Which of the following are inorganic nutrients?

Nitrogen assimilation

Among inorganic nutrients, nitrogen is of paramount importance as it accounts for ∼10% of the dry weight of cyanobacterial cells. Nitrate (NO3−) and ammonium (NH4+) are virtually universal sources of nitrogen for cyanobacteria, but urea or other organic nitrogenous compounds can be used by some strains. In addition, many strains can fix gaseous dinitrogen (N2). Plasma membrane-bound transport systems exist for both NO3− and NH4+, whereas N2 enters the cells by diffusion. Intracellular NO3 − must be reduced to NH4+. This is accomplished by the stepwise reduction to nitrite (catalyzed by nitrate reductase) and NH4+ (catalyzed by nitrite reductase), the reduction equivalents for both processes stemming from reduced ferredoxin. Ammonium (either taken up or endogenously generated) is assimilated by the glutamine synthetase/glutamate synthase enzyme system. The net action of this system is the formation of glutamate from α-ketoglutarate and NH4+, with the expenditure of ATP and the oxidation of ferredoxin. Glutamate can donate its amino moiety to various precursors of central metabolism by the action of specific transaminases. Many, but not all, cyanobacteria are able to fix N2; this is of great ecological significance because N2 is ubiquitous in the environment. The process is carried out by the enzyme nitrogenase and is a costly one, involving the consumption of both ATP and reduction equivalents (supplied by ferredoxin). In addition, nitrogenase will also inevitably reduce protons to H2 in what represents a wasteful decrease in efficiency. Nitrogenase is inherently and irreversibly inactivated by O2. Several strategies have evolved in cyanobacteria to circumvent this problem. Some strains will only carry out N2 fixation under anoxic conditions, but some will also do it in the presence of oxygen. Several strains have been shown to restrict temporally N2 fixation to the dark period, thus decreasing the exposure of nitrogenase to photosynthetic oxygen. Strains belonging to the Nostocales and Stigonematales have evolved a specialized cell type (the heterocysts; see ‘Heterocysts’) in which nitrogen fixation is spatially separated from photosynthesis and protected from O2 inactivation. Heterocystous strains display the highest specific rates of N2 fixation among all cyanobacteria. However, some nonheterocystous cyanobacteria, such as Trichodesmium, are able to fix substantial, biogeochemically significant amounts of N2 in the light; their mode of adaptation is unknown. The various mechanisms for nitrogen assimilation are tightly regulated so that the presence of less costly sources (NH4+) immediately inhibits NO3− (and NO2 −) uptake, or N2 fixation activity, and represses the expression of the enzymes involved in the reduction of alternative N2 sources. In the same way, the presence of abundant NO3− represses the expression of nitrogenase genes and results in the halting of new heterocyst differentiation.

Concepts related to production

Production of new organic matter from inorganic nutrients using sunlight as energy source, known as photosynthesis, is an essential prerequisite for life on earth. The organsims carrying out photosynthetic processes to build cellular material are therefore called autotrophs. Marine and freshwater algae contribute about half of annual primary productivity on the planet. Clarifying the processes contributing to the drawdown of inorganic carbon (CO2) is crucial for understanding carbon cycling and functioning of global ecosystems.

In its early phase, studies on the energy flow through aquatic systems revealed by estimates of primary productivity largely rested on the ecosystem concept (Goldman, 1968). The 50 year aniversary of invention of the radioactive carbon-14 method triggered a new interest in productivity measurements (Williams et al., 2002), boosting techniques (Fahey and Knapp, 2007) and concepts (e.g. Falkowski, 2012). New concepts were developed such as the “active pipe” concept for the global quantification (Tranvik et al., 2018), nutrient stoichiometry (Maranger et al., 2018) and integrative understanding (Hotchkiss et al., 2018; Seekell et al., 2018) of carbon metabolism. Advancements in carbon acquisition and concentrating mechanisms have been reviewed by Griffiths et al. (2017). Dokulil and Qian (2020) provide an article reviewing the history of photosynthesis, carbon acquisition and primary productivity.

2.3.3.6 pH

It is the negative logarithm of hydrogen ion activity that affects the absorption of ions and solidification of culture media. Optimum pH for culture media is 5.6–5.8 before sterilization. The values of pH lower than 4.5 or higher than 7.0 greatly inhibit growth and development of tissue in vitro. The pH of culture media generally drops by 0.3–0.5 units after autoclaving and keeps changing through the incubation of culture due to oxidation as well as differential uptake and secretion of substances by the cultured tissue. If the pH drops appreciably during plant tissue culture (pH below 5.0 it does not allow gelling of the agar and the medium becomes liquid), then a fresh medium should be prepared, whereas a pH greater than 6.0 gives hard medium (interferes with the absorption of nutrients). It is well established that pH of the medium affects the gelling efficiency of agar, uptake of ingredients, solubility of different salts, and chemical reactions (especially those catalyzed by enzymes) in the medium.

Edaphic Factors Affecting Plant Nutrients

For healthy growth, plants require many inorganic nutrients in varying amounts and forms. Macronutrients have concentrations of at least 500 mg kg− 1 in plants, while micronutrients are required in lower amounts, usually less than 100 mg kg− 1. Nitrogen (N), phosphorus (P), and potassium (K) are the most commonly measured and applied macronutrients by farmers, gardeners and other land managers. Macronutrients are not necessarily the most important nutrient in determining the ability of that plant to grow; macro- refers to the large quantity used by the plant. Liebig’s Law of the Minimum (or sufficiency levels) describes the fertilizer needs of plants by stating that plant growth (yield) is controlled by the most limited (needed) nutrient. In other words, an application of the most limited nutrient in the form needed by the plant will result in additional plant growth. The next most limited nutrient will then control the rate of growth of the plant. The exact requirements of each nutrient vary for each plant and environment. Recommended ranges are determined through edaphology research and experience.

Fertilizers are labeled with their N–P–K amounts and should be applied to the soil only when soil or tissue test results indicate that the nutrients are below optimum levels. Fertilizer applications for nutrients that are not limiting will not result in additional growth and may leave the plant rootzone by leaching or runoff impacting offsite environments. Due to the rapid fluxes of soil nitrogen content, on-site tissue nitrogen tests are often more accurate than soil tests. Specific edaphic factors that may be measured to monitor soil fertility include nitrate-N, available P, and soluble K (Table 2). Other commonly measured nutrients include calcium (Ca), magnesium (Mg), sulfate (SO42−), and iron (Fe). Recent debates in the scientific community have considered not just the total amount of each nutrient affecting plant growth and plant landscape distribution, but also the relative proportions of nutrients. Base-cation saturation ratio (BCSR) is considered an important tool for determining soil health in sustainable agriculture and some sectors of urban horticulture. For example, the calcium-to-magnesium ratio may be used to explain the occurrence of some endemic plants growing on serpentine soils in California, USA.

The soil reaction or pH is related to nutrient availability and optimum plant growth. Soil pH is one of the most common and important soil chemical properties measured. Some plants prefer acidic soils, while others are best suited for alkaline soils. There are few plants that will survive in extremely acid or alkaline soils however, and most flora and fauna prefer conditions between pH 5.5 and 8.5. The ideal soil pH is near 6.5 for most plants due to the high availability of all essential plant nutrients. Some pH-based nutrient problems can be avoided with the use of foliar-absorbed nutrients. Soil pH also affects the soil microbial population, which is necessary to convert soil organic matter to inorganic nutrient forms.

The combined physical, chemical, and biological benefits of organic matter make it one of the most important edaphic factors affecting plant growth. Most microbes prefer warm, moist, near-neutral pH conditions to mineralize organic matter. These conditions produce an active microbial community that will decompose manure, compost and plant residues into their final inorganic products of carbon dioxide, water, and plant-available inorganic nutrients. The nutrients found in these materials are generally found in lower quantities and are more slowly available to the plant than inorganic, synthetic fertilizers. Soils rich in organic matter are darker and tend to warm up faster in the spring. For these reasons, management of soil organic matter is essential for those practicing sustainable agriculture and organic production.

The ability of a soil to adsorb or hold cations is called the cation exchange capacity (CEC). Soil organic matter and clay minerals are usually negatively charged, thus allowing them to attract and retain positively charged cations from the soil solution. This is an important feature of soil materials because so many plant nutrients are cations (e.g., potassium (K+), calcium (Ca2 +), magnesium (Mg2 +), ammonium (NH4+) and many micronutrients). Cations are held loosely and temporarily by negatively charged exchange sites in and around soil particles that prevent leaching with every rainfall or irrigation event. However, their attraction is not permanent and the cations are easily available to plants and microbes as they are needed. Soils that have abundant humus and smectite or vermiculite clays are higher in CEC than sandy soils low in organic matter. Organic matter is also valuable in complexing or chelating some metals to make them more soluble and plant-available. In addition, soil organic matter has been shown to negate the toxicity of aluminum (Al) in some soils.

Several elements are toxic to plants and may require characterization to determine whether they are affecting plant growth and distribution on the landscape (Table 4). Aluminum, nickel (Ni), manganese (Mn), lead (Pb), copper (Cu), chromium (Cr), and cobalt (Co) are among the metals that have been analyzed and correlated with vegetation growth around mines or smelters. Some elements are plant micronutrients at low concentrations, but become toxic at high concentrations in the soil or solution. Examples of these elements are boron (B), copper, and zinc (Zn). In addition, excessive quantities of some nutrients can result in deficiencies in other nutrients due to preferential uptake.

Table 4. Toxic elements that may affect plant growth and soil quality

NA, no maximum concentration has been designated.

Soil mineralogy also influences the availability of some important nutrients such as phosphate and sulfate. Allophane or clay materials derived from volcanic glass can fix or permanently adsorb phosphate and sulfate anions. Iron and aluminum oxides are prevalent in leached, acidic soils of humid zones and can combine with phosphate to make insoluble phosphate minerals that are unavailable to plants. In arid regions, where calcareous soils are common, the calcium combines with and binds the phosphate. Phosphorus is generally most plant-available at near-neutral pH and in soils not dominated by allophane, iron or aluminum oxides, or calcium carbonate. Available or extractable phosphorus is analyzed to distinguish it from organic or mineral forms that are not accessible to plants.

Potassium and ammonium may also become “fixed” or unavailable in soils high in vermiculite. The size of these ions and their charge density allow them to become permanently imbedded in the interlayer region of high-charge vermiculite clay minerals that collapse around them in response to opposite electrical charges. Thus, soluble potassium and/or clay mineralogy is measured to determine the amount of potassium available for plant uptake.

Salinity, often measured by electrical conductivity (EC), can be a problem in closed basins with shallow water tables, arid regions where evapotranspiration rates exceed precipitation rates and other areas with poor quality water. Inorganic fertilizers are also salts and may contribute to soil EC and saline stress when applied in excess. Salinity refers to all ionic compounds in a soil solution while sodicity refers specifically to high sodium concentration relative to calcium and magnesium concentrations. Not all salts are equally destructive to plants and soil. For example, sodium may impact the soil by dispersing clay aggregates and forming an impermeable seal at the soil surface and along cracks that limits water and air movement into the soil. In addition, high sodium levels can be toxic to plants and impede water movement into plant roots. The sodium adsorption ratio (SAR) or exchangeable sodium percentage (ESP) are two measures of the sodium content affecting plant growth, especially when rainfall is insufficient to leach salts out of the soil profile. Closed basins with shallow groundwater are often sodic, saline and detrimental to plant growth.

Salts can affect plant growth in two ways, either directly as a toxic specific ion or indirectly by lowering the osmotic potential of the soil water, which hinders the plants' ability to uptake water. Increased salts lower the osmotic potential, which in turn lowers the total soil-water potential and requires additional energy to extract water from the soil solution. Specific ion toxicity can be a problem when boron (B), chloride (Cl−), selenium (Se), or sodium (Na) concentrations are excessive. Some plants are especially adapted to salinity. Adaptations include ion selectivity to prevent absorption of toxic ions through roots, sequester toxic ions into plant tissues or organelles away from critical plant tissues and plant functions, osmotic adjustment in plant tissues to ensure normal enzymatic activity, and osmotic adjustment in the rhizosphere to improve water uptake from soil matrix.

6.38.4.1 Biodegradation

Domestic STPs have been constructed for the efficient removal of organic and inorganic nutrients in the gram-per-liter range. Micropollutant removal was initially not intended by conventional biological treatment systems, and, therefore, insufficient removal is often observed. Joss et al. [29] suggested a classification scheme to characterize the biological degradation of 35 compounds including pharmaceuticals, hormones, and PCPs during wastewater treatment (Figure 2), illustrating the poor efficiency of municipal wastewater treatment in degrading micropollutants (only 4 out of 35 compounds were degraded by more than 90%).

Levine et al. [37] investigated the removal of antibiotics and other pharmaceuticals, hormones, PAHs, and an extended list of industrial and household chemicals in a biological treatment with efficient nitrogen removal. Persistent compounds such as triclosan and sulfamethoxazole were removed by more than 85%, while compounds such as carbamazepine, beta-sitosterol, beta-stigmastanol, and two flame retardants (tri(2-chloroethyl)phosphate and tri(dichloroisopropyl)phosphate) and two polycyclic musk fragrances (galaxolide and tonalide) were removed by less than 25%.

Competitive Interactions

As might be anticipated, competition between acidophiles for electron donors and acceptors, inorganic nutrients, and so on, is as important in acidic as in all other environments. One of the most detailed studies of this kind, given the importance of iron-oxidizing bacteria in commercial mineral processing and the genesis of AMD, has been the competition between At. ferrooxidans and Leptospirillum spp. for their communal substrate (electron donor), ferrous iron. Early assumptions that At. ferrooxidans was invariably the dominant iron-oxidizing bacterium in metal-rich, acidic environments have gradually been eroded, with increasing numbers of reports describing L. ferrooxidans (though in some cases this is probably L. ferriphilum) as the more abundant species. These include mine drainage waters and some (mostly stirred tank) commercial biomining operations. In general, Bacteria classified as At. ferrooxidans tend to grow more rapidly than Leptospirillum spp., and are better able to exploit acidic environments that contain relatively large concentrations of ferrous iron. In terms often used to differentiate heterotrophic bacteria, At. ferrooxidans is a copiotroph while Leptospirillum spp. are oligotrophs. This is also the reason why using ferrous iron-rich synthetic media to enrich for iron-oxidizing acidophiles favors At. ferrooxidans rather than L. ferrooxidans. On the other hand, the greater affinity for ferrous iron and the greater tolerance of ferric iron of L. ferrooxidans (and probably other Leptospirillum spp.) facilitates their dominance in stirred tank mineral leachates, where ferric iron concentrations can be many grams per liter and, conversely, in those extremely acidic environments (pH < 2.3) where ferrous iron concentrations are very small. In both situations, redox potentials (which are determined by the relative concentrations, rather than actual concentrations, of ferrous and ferric iron) are commonly above 750 mV, and Leptospirillum spp. are known to be far more efficient iron oxidizers than At. ferrooxidans under such highly oxidizing conditions. Other important factors that affect competition between these iron-oxidizing autotrophs are temperature and pH. Leptospirillum spp. in general (and L. ferriphilum in particular) tend to be more thermotolerant than At. ferrooxidans, which partly explains their greater importance within the warm interior of the Richmond mine at Iron Mountain (see ‘Acid mine streams and lakes’) and in stirred tanks used to bioprocess gold and cobaltiferous ores, which generally operate at around 40 °C. On the other hand, cold-tolerant iron-oxidizing acidophiles have been invariably identified as At. ferrooxidans-like. Leptospirillum spp. also tend to be more tolerant of extreme acidity (many stains grow at pH 1) than At. ferrooxidans, some strains of which do not grow below pH 1.8, though others, including the type strain, can grow at pH 1.5. The higher pH optima for their growth is one of the reasons why At. ferrooxidans is often more important in heap leaching of mineral ores, as engineered mineral heaps are generally not so acidic as stirred tanks.

In one of the few studies to describe competition between two other iron-oxidizing acidophiles, dissolution of pyrite at 45 °C by a mixed culture of the thermotolerant facultatively autotrophic bacterium Acidimicrobium (Am.) ferrooxidans and a thermotolerant strain of the obligate autotroph L. ferriphilum was examined. Numbers of the two bacteria (estimated using fluorescent in situ hybridization; FISH) remained very similar until the pH of the bioreactor was lowered from 1.5 to 1.2, at which point L. ferriphilum emerged as the dominant bacterium. However, when the thermotolerant sulfur oxidizer At. caldus was also included in the microbial consortium, Am. ferrooxidans was more abundant than L. ferriphilum at pH 1.5 and 1.2. The reason for this was probably the additional amount of organic carbon available for the heterotrophically inclined Am. ferrooxidans originating from the CO2-fixing At. caldus that used the RISCs produced by ferric iron attack of the pyrite in the bioreactor.

Coverage of Nutrients

Most compilations provide coverage of all the macronutrients and most of the vitamins and major inorganic nutrients. The principal gaps at present are the trace elements for which, despite their nutritional desirability, it is difficult for the compilers to provide representative values for the reasons discussed earlier. The coverage of the carotenoids in most databases is limited at present, but in the future, one should expect better coverage. The same will be possible for the different folate vitamins, although the analytical methods are possibly less well developed. Many databases give limited coverage of the carbohydrates in foods. Coverage of the nonnutrient biologically active components represents a greater problem because of the great number of potential constituents coupled with the variety of methods, but nutritional research interests in these components should eventually identify the most important constituents to include.

EFFECT OF PATHOGENS ON TRANSLOCATION OF WATER AND NUTRIENTS IN THE HOST PLANT

All living plant cells require an abundance of water and an adequate amount of organic and inorganic nutrients in order to live and to carry out their physiological functions. Plants absorb water and inorganic (mineral) nutrients from the soil through their root system. These substances are generally translocated upward through the xylem vessels of the stem and into the vascular bundles of the petioles and leaf veins, from which they enter the leaf cells. Minerals and part of the water are utilized by the leaf and other cells for the synthesis of the various plant substances, but most of the water evaporates out of the leaf cells into the intercellular spaces and from there diffuses into the atmosphere through the stomata. However, nearly all organic nutrients of plants are produced in the leaf cells, following photosynthesis, and are translocated downward and distributed to all the living plant cells by passing, for the most part, through the phloem tissues. When a pathogen interferes with the upward movement of inorganic nutrients and water or with the downward movement of organic substances, diseased conditions result in the parts of the plant denied these materials. The diseased parts, in turn, will be unable to carry out their own functions and will deny the rest of the plant their services or their products, thus causing disease of the entire plant. For example, if water movement to the leaves is inhibited, the leaves cannot function properly, photosynthesis is reduced or stopped, and few or no nutrients are available to move to the roots, which in turn become starved and diseased and may die.

Effect on Absorption of Water by Roots

Many pathogens, such as damping-off fungi (Fig. 3-2A), root-rotting fungi and bacteria (Figs. 3-2B–3-2D), most nematodes, and some viruses, cause an extensive destruction of the roots before any symptoms appear on the aboveground parts of the plant. Some bacteria and nematodes cause root galls or root knots (Figs. 3-2E and 3-2F), which interfere with the normal absorption of water and nutrients by the roots. Root injury affects the amount of functioning roots directly and decreases proportionately the amount of water absorbed by the roots. Some vascular parasites, along with their other effects, seem to inhibit root hair production, which reduces water absorption. These and other pathogens also alter the permeability of root cells, an effect that further interferes with the normal absorption of water by roots.

FIGURE 3-2. Examples of reduction of water absorption by plants. (A) Destruction of roots of young seedlings by the damping-off oomycete Pythium sp. (B) Roots and stems of pepper plants killed by Phytophthora sp. (C) Wheat roots at different stages of destruction by the take-all fungus Gaeumannomyces tritici. (D) Infection of crown and roots of corn plant with the fungus Fusarium. (E) Numerous galls caused by the bacterium Agrobacterium tumefaciens on roots of a cherry tree. (F) Root knot galls caused by the nematode Meloidogyne sp. on roots of a cantaloupe plant.

[Photographs courtesy of (A) Plant Pathology Department, University of Florida, (B) K. Pernezny, University of Florida, (C) W. McFadden, W.C.P.D., (D) Plant Pathology Department, Iowa State University, (E) Oregon State University, and (F) B. D. Bruton, USDA, Lane, Oklahoma.]

Effect on Translocation of Water through the Xylem

Fungal and bacterial pathogens that cause damping off, stem rots (Fig. 3-3A), and cankers (Fig. 3-3B) may reach the xylem vessels in the area of the infection and, if the affected plants are young, may cause their destruction and collapse. Cankers in older plants, particularly older trees (Fig. 3-3B), may cause some reduction in the translocation of water, but, generally, do not kill plants unless the cankers are big or numerous enough to encircle the plant. In vascular wilts, however (Figs. 3-3C–3-3F), reduction in water translocation may vary from little to complete. In many cases, affected vessels may be filled with the bodies of the pathogen (Figs. 3-4A–3-4D) and with substances secreted by the pathogen (Figs. 3-5D and 3-5E) or by the host (Fig. 3-5C) in response to the pathogen and may become clogged (Figs. 3-4A and 3-4C and 3-5C–3-5E). Whether destroyed or clogged, the affected vessels cease to function properly and allow little or no water to pass through them. Certain pathogens, such as the crown gall bacterium (Agrobacterium tumefaciens), the clubroot protozoon (Plasmodiophora brassicae), and the root-knot nematode (Meloidogyne sp.), induce gall formation (Figs. 3-2E and 3-2F) in the stem, roots, or both. The enlarged and proliferating cells near or around the xylem exert pressure on the xylem vessels, which may be crushed and dislocated, thereby becoming less efficient in transporting water.

FIGURE 3-3. Examples of reduction of upward translocation of water and mineral nutrients by (A) the stem of a cantaloupe plant infected with the fungus Phomopsis sp. (B) Canker on an almond tree caused by the fungus Ceratocystis fagacearum. (C) Vascular wilt of tomato caused by the fungus Fusarium. (D) Discolored vascular tissues of a tomato stem infected with the same fungus. (E) Wilted tomato plants infected with the vascular bacterium Ralstonia solanacearum. (F) Discolored vascular tissues of a tomato plant infected with the same bacterium.

[Photographs courtesy of (A) B. D. Bruton, USDA, Lane, Oklahoma, (B) B. Teviotdale, Kearney Agricultural Center, Parlier, California, (C,E, and F) Department of Plant Pathology, University Florida, and (D) L. McDonald, W.C.P.D.]

FIGURE 3-4. (A) Pseudomonas bacteria clogging a xylem vessel of a young plant shoot. (B) Bacteria moving from one vessel to another and to adjacent parenchyma cells through xylem pits. (C) Bacteria of the xylem-inhabiting Xylella fastidiosa in a vessel of a grape plant. (D) Marginal scorching of a grape leaf from a plant infected with X. fastidiosa, the cause of Pierce's disease of grape. (E) Xylella bacteria in a cross section of a xylem vessel of an infected grape leaf.

[Photographs courtesy of (A and B) E. L. Mansvelt, I. M. M. Roos, and M. J. Hattingh (1500×), (C) D. Cooke, provided by E. Hellman, Texas A&M University, (D) E. Hellman, and (E) E. Alves, Federal University of Lavras, Brazil.]

FIGURE 3-5. (A) Young squash plant showing early symptoms of vascular wilt caused by the bacterium Erwinia tracheiphila. (B) E. tracheiphila bacteria lining up the inside wall of a xylem vessel. (C) Tyloses in a xylem vessel. (D) Tyloses and gummy polysaccharides partially or totally clogging up xylem vessels of a squash plant. (E) Several xylem vessels totally clogged with gummy polysaccharides. (F) Cantaloupes in a field where the plants had been killed by the bacterium E. tracheiphila.

[Photographs courtesy of (A,B,D,E, and F) B. D. Bruton, USDA, Lane, Oklahoma, and (C) D. M. Elgersma.]

The most typical and complete dysfunction of xylem in translocating water, however, is observed in the vascular wilts (Figs. 3-3 and 3-5) caused by the fungi Ceratocystis, Ophiostoma, Fusarium, and Verticillium and bacteria such as Pseudomonas, Ralstonia, and Erwinia. These pathogens invade the xylem of roots and stems and produce diseases primarily by interfering with the upward movement of water through the xylem. In many plants infected by these pathogens the water flow through the stem xylem is reduced to a mere 2 to 4% of that flowing through stems of healthy plants. In general, the rate of flow through infected stems seems to be inversely proportional to the number of vessels blocked by the pathogen and by the substances resulting from the infection. Evidently more than one factor is usually responsible for the vascular dysfunction in the wilt diseases. Although the pathogen is the single cause of the disease, some of the factors responsible for the disease syndrome originate directly from the pathogen, whereas others originate from the host in response to the pathogen. The pathogen can reduce the flow of water through its physical presence in the xylem as mycelium, spores, or bacterial cells (Figs. 3-4A–3-4C and 3-5B) and by the production of large molecules (polysaccharides) in the vessels (Figs. 3-5D and 3-5E). In most host–pathogen combinations, the destruction of xylem vessels by fungi (Fig. 3-3A) results in the collapse and death of the plant, as does the invasion of xylem vessels by fungi (Figs. 3-3C and 3-3D) or bacteria (Figs. 3-3E and 3-3F and 3-5A–3-5F). In host combinations with the fastidious bacterium Xylella fastidiosa, growth, multiplication, and spread of bacteria in xylem vessels are slower and, instead of causing wilting and rapid death of the plant, a scorching of the margins of the leaves (Fig. 3-4D) and several other symptoms occur, but rarely does the plant die quickly. In all cases, however, in infected hosts the flow of water is reduced through reduction in the size or collapse of vessels due to infection, development of tyloses (Figs. 3-5C and 3-5E) in the vessels, release of large molecule compounds in the vessels as a result of cell wall breakdown by pathogenic enzymes (Figs. 3-5D and 3-5E), and reduced water tension in the vessels due to pathogen-induced alterations in foliar transpiration.

Effect on Transpiration

In plant diseases in which the pathogen infects the leaves, transpiration is usually increased. This is the result of destruction of at least part of the protection afforded the leaf by the cuticle, an increase in the permeability of leaf cells, and the dysfunction of stomata. In diseases such as rusts, in which numerous pustules form and break up the epidermis (Figs. 3-6A and 3-6B), in most leaf spots (Fig. 3-6E), in which the cuticle, epidermis, and all the other tissues, including xylem, may be destroyed in the infected areas, in the powdery mildews, in which a large proportion of the epidermal cells are invaded by the fungus (Fig. 3-6C), and in apple scab (Fig. 3-6D), in which the fungus grows between the cuticle and the epidermis—in all these examples, the destruction of a considerable portion of the cuticle and epidermis results in an uncontrolled loss of water from the affected areas. If water absorption and translocation cannot keep up with the excessive loss of water, loss of turgor and wilting of leaves follow. The suction forces of excessively transpiring leaves are increased abnormally and may lead to collapse or dysfunction of underlying vessels through the production of tyloses and gums.

FIGURE 3-6. Ways by which pathogens cause increased transpiration in infected plants. (A) The wheat leaf rust pathogen Puccinia recondita produces innumerable lesions (uredia) on wheat leaves and causes millions of breaks in the leaf epidermis through which transpiration goes on uncontrollably. (B) Uredospores breaking the epidermis and emerging from the surface of an infected leaf. (C) Grape berries infected with the powdery mildew fungus Uncinula necator, the mycelium of which penetrates and forms haustoria in almost every epidermal cell. (D) The apple scab fungus Venturia inaequalis grows between the cuticle and the epidermis, causing the cuticle to break in numerous places, allowing transpiration to occur. (E) Tomato leaves with numerous lesions caused by the fungus Septoria sp. and through which excessive transpiration occurs.

[Photographs courtesy of (A and E) W.C.P.D., (B) E. A. Richardson and C. W. Mims, University of Georgia, (C) J. Travis and J. Rytter, Plant Pathology Department, Pennsylvania State University, and (D) K. Mohan, University of Idaho.]

Interference with Translocation of Organic Nutrients through the Phloem

Organic nutrients produced in leaf cells through photosynthesis move through plasmodesmata into adjoining phloem elements. From there they move down the phloem sieve tubes (Fig. 3-7) and eventually, again through plasmodesmata, into the protoplasm of living nonphotosynthetic cells, where they are utilized, or into storage organs, where they are stored. Thus, in both cases, the nutrients are removed from “circulation.” Plant pathogens may interfere with the movement of organic nutrients from the leaf cells to the phloem, with their translocation through the phloem elements, or, possibly, with their movement from the phloem into the cells that will utilize them.

FIGURE 3-7. Necrosis of the phloem (P) in stems or petioles of plants is a common effect of viruses, such as the tobacco ringspot virus, on cowpea plants. As a result, roots starve and the plant declines (100×). Pa, parenchyma cells; X, xylem vessels.

Obligate fungal parasites, such as rust and mildew fungi, cause an accumulation of photosynthetic products, as well as inorganic nutrients, in the areas invaded by the pathogen. In these diseases, the infected areas are characterized by reduced photosynthesis and increased respiration. However, the synthesis of starch and other compounds, as well as dry weight, is increased temporarily in the infected areas, indicating translocation of organic nutrients from uninfected areas of the leaves or from healthy leaves toward the infected areas.

In stem diseases of woody plants in which cankers develop (Figs. 3-8A–3-8C), the pathogen attacks and remains confined to the bark for a considerable time. During that time the pathogen attacks and may destroy the phloem elements in that area, thereby interfering with the downward translocation of nutrients. In diseases caused by phytoplasmas, as well as in diseases caused by phloem-limited fastidious bacteria, bacteria exist and reproduce in the phloem sieve tubes (Fig. 3-8D), thereby interfering with the downward translocation of nutrients. In several plants propagated by grafting a variety scion onto a rootstock, infection of the combination with a virus (e.g., infection of an apple or stone-fruit rootstock with tomato ringspot virus) leads to formation of a necrotic plate at the points of contact of the hypersensitive scion variety with the rootstock (Fig. 3-8E), which leads to the death of the scion. However, infection of a pear scion grafted on an oriental rootstock with the pear decline phytoplasma, or of a citrus variety propagated on sour rootstock with the citrus tristeza virus, results, in both cases, in the necrosis of a few layers of cells of each rootstock in contact with the tolerant variety. In these cases, the rootstock is the component of the scion/stock combination that is hypersensitive to and becomes killed by the appropriate pathogen.

FIGURE 3-8. Examples of diseases in which the pathogen interferes with the downward translocation of organic nutrients. (A) Young canker caused by the fungus Nectria in which the bark of the branch has been invaded and killed by the fungus. (B) Two advanced Nectria cankers in which both the phloem and a great deal of the xylem have been killed by the fungus. (C) Blister canker on a pine tree in which the bark and phloem have been killed by the fungus Cronartium ribicola. (D) Phytoplasmas filling a phloem sieve element block the downward translocation of photosynthates. (E) The graft union of a pear grafted on oriental pear rootstocks, which results in the death of pear phloem. (F) Potato tuber showing vein necrosis caused by the potato leaf roll virus.

[Photographs courtesy of (A) USDA Forest Service, (B) A. Jones, Plant Pathology Department, Michigan State University, (C) Oregon State University, and (F) Cornell University.

In some virus diseases, particularly the leaf-curling type and some yellows diseases, starch accumulation in the leaves is mainly the result of degeneration (necrosis) of the phloem of infected plants (Fig. 3-8F), which is one of the first symptoms. It is also possible, however, at least in some virus diseases, that the interference with translocation of starch stems from inhibition by the virus of the enzymes that break down starch into smaller, translocatable molecules. This is suggested by the observation that in some mosaic diseases, in which there is no phloem necrosis, infected, discolored areas of leaves contain less starch than “healthy,” greener areas at the end of the day, a period favorable for photosynthesis, but the same leaf areas contain more starch than the “healthy” areas after a period in the dark, which favors starch hydrolysis and translocation. This suggests not only that virus-infected areas synthesize less starch than healthy ones, but also that starch is not degraded and translocated easily from virus-infected areas, although no damage to the phloem is present.

Central Problem

The central problem for the lower trophic level of ocean ecosystems is obtaining light (energy) and inorganic nutrients (mass). Odum's definition requires that for a functioning system to be a distinct ecosystem it must possess characteristic trophic structure and material cycles. That is, how one kind of ocean ecosystem captures light and passes that energy on in the form of primary productivity, secondary productivity, and so forth is different from how another kind of ocean ecosystem processes and transfers its energy. Likewise, how mass (C, N, P, and Si), initially in the form of inorganic compounds, is taken up and transferred through the food web and eventually released back to the environment is different and very poorly understood.

What controls the supply of light and nutrients to an ocean ecosystem? Sverdrup in 1955 was the first oceanographer to note that the spatial and temporal patterns of physical processes, particularly the seasonal patterns of mixing, stratification, and upwelling as well as the seasonal pattern of irradiance, control the patterns of biological organization. The division of ocean ecosystems into six distinct types is based fundamentally on the pioneering work of Sverdrup and that of others in the intervening years.

Introduction and aim of the chapter

Most domestic, industrial and agricultural activities generate wastewater streams which, depending on the specific activity, contain varying concentrations of organic matter, nutrients, inorganic (e.g., heavy metals) and organic (e.g., pesticides, pharmaceutical residues) micropollutants and pathogenic microorganisms. According to estimations from the United Nations (UN Water, 2017), more than 80% of all wastewater generated globally is discharged to the environment without adequate treatment. This threatens not only ecosystem health through phenomena such as oxygen depletion, eutrophication, ecotoxicity, but also threatens human health of those people depending on the water resources where the wastewaters are discharged. In addition, valuable and non-renewable resources such as phosphorus and metals are in this way lost to the environment.

In a reaction to these problems, wastewater treatment technologies were developed from the late 1800s onwards (Lofrano and Brown, 2010). While originally focused on improving hygienic conditions, through time the focus shifted to environmental protection and in recent years to resource recovery, each time posing more stringent demands on effluent quality. Many of the commonly used wastewater treatment technologies are, however, very resource intensive, both during the building phase (construction materials) and during their operational life (energy for pumping and aeration, sludge conditioning chemicals) (Garfí et al., 2017). In the 1960s, German researchers developed a new, resource-extensive technology which was entirely based on the self-purification capacity of nature: constructed wetlands (CWs). CWs are defined as manmade wetland ecosystems, engineered in such a way that they make optimal use of the biological, physical and chemical processes offered by wetland vegetation, microorganisms and soil to purify water. An alternative term is treatment wetlands, to distinguish them from other “constructed” (i.e., artificial) wetlands whose primary purpose is nature restoration rather than water purification.

Apart from wastewater treatment, CWs potentially also offer other ecosystem and circular economy services such as biomass production, wildlife habitat provision, evaporative cooling and even recreational possibilities (Masi et al., 2018). In this way, CWs are a prime example of ecological engineering, a term coined in 1962 by the American ecologist Howard T. Odum, and defined as “design, construction and management of ecosystems that have value to both humans and the environment”. Likewise, CWs can be considered as green infrastructure under the umbrella concept of nature-based solutions, which are equally defined as “working with nature to address societal challenges, providing benefits for both human well-being and biodiversity” (Nature-based Solutions Initiative, n.d.).

From their initial conceptualization for domestic wastewater treatment, the use of CWs has been expanding rapidly, and CWs are now being used across the globe, in every possible type of climate (desert to arctic, e.g., Stefanakis et al. (2018) and Yates et al. (2012)), for a wide range of wastewater types, i.e., domestic, urban, industrial and agricultural, and at scales varying from a few m2 to several km2. This expansion is chiefly due to the positive balance between investment and operation costs on the one hand, which are typically lower than other wastewater treatment technologies, and the obtained treatment efficiencies on the other hand, which are competitive with other biological treatment technologies, though obviously cannot reach similar quality levels as energy-intensive systems like membrane filtration. The major drawback of CWs—when ignoring other ecosystem services apart from wastewater treatment—is the rather large area they occupy compared with conventional industrial plants. For this reason, CWs are mainly applied in rural and semi-urban areas, although recent developments (see section “Alternative lay-outs”) have triggered urban applications as well.

This chapter gives an overview on the use of constructed wetlands for urban wastewater treatment. Urban wastewater consists at least of domestic wastewater, in many cases mixed with (possibly already partially treated) industrial wastewater and/or stormwater runoff (Fig. 1). To this end, the different types of constructed wetlands and their characteristics will be summarized, the most important treatment processes occurring in CWs will be discussed, and the essential criteria for choosing, designing and operating CWs will be listed.