Info

FIGURE 9.5 An example of self-assembly in nanotechnology, which occurs in step E of the diagram. Led: Light emitting diode. (From Gracias, D. H., J. Tien, T. L. Breen, C. Hsu, and G. M. Whitesides. 2000. Science 289:1170. With permission.)

components self-organize into ecosystems that provide some service to humans. In nanotechnology, molecular self-assembly is used to create desired products and functions (Rietman, 2001; Service, 1995; Whitesides, 1995; Whitesides et al., 1991). Chemical molecules and their environments are manipulated to facilitate the self-organization of devices in this form of engineering (Figure 9.5). Perhaps in the future, engineers from these widely different scales may be able to share ideas about self-organization as an engineering design approach.

Terraforming and Global Engineering

The largest scale of ecological engineering is terraforming, which is the modification of a planetary surface so that it can support life (Fogg, 1995). While this application is still in the realm of science fiction, it is receiving credible attention. Some interesting theory about biosphere-scale ecological engineering is being discussed, especially in terms of Mars (Allaby and Lovelock, 1984; Haynes and McKay, 1991; McKay, 1999; Thomas, 1995). Mars has a thin atmosphere and probably has water frozen in various locations. The principal factor limiting life seems to be low temperature. One idea to terraform Mars is to melt the polar ice cap in order to initiate a greenhouse effect that would raise temperature (Figure 9.6). Then, living populations would be added, perhaps starting with microbial mats from cold, dry regions of the earth that might be preadapted to the Martian surface. The mats are dark-colored and would facilitate planetary warming by lowering the albedo and absorbing solar radiation. These actions are envisioned to set up climate control, as described by the Gaia hypothesis on earth (Margulis and Lovelock, 1989). Arthur C. Clarke (1994), the famous science fiction author, has extended the theory with many imaginative views of the stages of succession involved in terraforming Mars.

While actual terraforming may not be expected to be possible for hundreds of years in the future, some practical applications are being debated for engineering at this scale on the earth. There is much interest in understanding feedbacks between the biota and climate systems (see, for example, Woodwell and MacKenzie, 1995). Some applied planetary engineering has been suggested to deal with the present climate change in the form of tree plantings to absorb and sequester carbon dioxide (Booth, 1988), though these calculations are not promising as a long-term solution to the greenhouse effect (Vitousek, 1991). A more uncertain plan is ocean fertilization with iron as a planetary scale CO2 mitigation plan. John Martin (1992) first suggested the "iron hypothesis" to explain limitation of open ocean primary productivity based on small-scale bottle experiments. He later proposed that large-scale iron fertilization could generate a significant sink for global CO2 and boldly stated, "give me a half a tanker of iron and I will give you the next ice age" (Dopyera, 1996)! Since his proposal (and his untimely death), two large-scale experiments (Transient Iron Addition Experiment I and II or IRONEX I and II) in the southern Pacific Ocean have basically confirmed Martin's hypothesis. Proposals about commercial iron fertilization for CO2 mitigation are currently being debated (Chisholm et al., 2001; Johnson and Karl, 2002; Lawrence, 2002).

From Biosensors to Ecosensors

Biosensors are a growing form of technology becoming widely used in medical applications (Schultz, 1991). As noted by Higgins (1988)

a biosensor is an analytical device in which a biological material, capable of specific chemical recognition, is in intimate contact with a physico-chemical transducer to give an electrical signal.

FIGURE 9.6 Hypothetical sequence of events caused during terraforming on Mars, initiated by volatilization of the northern polar ice cap. (Adapted from Wharton, R. A., Jr., D. T. Smeroff, and M. M. Averner. 1988. Algae and Human Affairs. C. A. Lembi and J. R. Waaland (eds.). Cambridge University Press, Cambridge, U.K.)

FIGURE 9.6 Hypothetical sequence of events caused during terraforming on Mars, initiated by volatilization of the northern polar ice cap. (Adapted from Wharton, R. A., Jr., D. T. Smeroff, and M. M. Averner. 1988. Algae and Human Affairs. C. A. Lembi and J. R. Waaland (eds.). Cambridge University Press, Cambridge, U.K.)

FIGURE 9.7 An example of a system for toxicity assessment with continuous monitoring sensors. (Adapted from American Society for Testing and Materials. 1996. Annual Book of ASTM Standards. American Society for Testing and Materials, West Conshohocken, PA.)

Biological materials offer unique capabilities in specificity, affinity, catalytic conversion, and selective transport, which make them attractive alternatives to chemical methods of sensing. This is an interesting area that involves the interfacing of biology with electronics. The three basic components of a biosensor are (1) a biological receptor, (2) a transducer, such as an optical fiber or electrode, and (3) associated signal processing electronics. Environmental applications of biosensors have focused on continuous monitoring for water quality evaluation (Grubber and Diamond, 1988; Harris et al., 1998; Rawson, 1993; Riedel, 1998). An example employing respiratory behavioral toxicity testing with fish (American Society for Testing and Materials, 1996) is shown in Figure 9.7. In this case gill movements are sensed with electrodes placed in the fish tank and related to pollutant concentrations in the water. The system can predict toxicity of a water stream with associated interfacing. In the future, biosensors may be able to be scaled up to ecosensors by ecological engineers. As noted by Cairns and Orvos (1989), most environmental uses of biosensors rely on single-species indicators of pollution stress that may not be adequate for all purposes. Ecosensors could be devised that utilize information on multispecies community composition or on ecosystem metabolism, as mentioned in the next section on technoecosystems.

Technoecosystems

H. T. Odum (1983) defined technoecosystems as "homeostatically coupled" hybrids of living ecosystems and hardware from technological systems. This is a vision of a living machine but with added control. The simplest version would be the turbi-dostat (Myers and Clark, 1944; Novick, 1955) which is a continuous culture device for studying suspended populations of algae or bacteria. In this device, turbidity of the suspension is proportional to density of the microbial population. A photocell senses turbidity and is connected to a circuit that controls a valve to a culture media reservoir. If the turbidity is higher than a given threshold, then the circuit remains off, leaving the valve to the reservoir closed. However, if the turbidity is lower than the threshold, then the circuit opens the valve which adds culture media to the suspension. The added media causes growth of the population, which in turn causes an increase in turbidity. The increased turbidity thus causes the circuit to turn off, halting the addition of media. In this fashion the turbidostat provides for density dependent growth of the microbial population. The key to the turbidostat and other technoecosystems is feedback through a sensor circuit which allows for self-control. This action is similar to the concept of biofeedback from psychobiology (Basmajian, 1979; Schwartz, 1975). Biofeedback allows humans or other animals to control processes such as heart rate, blood pressure, or electrical activity of the brain when provided with information from a sensor about their physiological function.

A variety of simple technoagroecosystems have been developed including irrigation systems that sense soil water status (Anonymous, 2001), aquacultural systems that sense growth conditions for fishes (Ebeling, 1994), and computerized greenhouses (Goto et al., 1997; Hashimoto et al., 1993; Jones, 1989). Ecological engineers may design more complex technoecosystems. For example, studies by R. Beyers and J. Petersen were described in Chapter 4 for microcosms which sensed ecosystem metabolism and regulated light inputs. Wolf (1996) constructed a similar system which regulated nutrient fertilizer inputs for experimental bioregeneration. Robert Kok of McGill University envisioned even more complicated hardware interfaces in his "Ecocyborg Project." Along with his students and colleagues Kok published many designs and analyzes for ecosystems with artificial intelligence control networks (Clark et al., 1996, 1998, 1999; Kok and Lacroix, 1993; Parrott et al., 1996). Blersch (in preparation) has built this kind of design around a wetland soil microcosm (Figure 9.8). The microcosm is part of a hardware system that attempts to maximize denitrification in the microcosm by controlling limiting factors. Based on a sensing of the change in the microcosm's redox potential, either nitrogen or carbon is added to accelerate microbial metabolism. Denitrification is monitored as the rate of consumption of nitrogen addition, and microbial metabolism is monitored as the rate of decline in redox potential. Artificial intelligence is being investigated with a logic system that evaluates inputs from the redox probe in the actual microcosm and inputs from a simulation model of the system that is run simultaneously with the microcosm. The goal is to achieve the maximum denitrification rate through the use of the decision algorithm to optimize the input of elements that stimulate microbial metabolism.

Real-time

Model Simulation

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment