Sunday, September 06, 2015

Eat Less Meat to Save Ourselves.

 A report has been released by the U.N., in which it is urged that we reduce consumption of meat and dairy products as a means to mitigate climate change, hunger and fuel poverty  It is stressed that food, transportation and housing must be made more sustainable if we seriously intend to ameliorate biodiversity loss and climate change, and as a matter of urgency. Some 30% of global CO2 emissions is a result of internationally traded goods, while the mining sector uses 7% of the world's energy: a fraction that is expected to increase in line with "growth", which has serious connotations regarding international policy. A doubling of income is predicted to cause an 81% increase in CO2 emissions, which is an alarming prospect in the context of the rising population, predicted to be over 9 billion by 2050. 70% of all the world's freshwater consumption is taken by agriculture, which also accounts for 38% of the total use of land, and 14% of global greenhouse gas emissions. These figures may in fact be optimistically low, since another study reports that some 85% of freshwater is used for agriculture, which either directly or indirectly, is responsible for half of all greenhouse gas emissions. It has been estimated that it will be necessary to increase food production by 70% in 2050 if the population of the world is to be fed, but its expected increase from 7.3 billion now to perhaps 9.6 billion in 2050 will overwhelm any efficiency gains in agriculture. The production of animal products is particularly demanding in terms of land for grazing animals, and water, and a rising global middle class which is increasingly meat-hungry.

The above 70% increase in food production assumes that the western diet will spread to the Global South, with no reduction in consumption by the northern nations. 30-40% of cereals are presently fed to animals, which could rise to 50% if levels of meat and dairy consumption increase as predicted. It has been reckoned that 3.5 billion additional people could be fed if all cereals were given over for human consumption. Some 400 million tonnes of cereals could be made available for human diets, if meat and dairy were restricted to the levels being consumed in the year 2000, and this is sufficient to feed an extra 1.2 billion people. Moreover, it would be a better option to take meat into our diet from animals not fed on cereals, but grazed on grass, which is not a viable food source for humans. This would also reduce greenhouse gas emissions, especially of nitrous oxide, primarily from nitrogen fertilizers: 74% of N2O emissions in the U.S. are from agricultural soils.

It has been concluded that it is possible to provide a healthy and sustainable diet for 9 billion people in 2050, giving each person a daily 3,000 kcal, of which 500 kcal is from animals. This would require less meat and dairy being eaten by wealthier consumers, while allowing those in Asia and sub-Saharan Africa more such protein sources. Food waste is another critical issue, since in the U.K. we import 40% of the food we eat, and yet half of what is made available to us is discarded. Typically 280-380 kg of food goes to waste in the U.S. and Europe, but a more modest 125-165 kg in poorer parts of the Global South.  It is thought that global demand for food could be cut by 25% through measures to curb food waste, while across the world, more varied and healthful diets could be provided.

The soil itself has been described as being our "silent ally" in the "2015 International Year of Soils" programme, launched by the United Nations Food and Agriculture Organization (FAO), which emphasises that healthy soil is essential and underpinning in providing food, fuel fibre, and even medicine. As the FAO stresses: 'soils are also essential to our ecosystems, playing a key role in the carbon cycle, storing and filtering water, and improving resilience to floods and droughts, and yet we are not paying enough attention to this important “silent ally”.' In regard to climate change, soils provide the largest pool of organic carbon, and hence it is vital not to degrade soil organic matter through unsustainable agricultural practices; soils are furthermore essential in the storage and distribution of water, meaning that degradation of the soils can only exacerbate the problems of heavily water stressed regions such as the Middle East.

It is reckoned that some 33% of all soil resources are degraded, and that "Unless new approaches are adopted, the global amount of arable and productive land per person will in 2050 be only one-fourth of the level in 1960." This level of degradation is a combined result of erosion, compaction, soil sealing, salinization, depletion of soil organic matter and nutrients, acidification, and pollution, which are mainly caused by land management practices that are patently non-maintainable.

Soil degradation also threatens biodiversity, since the soil food web contains perhaps one quarter of all the biodiversity on Earth - within the earth, which covers its surface as a "fragile, living skin". The living organisms that make up the soil food web, including earthworms, "the tractors of the soil"; bacteria, nematodes and other microbes; the plant roots, and their associated mycorrhizal fungi, are the vanguard for the recycling of nutrients, and enhance the growth of plants through their greater absorption of nutrients. By supporting this hidden biodiversity below the ground, the more visible biodiversity above the ground is further buttressed. If we avoid treating the soil "like dirt", we may nurture the essential organisms that are critical in the ability of soil to absorb carbon and water, and attenuate the acceleration of climate change.

Friday, September 04, 2015

Cleaning the Earth Nature's Way - Phytoremediation.

Phytoremediation1,2 may be defined as the treatment of environmental problems by using plants in situ so to avoid the need to excavate the contaminant material for disposal elsewhere. It can be applied to the amelioration of contaminated soils, water, or air, using plants that can contain, degrade, or eliminate metals, pesticides, solvents, explosives, crude oil and its derivatives (refined fuels), and related contaminating materials. Phytoremediation has been used successfully for the restoration of abandoned metal-mine workings, and cleaning up sites where polychlorinated biphenyls have been dumped during manufacture, and for the mitigation of on-going coal mine discharges. Phytoremediation uses the natural ability of particular plants (“hyperaccumulators”, described below) to bioaccumulate, degrade, or otherwise reduce the environmental impact of contaminants in soils, water, or air. Those contaminants that have been successfully mitigated in phytoremediation projects worldwide are metals, pesticides, solvents, explosives, and crude oil and its derivatives, and the technology has become increasingly popular and has been employed at sites with soils contaminated with lead, uranium, and arsenic. A major disadvantage of phytoremediation is that it takes a relatively long time to achieve, because the process rests upon the ability of a plant to thrive in an environment that is not normally ideal for plants.

Advantages and limitations of phytoremediation.
  • Advantages:
    • In terms of cost, phytoremediation is lower than that of traditional processes both in situ and ex situ.
    • The plants can be easily monitored.
    • There is the possibility of the recovery and re-use of valuable metals (by companies specializing in “phyto-mining”).
    • It is potentially the least harmful method because it uses naturally occurring organisms and preserves the environment in a more natural state.
    • Trees may be used in phytoremediation, since they grow on land of marginal quality, have long life-spans and a high flood tolerance. Willows and poplars are most commonly used, and can grow 6-8 feet (ca 2 metres) per year. For deep contamination, hybrid poplars with roots extending 30 feet deep have been used, which penetrate microscopically sized pores in the soil matrix and each tree can cycle 100 L of water per day, functioning almost as a solar powered and self-contained pump and treatment system.
    • Phytoscreening is possible, in which plants may be used as biosensors for particular types of contaminants, thus giving a signal of underlying contaminant plumes, e.g. trichloroethene has been detected in the trunks of trees.
    • Genetic engineering may confer improvements to phytoremediation, e.g. genes encoding a nitroreductase from a bacterium, when inserted into tobacco, increased the resistance of the plant to the toxic effects of TNT and the uptake of the material. Plants may be genetically modified to grow in soils even when the pollution levels in the soil are lethal for non-treated plants, and to absorb a greater concentration of the contaminant.
  • Limitations:
    • Phytoremediation is limited to the surface area and depth occupied by the plant roots.
    • Slow growth and low biomass require a long-term commitment.
    • Using plants, it is not possible to prevent entirely the leaching of contaminants into the groundwater (without the complete removal of the contaminated ground, which in itself does not resolve the problem of contamination).
    • The survival of the plants is affected by the toxicity of the contaminated land and the general condition of the soil.
    • Bio-accumulation of contaminants, especially metals, into the plants which then pass into the food chain, from primary level consumers upwards, or requires the safe disposal of the affected plant material, i.e. the plants might be eaten by animals.
    • The procedure is slow.
Hyperaccumulators and biotic interactions. 
If a plant is able to concentrate a particular contaminant, to a given minimum concentration (> 1000 mg/kg of dry weight for nickel, copper, cobalt, chromium or lead; or > 10,000 mg/kg for zinc or manganese), it is categorized as a hyperaccumulator. This capacity for accumulation is a result of genetic adaptation over many generations in hostile environments. Metal hyperaccumulation can affect various different factors, such as protection, interferences between different species of plants, mutualism (e.g. mycorrhizae, pollen and seed dispersal), commensalism, and biofilm.

Different possible phytoremediation methods.
Various processes that are mediated by plants or algae might be used to address environmental problems:
  • Phytoextraction — uptake and concentration of substances from the environment into the plant biomass.
  • Phytostabilization — reducing the mobility of substances in the environment, for example, by limiting the leaching of substances from the soil.
  • Phytotransformation — chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation), or immobilization (phytostabilization).
  • Phytostimulation — enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. This process is also known as rhizosphere degradation. Phytostimulation can also involve aquatic plants supporting active populations of microbial degraders, as in the stimulation of atrazine degradation by hornwort.
  • Phytovolatilization — removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and/or less polluting substances.
  • Rhizofiltration — filtering water through a mass of roots to remove toxic substances or excess nutrients. The pollutants remain absorbed in or adsorbed to the roots. 
In phytoextraction (or phytoaccumulation) plants or algae are used to extract contaminants from soils, sediments or water into harvestable plant biomass (those organisms that take larger-than-normal amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been used more often for extracting heavy metals than for organic contaminants. The plants absorb contaminants through the root system which they then contain in the root biomass and/or move them into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. The process can be repeated to affect further decontamination. There are two forms of phytoextraction:
  • Natural hyper-accumulation, where plants take up the contaminants in soil unassisted.
  • Induced (assisted) hyper-accumulation, in which a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily. In many cases natural hyperaccumulators are metallophyte plants that can tolerate and incorporate high levels of toxic metals.

Examples of phytoextraction:
  • Arsenic, using the Sunflower (Helianthus annuus), or the Chinese Brake fern (Pteris vittata), a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.
  • Cadmium, using willow (Salix viminalis): willow has a significant potential as a phytoextractor of cadmium (Cd), zinc (Zn), and copper (Cu), as willow has some specific characteristics like high transport capacity of heavy metals from root to shoot and huge amount of biomass production; can be used also for production of bioenergy in the biomass energy power plant.
  • Cadmium and zinc, using Alpine pennycress (Thlaspi caerulescens), a hyperaccumulator of these metals at levels that would be toxic to many plants, although its growth appears to be inhibited by copper.
In phytostabilization the intention is to stabilize, or contain the pollutant over the long-term. There may be a number of contributing factors to this, e.g. the reduction of wind (soil) erosion by the body of the plant, but the roots of the plant can resist water (soil) erosion, immobilize the pollutants by adsorption or accumulation, and provide a zone around the roots where the pollutant can be deposited in an immobilized form. In contrast with phytoextraction, phytostabilization aims mainly to sequester pollutants in soil around the roots but not in the plant tissues. Hence the pollutants are increasingly less bioavailable, such that exposure to livestock, wildlife, and humans is reduced. Mine tailings may be stabilized by growing a vegetative cap.

 Some plants, e.g. cannas, are able to detoxify organic pollutants - pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances  - by metabolising them. The metabolic functions of microorganisms living in association with plant roots may also metabolize these substances, as present in soil or water. Due to the complex and recalcitrant nature of many of these compounds, they cannot be broken down entirely (mineralised) to basic molecules (H2O, CO2, etc.) by plants and hence the term phytotransformation represents molecular alterations rather than the complete decomposition of the compound. Phytotransformation may be viewed1 as a "Green Liver" because plants behave analogously to the human liver in processing these xenobiotic compounds, introducing polar groups such as –OH to them. This is known as Phase I metabolism, similar to the way that the human liver increases the polarity of drugs and foreign compounds. In plants, it is enzymes such as nitroreductases which carry out these transformations, whereas in the human liver it is enzymes such as the Cytochrome P450s that perform the task. Phase II metabolism in the second step in phytotransformation, in which the polarity of the xenobiotic molecule is increased by combination with plant biomolecules such as glucose and amino-acids. This is called “conjugation”, and is once more similar to processes such as glucoronidation (addition of glucose) and glutathione addition reactions, catalysed by appropriate enzymes. The effect of the two metabolic steps may serve to detoxify the xenobiotic and aid its mobilization via aqueous channels. In Phase III metabolism, the xenobiotic becomes sequestered, by incorporation in a complex “lignin-type” structure, where it is kept apart from the normal functioning of the plant. The phytotransformation of trinitrotoluene (TNT) has been well studied, and a detailed mechanism proposed for it.

Phytostimulation and rhizoremediation. 
This term identifies the process where compounds released from plant roots enhance microbial activity in the rhizosphere, which is the narrow region of soil around the roots of plants, and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. In rhizoremediation, microorganisms degrade soil contaminants in the rhizosphere. It is usual that those soil pollutants which are remediated by this method are highly hydrophobic organic xenobiotics that are hence unable to enter the plant. Rather than the plant being a main protagonist in this process, it creates a haven in which microorganisms in the rhizosphere are able to perform the degradation. The plant acts as a solar-powered pump, which draws in both water and the xenobiotic agent, simultaneously producing substrates (e.g. root exudates and root turnover) that assist the growth of the microbes which act as pollutant degrading agents. Microbial activity is stimulated in the rhizosphere through a number of different routes: (i) exudates, e.g. sugars, carbohydrates, amino acids, acetates, and enzymes, nourish indigenous microbe populations; (ii) root systems bring oxygen into the rhizosphere, meaning that aerobic transformations are supported; (3) the available organic carbon is enhanced through the growth of fine-root biomass; (4) mycorrhizae fungi, which are an essential component of the rhizosphere, provide unique enzymatic pathways lending the capacity to degrade pollutant molecules that would not be degraded by bacteria alone; and (5) the presence of plants (and their roots) creates a domain for microbial populations, which are activated in the rhizosphere. There have been five enzyme systems identified in soils: (i) dehalogenase (which acts in dechlorination reactions of chlorinated hydrocarbons); (ii) nitroreductase (essential for the initial step of nitroaromatic degradation); (iii) peroxidase (a critical catalyst for oxidation reactions); (iv) laccase (able to begin the decomposition of otherwise robust aromatic ring structures); (v) nitrilase (another key factor in oxidation processes). The method is limited in that when there are high concentrations of pollutants present, the plants may be overwhelmed and die. The successful use of phytostimulation has been demonstrated in the remediation of chlorinated solvents from groundwater, petroleum hydrocarbons from soil and groundwater and PAHs from soil. 

Probably, this is the most controversial of the phytoremediation technologies, since it involves the release of contaminants either directly, or in a metabolically modified form, into the atmosphere. Phytovolatilization3 has been used principally for the removal of Hg2+ ions which are transformed into less toxic elemental mercury4. Tritium (3H), a radioactive isotope of hydrogen with a half-life of about 12 years, decaying to helium, has also been removed by phytovolatilization5. A good deal more research is necessary before this strategy becomes mainstream, since there are various negative features to be addressed. For example, mercury that is released into the atmosphere from plants is likely to be recycled by precipitation and thus returned the ecosystem, and the method is restricted both to sites where the concentration of contaminants is toward the low side, and where the contamination is no deeper than the roots of the plants being used. 

Rhizofiltration6 involves filtering contaminated water through a mass of roots for the extraction of contaminants, or excess nutrients, e.g. phosphorus. The contaminated water can either be collected from a waste site and taken to where plants are being hydroponically cultivated, or the plants may be planted in the area directly. In both cases, the roots draw up the water and its associated contaminants. This process is very similar to phytoextraction in that the contaminants become sequestered in the form of harvestable plant biomass. Then new plants are grown and harvested until a satisfactory degree of decontamination is achieved. It is the concentration and precipitation of heavy metals that is sought principally. While noting these similarities, the fundamental difference between the two approaches is that rhizofiltration is used in aquatic environments, while phytoextraction is applied to the decontamination of soils. There are limitations to rhizofiltration. As usual in phytoremediation methods, any contaminant that is below the rooting depth will not be extracted, and if the level of contamination is too high the plants will not grow. Depending on the type of plant and contaminant, the process may need to be continued over a protracted period, before regulatory levels are achieved. It is generally true that many different kinds of contaminants will be present – in some cases a mixture of organics and heavy metals – and thus the use of rhizofiltration alone is unlikely to succeed. Importantly, the plants chosen should be non-fodder crop to minimize poisoning animals, which might eat them in contaminated form.  That noted, the effective removal of heavy metal cations, e.g. Cu2+, Cd2+, Cr6+, Ni2+, Pb2+, and Zn2+ from aqueous solutions has been demonstrated7, and the removal of low-level radionuclides, from liquid streams8. In that latter application, a “feeder layer” of soil is suspended above the stream through which plants grow, from which the plant roots extend downward into the water. In this way, fertilizer can be used to help the plants to grow, while avoiding adding to the contamination of the stream, while the latter is cleansed of heavy metal cations9. Rhizofiltration is cost-effective when large volumes of water must be treated containing low concentrations of contaminants. Inclusive of the costs of the capital outlay and final waste disposal, the cost of removing radionuclides from water using sunflowers was reckoned (at 1996 prices) at $2─6 per thousand gallons of water treated10.

(1) Burken, J.G. (2004), "2. Uptake and Metabolism of Organic Compounds: Green-Liver Model", in McCutcheon, S.C.; Schnoor, J.L. (Eds.), Phytoremediation: Transformation and Control of Contaminants, A Wiley-Interscience Series of Texts and Monographs, Hoboken, NJ: John Wiley, p. 59, doi:10.1002/047127304X.ch2, ISBN 0-471-39435-1.
(5) Dushenkov, S. (2003) “Trends in phytoremediation or radionuclides.” Plant and Soil, 249, 167. (6)
(7) EPA, (1998) “A Citizen's Guide to Phytoremediation,.” U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response,” EPA 542-F-98-011, August. 
(8) Dushenkov, V., Motto, H., Raskin, I. and Nanda Kumar, P.B.A. (1995) "Rhizofiltration: the Use of Plants to Remove Heavy Metals From Aqueous Streams." Environmental Science Technology 30, 1239. 
(9) Raskin, I., Smith, R.D. and Salt, D.E. (1997) "Phytoremediation of Metals: Using Plants to Remove Pollutants from the Environment." Current Opinion in Biotechnology. 8, 221. 
(10) Cooney, C. M. (1996) "Sunflowers Remove Radionuclides From Water in Ongoing Phytoremediation Field Tests." Environmental Science and Technology 30, 194.