Chapter 30: Nuclear Techniques for Qualifying Source for Appointment, Tracing Carbon Dioxide, and Detecting Potent Toxins in the Ocean

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on July 7, 2012.

Extensive dedicated research of decades on the subject of climate change leading up to substantial scientific deductions about shrinking glaciers and detrimental desolation instigated by fierce storms seem to have a positive influence on most people around the world who accepted the possibilities of global climate change caused by rising levels of carbon dioxide (CO2).  This is based on the fact that CO2 is a greenhouse gas and the warmth of the climate is dictated by the magnitude of CO2 in the atmosphere.  Simply put – the more CO2 in the atmosphere, the warmer the climate becomes.  Here is a simplistic description of how the carbon cycle functions.

The basic cycles of nature is defined as the interaction among plants, animals, and soil.  Incidentally, in the carbon cycle, plants absorb CO2 from the atmosphere and use it, combined with water they get from the soil, to make the substances they need for growth.Slide1The process of photosynthesis incorporates the carbon atoms from CO2 into sugars. Animals, such as the rabbit pictured in the figure 30-01, eat the plants and use the carbon to build their own tissues. Other animals, such as the fox, eat the rabbit and then use the carbon for their own needs. These animals return carbon dioxide into the air when they breathe, and when they die, since the carbon is returned to the soil during decomposition. The carbon atoms in soil may then be used in a new plant or small microorganisms. Ultimately, the same carbon atom can move through many organisms and even end in the same place where it began. Herein lies the fascination of the carbon cycle; the same atoms can be recycled for millennia.

The ocean plays a vital dominant role in the Earth’s carbon cycle. The total amount of carbon in the ocean is about 50 times greater than the amount in the atmosphere, and is exchanged with the atmosphere on a time-scale of several hundred years. At least 1/2 of the oxygen we breathe comes from the photosynthesis of marine plants. Currently, 48 percent of the carbon emitted to the atmosphere by fossil fuel burning is sequestered into the ocean. But the future fate of this important carbon sink is quite uncertain because of potential climate change impacts on ocean circulation, biogeochemical cycling, and ecosystem dynamics.

Carbon atoms are constantly being cycled through the earth’s ocean by a number of physical and biological processes. The flux of carbon dioxide between the atmosphere and the ocean is a function of surface mixing (related to wind speed) and the difference the concentration of carbon dioxide in the air and water. The concentration in the ocean depends on the atmosphere and ocean carbon dioxide partial pressure which, in turn, is a function of temperature, alkalinity (which is closely related to salinity), photosynthesis, and respiration. Carbon is also sequestered for long periods of time in carbon reservoirs (sinks) such as deep-ocean and ocean sediment.Slide2Prior to the Industrial Revolution, the annual uptake and release of CO2 by the land and the ocean had been on average just about balanced. In more recent history, atmospheric concentrations have increased by 80 ppm (parts per million) over the past 150 years. However, only about half of the carbon released through fossil fuel combustion in this time has remained in the atmosphere, the rest being sequestered the ocean. Global observations of the spatial and temporal patterns of carbon exchange and understanding the underlying processes that regulate this exchange is critical for predicting the future behavior of these carbon sinks.

The graph presented under figure 30-02 illustrates the annual flow of carbon between the atmosphere, vegetation and soils.

1.       LONG-LIVED RADIONUCLIDES TO UNDERSTAND ENVIRONMENTAL PROCESSES:

Radionuclides provide tools to investigate ocean resources, oceanographic processes and marine contamination on a quantitative basis and at the same time can help in addressing coastal zone management problems. Given that radionuclides contain a “clock” owing to their decay over time, they can be used to study temporal bio-geochemical processes in the marine environment.

Long-lived radionuclides, due to their high variability in nature and different physical and chemical properties, have been used to study biogeochemical processes (e.g. migration, oceanographic or sedimentation investigations). The radioactive characteristics of these nuclides and the variation of mother-to-progenies ratios as a function of time are used in dating measurements (e.g. carbon-14 or uranium-lead dating) and for the investigation of time dependent natural processes, such as migration or sedimentation studies.

Such long-lived radionuclides can serve as natural and man-made tracers and radioactive clocks in the environment, which allow researchers to date and study large scale environmental processes, as well as obtain information otherwise not accessible. Furthermore, with uncertainty surrounding future climatic scenarios and potential environmental responses, research is increasingly turning to radionuclide-based tracers and dating methodologies for improving the understanding of environmental processes and changes in marine, freshwater and terrestrial environments.

During the past two decades, owing to the rapid development of inorganic mass spectrometric instrumentation, the use of inductively coupled plasma mass spectrometers (ICP–MSs), especially inductively coupled plasma sector field mass spectrometers (ICP–SFMSs) equipped with double focusing sector field analyzers, has become a complementary and alternative tool to traditional radio-analytical methods (e.g. alpha spectrometry and liquid scintillation) for the analysis of long-lived radionuclides. Mass spectrometric techniques dominate the field of isotopic analysis because they are less time consuming, can have a lower detection limit, and are very precise and accurate. ICP-MS is sometimes the only technique capable of determining an isotopic “fingerprint”, especially for minor isotopes of an element.Slide3

Sources of environmental contamination can be identified by an isotopic abundance and/or an isotopic ratio analysis which serves as a kind of “fingerprint” of the contamination. Chemicals produced from distinct sources by essentially different processes are expected to exhibit specific isotopic compositions that can be used to identify sources.  Once the different sources (e.g. anthropogenic or natural geogenic) are identified, the isotopic abundances and the isotopic ratios can be used to quantify source apportionment. Isotopic signatures are the basis for the investigation of historical and environmental changes of the selected sampling sites.

2.       NUCLEAR TECHNIQUES FOR ASSESSING PARTICLES FLUXES FROM OCEAN TO SEAFLOOR:

Nuclear techniques are being deployed in various parts of the world to help control the growing CO2 in the atmosphere as well as in ocean.  More specifically, the ocean is a major carbon sink and the trapping of increasing quantities of CO2 is provoking its acidification.  “Sinking particles” are the ultimate removal mechanism of carbon and other elements as well as contaminants from the upper ocean. This includes atmospheric carbon, which is converted from CO2 to biomass and sequestered to deep-water via particle sinking, contaminants and radioactive elements. By analyzing suspended particulate matter from various ocean depths, various factors controlling the transfer of carbon from the surface to the deep ocean can be assessed.  The natural radionuclide thorium-234 (234Th) has increasingly been used over the past years to quantify particle fluxes and carbon export from the upper ocean in both open-ocean and coastal environments.  234Th is a particle reactive isotope that is produced in seawater by radioactive decay of its dissolved conservative parent uranium-238 (238U). The disequilibrium between 238U and the measured total 234Th activity reflects the net rate of particle export from the surface ocean on time scales of days to weeks.

234Th is used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean.

Since the first reported measurements of 234Th in the ocean, decreases in 234Th activities between open-ocean and coastal waters indicated that 234Th activity distributions were strongly influenced by the marine particle cycle. This prompted the use of 234Th as a new in situ tracer of oceanic “Scavenging”, a term used to describe the complex processes related to the association of particle reactive elements with particle surfaces and their removal in the oceans. Using simple activity balance equations, the ratio of the particle reactive radionuclide 234Th (Half-life =24.1 days) to its soluble and long lived parent, 238U, can be used to quantify the rate of uptake of 234Th onto particles and their export flux out of the surface ocean. A larger disequilibrium in total activities (234Th: 238U activity ratios > 1) reflects a higher export flux.Slide4There has been considerable expansion in the use of 234Th and its ratio to particulate organic carbon (POC) as a C flux tracer since the first measurement of 234Th in the ocean 35 years ago.  Single 234Th activity profiles coupled with a few filters for POC and 234Th have been superseded by programs with hundreds of 234Th measurements per cruise and more sophisticated methods for C/ 234Th.  Arguably one of the largest uncertainties in the application of this approach is the correct determination of C/ 234Th on particles that are representative of the sinking flux.  Differences in C/ 234Th are methods-related, as well as varying regionally, temporally and with depth, making it difficult to compare studies. No single process or model can account for the wide range in C/ 234Th observations, and multiple processes are likely to play a role. Nonetheless, this does not mean the 234Th approach cannot be used as an empirical flux tool.  The 234Th flux approach would fail if we are missing a component of the export flux that has both different C/234Th, and on a flux weighted basis, accounts for a significant fraction of POC export. Many of the data thus far, suggest little difference in C/ 234Th between particles collected via pumps or traps or by sinking velocity.

Since the local plankton community influences both particle surface properties and solution ligand characteristics, regional and depth dependent fractionation of C/ 234Th is the norm. Using single sampling methods regional C/ 234Th differences and generally a constant or increasing C/ 234Th with increasing particle size can be seen. Furthermore, decreasing C/ 234Th with depth also can be seen. Thus, choosing C/ 234Th at depths corresponding to the depth of export is critical. Moreover, C/Th variability is in many cases smaller at depth, thus reducing uncertainties when applying this method.

The technique was recently applied in an international project in the coastal Arctic Sea for assessing the impact of permafrost melting due to climate warming, and the consequent increase in organic material outflow through rivers from the coast to open waters (Figure: 30-04).  The graph illustrates the development of an n-situ large pump to collect particulate material used to measure radionuclides in Artic waters.

3.       EARLY WARNING OF MARINE HARMFUL ALGAL BLOOMS:

It is recognized that algae are an important source of nutrition for oceanic life, placed at the base of the food cycle. The existence of the Red Tides which is known as Harmful Algal Blooms (HABs) in coastal areas disrupts the marine food-web, causing the intoxication, stranding and death of many marine mammals, birds and turtles.Slide5HABs pose a serious and recurrent threat to marine ecosystems, fisheries, human health, and coastal aesthetics worldwide. These phenomena are caused by growth and accumulation of microscopic algae, some of which produce potent toxins. The significant public health, economic, and ecosystem impacts of HABs suggest that these phenomena would be legitimate targets for direct control or mitigation efforts.

Outbreaks of algal blooms result from an increase in nutrient levels in the water, induced by coastal upwelling and agricultural runoffs.  Toxins produced by HABs are collect in shellfish which results in contaminated seafood entering the food chain and affecting not only on the marine ecosystem, but also human health. HABs are also a major threat to fishermen’s livelihoods: the red tides can cause massive fish kills and therefore, significantly affect local and commercial fisheries. In addition, the closure of fishing grounds due to HAB events creates considerable financial hardship for fishermen.

Although HABs can sometimes look like a massive, coloured patch of water approaching the shore, most of the time it is rather difficult to see the blooms with the naked eye. This increases the risk of contaminated sea products entering the human food chain.

Nuclear technology can be used to identify HABs early, and to pinpoint outbreaks with more accuracy. This protects the food chain, and helps to limit the amount of time that fishing grounds must be closed. Through its technical cooperation (TC) programme, the International Atomic Energy Agency (IAEA) is helping countries use nuclear technology to identify HABs events, and to limit their impact.Slide6Several TC projects are focusing on the development of early warning systems for HABs, with the goal of minimizing the damage they cause. Nuclear techniques such as receptor-binding assay (RBA), used in combination with other methods, allow the concentration of toxins in seawater and in marine products to be measured quickly and efficiently. In addition, the nuclear approach provides a faster and more precise detection of toxins than the conventional mouse bioassay method. This faster, more accurate procedure helps to prevent contaminated products from entering the local food market and reduces the risk of intoxication through the ingestion of contaminated seafood.

Here is some background on RBA.  Toxins exert their devastating impacts in a variety of ways, but most commonly they interfere with the transmission of nerve impulses. The activation of a nerve impulse is an electrical phenomenon in which a series of connected nerve fibers are sequentially polarized and depolarized. Actually these cells are not “connected” in the usual meaning of the word, but are “connected” through a space or junction called the “synaptic cleft”. A nerve impulse is passed from one nerve cell to the other across the cleft by a “neurotransmitter”. Usually, this is a small, low molecular weight chemical, such as acetylcholine or epinephrine (adrenaline). A number of chemicals have been identified as neurotransmitters in the past 20 years.

As far as the receptors are concerned, propagation of nerve impulses can be a rather complicated process and there are plenty of opportunities for things to go wrong. In order for cells not to depolarize all the time, the receptor site must accept only very specific chemicals. By and large, the receptor site has a structure that is fairly specific for its designated neurotransmitter. Nerve Toxins are harmful in a number of ways, among them:

  • They can mimic neurotransmitters (they have chemical structures very similar to the correct neurotransmitter);
  • They interfere with or block the release of the ions through the cell; and
  • They can block the action of a neurotransmitter at the receptor– they don’t let the neurotransmitter attach allowing the cell to depolarize.

With regards to assay, very early on, researchers took advantage of toxin binding to nerves as the basis of assays for the toxins and/or neurotransmitters. However, these usually required very complex setups and the isolation of nerve receptors from a variety of animals (e.g., the giant squid axon). Needless to say these procedures were time consuming, very expensive, and required highly trained and skilled people. While they were used widely, and still are to some extent today, by medical and pharmacological researchers, recent advances in molecular biology have permitted much simpler methods. These new methods can be faster, cheaper, far more sensitive, and most importantly use very small amounts of toxic extracts. It is now possible to clone receptors from one species and have them reproduced in cells of another species. It is this method that we use to produce receptors used in our receptor binding assay for domoic acid determination.

Receptor binding assays measure the binding of a toxin to its nerve cell receptor by either marking the toxin or a suitable derivative of the toxin. This is most commonly done now using radioactive markers. In the procedure, the radioactive toxin will be displaced or “bumped off” its receptor by toxin present in an unknown sample, thereby reducing the total radioactivity. The amount of radioactively labelled toxin that is displaced is proportional to the amount of toxin in the unknown sample. The toxin present in an unknown sample can then be quantified by comparison to a standard curve obtained using pure toxin. The advantage of this technique is that it can be made highly specific and sensitive for a particular toxic activity. Currently, receptor binding assays have been developed for domoic acid and PSP toxins. Below is a general description of the receptor binding assay.

Resources:

  1. Oracle – What is the carbon cycle;
  2. NASA – Carbon Cycle;
  3. Nuclear Technology Review 2011;
  4. Nuclear Technology Review 2010;
  5. Marine Chemistry – An Assessment of Particulate Organic Carbon;
  6. IAEA Nuclear Techniques help to detect harmful algal blooms; and
  7. Harmful Algal Blooms – Receptor Binding Assay.

 

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