Project: The Continuous Greening Cycle
Desert covers one-fifth of the world's land area, and is inhabited by one-sixth of the world's population, i.e. over a billion people. More serious, however, is desertification - a decrease in soil productivity resulting in loss of fertile land. 40% of the Earth's land area is arid and vulnerable to desertification, and its consequences to the inhabitants - including starvation and poverty - form a major problem.
Desertification results from a combination of natural and human-related factors.
Arid regions tend to experience large fluctuations in precipitation. In those regions, rainfall tends to be infrequent and brief but in heavy volume, leading to erosion of and leeching of nutrients from the soil. Also, drought sometimes occur for long periods, hindering the growth of vegetation. According to some reports, precipitation is correlated to plant cover; consequently a vicious cycle of reduced precipitation and loss of vegetation is induced, finally leading to desertification.
Chief among the human causes of desertification are over-grazing and over-cultivation. As the human population in semi-arid regions increase, a need for higher food production arises, driving humans to expand their agricultural and grazing activities into previously unpopulated areas. These areas, already naturally low in productivity, are stressed beyond capacity and suffer a rapid loss of fertility. Since the process of restoring such exhausted land is often beyond the capability of the communities depending on them, the people move on, expanding further into new areas and leaving a trail of desertification-prone land.
Existing measures to counter desertification include reforestation and implementation of irrigation systems. However, reforestation is difficult because the land is already exhausted of nutrients and water content. On the other hand, installation and maintenance of irrigation systems are costly and the energy requirement of operating such systems is prohibitive.
Clearly, a low cost, low energy, sustainable method of countering desertification is needed, and bioengineering may have the answer. In searching for a way to apply bioengineering to desert greening, one thing that popped into mind almost immediately was the idea of a cycle. The concept of micro-machines that obtain sustenance from the plants, then produce something that aids plant growth in return, seemed promising. Also, it demonstrates an advantage of using a biological solution - engineered microbes can self-multiply, expanding the area of effect. After much brainstorming and discussion, we came up with our project - the Continuous Greening Cycle.
We had 3 objectives in mind when planning this project:
- To address the environmental issue of desertification
- To investigate the feasibility of engineering a cyclic biological system
- To contribute to iGEM and Synthetic Biology by developing and characterizing new parts
We envisioned a Continuous Greening Cycle in which engineered microorganisms decompose plant fiber into nutrients through the action of cellulolytic enzymes. They then produce polymers with hygroscopic properties such as gamma polyglutamic acid (γ-PGA) that retain water in the soil to help plants grow. When the plants die, they contribute to the biomass from which microorganisms derive their substrate, thus beginning the cycle anew.
Check out the following page for an animation of the cycle: Project Animation
The first step of our cycle is cellulose degradation. For that, we need cellulases, a family of enzymes that break β-glucosidic bonds in cellulose. We decided to implement cellulase production in baker's yeast, Saccharomyces cerevisiae, as cellulase production yeast is well-documented. Check out the Cellulase page for more info.
Polyglutamic acid (PGA) synthesis
The second step in the cycle production of the water-retaining substance, γ-polyglutamic acid (also known as poly-γ-glutamate or γ-PGA). We chose Escherichia coli for expression of PGA synthesis genes, as success with this chassis has been reported. For more details, check out the γ-PGA page.
Construction of the parts for cellulose degradation and PGA synthesis outlined above formed the majority of our wet lab work. We produced a collection of new BioBricks by PCR from existing plasmids or genome DNA, and also made some constructs to test the new parts. For more info, please see Parts.
We ran several tests to confirm the working of our parts, as well as characterize them quantitatively. See the Tests page for more info.
We also attempted to construct a model and simulate it using software in order to determine the feasibility of the cycle as well as identify important parameters involved. See the Modeling page for more info.
- MIILENNIUM ECOSYSTEM ASSESSMENT，http://www.maweb.org/en/index.aspx
- Biochemistry and molecular genetics of poly-γ-glutamate synthesis, M Ashiuchi (2002)
- Secretion of a Bacterial Cellulase by Yeast, Skipper et al (1985)
- Expression of two Trichoderma reesei endoglucanases in the yeast Saccharomyces cerevisiae, Panttilä et al (1987)