absorb is orthophosphate, which is a nonrenewable resource and derived from phosphate rock mining. Plants can only utilize �% of inorganic phosphorus (Pi) fertilizers due to the inaccessible form developed from its reactivity with aluminum and iron oxides and transformation of soil microbes (Jez et al., 2016). However, the remaining phosphorus in soil is fixed by roots and soil particles or transported to lakes and rivers through farm drainage and surface runoff, and leads to severe environment pollution such as eutrophication of water (Andersson et al., 2013). Pi-fertilizer is over-applied in order to make crops more productive due to the growing world population (Wu et al., 2013). According to the situation mentioned above, breeding of new crop varieties with high phosphorus acquisition and use efficiency should be implemented, which will not only reduce the high cost of Pi-fertilizers, but also render the agriculture development sustainable (Liu et al., 2016).

For acclimatization to external environments with low levels of phosphorus, plants have developed a series of response mechanisms of phenotypic, physiological, biochemical, metabolic, and molecular alternations to absorb more Pi from soil and reallocate the phosphorus in plants (Zhang et al., 2014). In general, plants often reduce the growth of shoots and roots under Pi deprivation, thus resulting in an increase of the root/shoot ratio (Royo et al., 2015). At the physiological and biochemical levels, Pi content is a very important parameter, and also decreases under conditions of phosphorus stress (Wang H. et al., 2014). Meanwhile, plant roots secrete multiple organic acids such as citrate and malate into rizospheres for exchanging via ligand exchange reactions the phosphate that is adsorbed onto oxides surfaces (Clarholm et al., 2015 Ding et al., 2016). Moreover, phospholipids located in the membrane are degraded to available phosphorus and replaced by galactolipids and sulfolipids, large amounts of which are synthesized under phosphorus deprivation (Okazaki et al., 2013). In addition, many metabolic processes (e.g., photosynthesis, respiration, amino acids, and lipid metabolism) were suppressed and molecules containing phosphorus (e.g., nucleases, ribonucleases, phosphoesterases, and purple acid phosphatases) were degraded under Pi-limiting conditions, meanwhile, genes involved in all of which and Pi transport were differetially expressed between normal ans low phosphorus treatment.

Every species exhibits an individual pattern of responding to phosphorus starvation stress that is linked to the growth strategy. These responses, under the condition of low phosphorus, depend on tissues, and also on genotypes (Byrne et al., 2010) the selection of varieties with high phosphorus efficiency is a feasible method for improving plant PUE. Previous studies have reported that monocotyledons are more P-efficient than dicotyledons (Vergutz et al., 2012). C4 plants are more sensitive to low phosphorus than C3 plants, and the photosynthesis rate of C4 plants decreases sharply under low phosphorus conditions (Sage and Mckown, 2005). Barley, one of the earliest domesticated crops (Pourkheirandish et al., 2015) and the world's fourth most ample cereal crop (Consortium, 2012), is widely used as fodder for animals and raw materials for the beer industry (Mascher et al., 2017). Barley is also greatly accommodated and is a staple plant, even known as “the last crop before the desert,” in some territories with a volatile climate. In addition, barley is now cultivated all over the world and its ability to increase yields in harsh circumstances is critical for future decades (Keller and Krattinger, 2017). Previous studies have demonstrated that barley has unique adaptive mechanisms in response to abiotic stress (Zeng et al., 2014 Quan et al., 2016). In previous experiments, we have identified two barley genotypes with contrasting phosphorus efficiency, which provide excellent germplasms for subsequent molecular research (Ren et al., 2016).

Phosphorus transporters are major proteins for uptake and translocation of phosphorus, and were firstly identified in Arabidopsis thaliana and Solanum tuberosum treated with a yeast mutant without the pho84 gene (Muchhal et al., 1996 Leggewie et al., 1997). In Arabidopsis, the majority of PHT1 genes were expressed in different parts of roots (Abel, 2017). Shin et al. (2004) found that both Pht11 and Pht14 can improve the uptake of phosphorus in Arabidopsis (Shin et al., 2004). In addition, two other genes of the PHT1 family, AtPHT18 and AtPHT19, are involved in transporting phosphorus from roots to shoots (Lapis-Gaza et al., 2014). In a recent study, a SULTR-like phosphorus distribution transporter (SPDT) was reported to distribute phosphorus to the grain (Yamaji et al., 2017). In terms of regulation of phosphorus genes, WRKY45 was reported to activate the expression of PHT11 in Arabidopsis (Wang H. et al., 2014). Moreover, miR399 and miR827 can also positively regulate phosphorus genes (Chien et al., 2017). In barley, eight pht1 homologous genes have been identified (Smith et al., 1999 Rae et al., 2003). Among these PHT1 genes, HvPHT16 was identified to redistribute the phosphorus for expression in shoots and roots (Preuss et al., 2010), and HvPHT18 was detected as a mycorrhiza-specific gene (Glassop et al., 2005). In addition, the promoters of HvPht11 and HvPht12 were highly expressed under phosphorus starvation conditions (Nussaume et al., 2011). Schünmann et al. (2004) also reported that the expression of phosphorus transporters was regulated by a MYB-type transcription factor (Schünmann et al., 2004). In terms of phosphorus efficiency, previous researches mainly focus on one of three major phosphorus metabolic processes (Pi uptake, transport and utilization), while interaction and transcriptional regulation among them still need to be further investigated.

Scenic changes clearly occur in growth and development processes under Pi-limiting conditions, as well as in different omics profiles. Next-generation sequencing (NGS) technology (RNA-seq) can generate digital data of gene expression with removal of the limits of predesigned probes (Secco et al., 2013), and have been conducted not only on Arabidopsis (Lan et al., 2012), rice (Oono et al., 2013), maize (Li et al., 2012), but also on de novo assembly for many organisms in the past (Du et al., 2017). The results of incomplete sequences and low quality transcripts gained from RNA-seq are a limitation of alternative splice analysis and corrected annotations, respectively (Tilgner et al., 2015). However, the advent of single-molecule real-time (SMRT) sequencing�veloped by Pacbio Biosciences𠅋rought an end to these restrictions by generating longer or full-length sequences, and by improving the annotation information of the known genome (VanBuren et al., 2015 Lan et al., 2017). However, SMRT sequencing can provide inaccurate information of genes, an important issue that was reported to account for the high error rate in recent studies. Nevertheless, the problem can be corrected by RNA-seq reads and circular-consensus (CCS) reads. In recent years, the method of combining RNA-seq and SMRT sequencing has been frequently applied to generate comprehensive information at the transcriptional level, which provides the scientific basis for perfecting the genome database and molecular breeding (Xu et al., 2015).

In the present study, SMRT sequencing technology was carried out in the two barley varieties with varying phosphorus efficiency under different treatments, at three time points, for analyzing sophisticated responses of barley plants to Pi starvation and recovery. A rich and integrated data-set was constituted for improving genome annotation and extending our awareness of molecular responses. This research was achieved by means of a previously unused method in barley, unveiling important transcripts responding to external Pi stress, and the results can be exploited for breeding new crop varieties with high PUE.


18.10: The Phosphorus Cycle - Biology

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In the phosphorous cycle, phosphorous atoms exist primarily in rock, not a gaseous state. When rock erodes, the phosphorous is released and becomes dissolved in streams, lakes and ground water.

Plants and algae use these free inorganic phosphates in the soil or water to produce organic molecules. Then by consuming the plants, heterotrophs tap into the phosphorous stores to build their own compounds. When organisms die, decomposers break up the phosphorous molecules and release inorganic phosphate to be used again by plants and algae, a process called phosphate mineralization.

While phosphates cycle quickly through organisms in the food chain, they have a longer residence time in water. Dissolved phosphate ions react to form insoluble compounds that precipitate in the ocean and become sediment, then rock, and as result of tectonic uplift, eventually return to the environment.

However, since the amount of phosphorous is limited in the environment, it must be provided to agricultural crop plants as fertilizer to get maximum yield. Excess phosphorous becomes run-off in aquatic ecosystems resulting in a variety of environmental problems such as algal blooms.

27.10: The Phosphorus Cycle

Unlike carbon, water, and nitrogen, phosphorus is not present in the atmosphere as a gas. Instead, most phosphorus in the ecosystem exists as compounds, such as phosphate ions (PO4 3- ), found in soil, water, sediment and rocks. Phosphorus is often a limiting nutrient (i.e., in short supply). Consequently, phosphorus is added to most agricultural fertilizers, which can cause environmental problems related to runoff in aquatic ecosystems.

Biological Phosphorus Cycle

Phosphorus is present in many important biological structures, such as DNA, cell membranes, bones and teeth. It is not present in the atmosphere in a gaseous form, but is found in minerals, sediment, volcanic ash, and aerosols. As rocks and sediment weather over time, they release inorganic phosphate, which gradually reaches soil and surface water. Plants absorb and incorporate these phosphates into organic molecules. Animals obtain and incorporate phosphates by consuming plants and other animals. When plants and animals die or excrete waste, organic phosphates return to the soil and are broken down by bacteria&mdashin a process called phosphate mineralization&mdashinto inorganic forms that can again be used by plants.

Geochemical Phosphorus Cycle

Natural runoff can transport phosphates to rivers, lakes, and the ocean, where they can be ingested by aquatic organisms. When aquatic organisms die or excrete waste, phosphorus-containing compounds may sink to the ocean floor and eventually form sedimentary layers. Over thousands of years, geological uplift can return phosphorus-containing rocks from the ocean to land.

Human Impacts on the Phosphorus Cycle

Like nitrogen, phosphorus is often a limiting factor in plant growth in natural environments, which has led to the agricultural practice of adding phosphorus to fertilizers in order to increase crop yield. However, agricultural runoff from this practice can stimulate the rapid growth of aquatic producers, causing a variety of environmental problems.

Watson, Andrew J., Timothy M. Lenton, and Benjamin J. W. Mills. &ldquoOcean Deoxygenation, the Global Phosphorus Cycle and the Possibility of Human-Caused Large-Scale Ocean Anoxia.&rdquo Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences 375, no. 2102 (September 13, 2017). [Source]

White, Angelicque, and Sonya Dyhrman. &ldquoThe Marine Phosphorus Cycle.&rdquo Frontiers in Microbiology 4 (May 21, 2013). [Source]


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Phosphorus Cycle - PowerPoint PPT Presentation

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Phosphorus rubber?

Goodyear's 1839 discovery of the vulcanization of natural rubber obtained from rubber trees marks the beginning of the modern rubber industry. A variety of synthetic rubber products were subsequently developed. In the journal Angewandte Chemie, scientists have now introduced a new, interesting variant: a phosphorus-containing rubber with a structure that corresponds to that of natural rubber.

The similar properties of double bonds between carbon atoms (C=C) and phosphorus-carbon double bonds (P=C) led to the idea to try general polymerization techniques on the latter. After a number of successful attempts, researchers working with Derek P. Gates at the University of British Columbia (Vancouver, Canada) wanted to apply this concept to molecules that contain both P=C and C=C double bonds: phosphorus analogs of the building block of rubber, isoprene (2-methylbuta-1,3-diene) and its close relative, 1,3-butadiene.

Starting with phosphorus-containing precursors, the team was able to synthesize the first examples of poly(1-phospha-isoprene) and poly(1-phospha-1,3-butadiene). Precise characterization with a variety of spectrometric techniques gave some insight into the molecular structures of the resulting polymers. Like in the polymerization of isoprene and related dienes (compounds with two carbon-carbon double bonds), one of the double bonds in each building block is retained. The polymerization mainly occurs through the C=C double bonds and only a tiny proportion happens at the P=C double bonds. This means that only a few phosphorus atoms are incorporated into the polymer backbone. The majority of the phosphorus atoms form side chains in which the P=C double bonds are maintained, leaving them available for further reactions or alterations to the polymers.

"Our functional phosphorus-containing materials are rare examples of polymers containing phosphaalkene moieties and offer many prospects for further derivatization and crosslinking," according to Gates. For example, the researchers were able to bind gold ions to the polymers. "As a macromolecular ligand for gold ions, the new polymers may be of future interest in catalysis and nanochemistry. Furthermore, the successful polymerization of P=C/C=C hybrid monomers opens the door to incorporate P-functionalities into commercial rubbers such as butyl rubber or styrene-butadiene rubber that traditionally use isoprene or butadiene comonomers. Such new copolymers promise unique architectures, properties, and functionality when compared to their carbon-only analogues."


Abstract

The Great Oxidation Event (GOE) is one of the most significant changes in seawater and atmospheric chemistry in Earth history. This rise in oxygen occurred between ca. 2.4 and 2.3 Ga and set the stage for oxidative chemical weathering, wholesale changes in ocean chemistry, and the evolution of multicelluar life. Most of what is known about this important event and the subsequent oxygenation history of the Precambrian Earth is based on either geochemistry or “data mining” published literature to understand the temporal abundance of bioelemental sediments. Bioelemental sediments include iron formation, chert, and phosphorite, which are precipitates of the nutrient elements Fe, Si, and P, respectively. Because biological processes leading to their accumulation often produce organic-rich sediment, black shale can also be included in the bioelemental spectrum. Thus, chemistry of bioelemental sediments potentially holds clues to the oxygenation of the Earth because they are not simply recorders of geologic processes, but intimately involved in Earth system evolution.

Chemical proxies such as redox-sensitive trace elements (Cu, Cr, V, Cd, Mo, U, Y, Zn, and REE's) and the ratio of stable isotopes (δ 56 Fe, δ 53 Cr, δ 97/95 Mo, δ 98/95 Mo, δ 34 S, Δ 33 S) in bioelemental sediments are now routinely used to infer the oxygenation history of paleo-seawater. The most robust of these is the mass-independent fractionation of sulfur isotopes (MIF), which is thought to have persisted under essentially anoxic conditions until the onset of the GOE at ca. 2.4 Ga. Since most of these proxies are derived from authigenic minerals reflecting pore water composition, extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously.

Paleoenvironmental context is critical to understanding whether geochemical trends during Earth's oxygenation represent truly global, or merely local environmental conditions. To make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed. Because relatively few studies consider alteration in detail, our ability to measure geochemical anomalies through the GOE now surpasses our capacity to adequately understand them.

In this review we highlight the need for careful consideration of the role sedimentology, stratigraphy, alteration, and basin geology play in controlling the geochemistry of bioelemental sediments. Such an approach will fine-tune what is known about the GOE because it permits the systematic evaluation of basin type and oceanography on geochemistry. This technique also provides information on how basin hydrology and post-depositional fluid movement alters bioelemental sediments. Thus, a primary aim of any investigation focused on prominent intervals of Earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean-atmosphere evolution.

Highlights

► only review paper that advocates a balanced, sedimentologic-based approach to understanding the Great Oxidation Event (GOE). ► Comprehensive review of the evidence for the GOE emphasizing the types of sedimentary deposits that formed. ► Addresses shortcomings with interpretations based solely on geochemistry. ► Provides directives and a clear protocol for evaluating geochemical proxies in a sedimentological and basin evolution context.


Elevated CO2 enhances nitrogen fixation and growth in the marine cyanobacterium Trichodesmium

The increases in atmospheric pCO2 over the last century are accompanied by higher concentrations of CO2(aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)-fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO2(aq). We examined the influence of pCO2 [preindustrial∼250 ppmv (low), ambient∼400, future∼900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom-forming cyanobacterium contributing substantial inputs of ‘new N’ to the oligotrophic subtropical and tropical oceans. High pCO2 enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO2 cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO2. Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO2 to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.


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