Description
Abstract
With the development of China's aquaculture industry, as well as the improvement of people's living standards, shrimps and crabs are increasingly coming to people's tables as tasty delicacies. This has resulted in approximately 5.6 million tons of shell waste in 2023, in China, which poses severe environmental challenges, including pollution and resource wastage. Our team aims to address this problem by employing synthetic biology to convert chitin from shrimp and crab shells into valuable chitooligosaccharides using enzymatic treatment. This approach not only offers a sustainable solution to waste management but also transforms a problematic byproduct into economically viable resources.
Inspiration
Most of our team members are from of Suzhou, China, where is a city with a well-developed aquaculture industry , with river crabs, shrimp, and other species prominent. Every May to October, crayfish, hairy crabs, etc. become popular delicacies. While enjoying the delicious food, we also pay attention to the astonishing amount of shrimp and crab shell waste generated. And our iGEM journey begins here.
Our Concern
Globally, shrimp and crab shell waste contribute significantly to environmental pollution. In developed countries, the cost of disposing of these shells can be high, while in developing nations, they are often discarded irresponsibly, exacerbating pollution issues[3]. The disposal of these shells alters soil, water, and marine ecosystems, leading to serious ecological ramifications [4].
Importance of this problem
Where do the food waste remnants, specifically the shrimp and crab shells, end up? Our team set out to explore this issue. We discovered that the recovery rate of these shells is disappointingly low. From the public suevey and our investigation of lobster restaurants, we found that most of people and the individual owners are not aware of recycling shrimp and crab shells, and most of them just dispose of them together as kitchen waste.These Shrimp and Crab Aquaculture Enterprises also have no plans to recycle this year, but most people have expressed a desire to recycle shrimp and crab shells. However, the recycling cost for shrimp and crab shells is prohibitively low. Wet shrimp shells are priced at less than $0.10 per kilogram[1], and when factoring in storage and transportation expenses, the economic incentives are minimal, failing to captivate businesses. Consequently, there is an immediate necessity for the development of an efficient method to utilize shrimp and crab shells, enhancing the recycling efficiency of these shells. This approach would facilitate the production of chitooligosaccharides, which could then be more readily processed and applied in a broader spectrum of applications.
Current solutions and the problem they cause
The implications of shell waste extend beyond environmental concerns. They also encompass economic dimensions, as discarded shells represent lost opportunities for resource recovery. By utilizing synthetic biology, our project seeks to mitigate these issues through the enzymatic conversion of chitin into high-value chitooligosaccharides. These compounds have applications in various fields, including medicine, agriculture, and environmental management, highlighting the dual benefit of addressing waste while generating economic value[5, 6].
Chitin & Chitooligosaccharides
The main components of shrimp and crab shells, one of the sources of shellfish, are 20%-30% chitin, 40%-50% inorganic substances such as calcium carbonate and 30%-40% protein. Among them, there is a focus on chitin utilization and recycling.
Chitin is a biomass resource second only to cellulose in nature. It is mainly composed of N-acetyl-D-glucosamine (GlcNAc). The polymer macromolecule obtained after high deacetylation of chitin is called chitosan, and the main component of chitosan is D-glucosamine (GlcN). Generally, the low molecular weight product with a degree of polymerisation (DP) of 2-10 obtained after the degradation of chitosan is called chito-oligosaccharide (COS)[7].(Fig 4)
Figure 4 Chemical structure of chitin, chitosan, and chito-oligosaccharides[8]
Figure 5 Schematic diagram of the processing of chitin to chito-oligosaccharide
A large number of studies have found that chitooligosaccharides is a class of functional carbohydrates with physiological activities such as antibacterial, antioxidant, anti-tumour, cholesterol-lowering, blood pressure-lowering, anti-infective, and anti-inflammatory, which show great potential for application[9]. These functions have significant differences with different degrees of polymerisation. Currently, the price of chito-oligosaccharides with a single degree of polymerisation on the market is high, and the higher the degree of polymerisation, the higher the price. The functional effects of COS are also closely linked to their DP values and can directly affect their application value and effectiveness, so the biological activity of a single COS with a well-defined DP is of great interest. The hydrolysis products of chitosanases found so far are oligomeric chitooligosaccharides with varying degrees of polymerisation, mostly dominated by chitobiose and chitotriose. Yet there is an increasing demand for single-component functional sugars on the market today. Table 1 shows the current market price of pure chitosan Chitooligosaccharides in the Chinese market. The price of 10 mg is more than 500 CNY, which is very expensive for Chinese.
Table 1: The Price of Pure Chitosan Chitooligosaccharides
| Chitooligosaccharides | Content (mg) | Price (CNY) |
|---|---|---|
| (GlcN)2 | 10 | 518 |
| (GlcN)3 | 10 | 585 |
| (GlcN)4 | 10 | 650 |
| (GlcN)5 | 10 | 1235 |
| (GlcN)6 | 10 | 1560 |
At present, conventional production methods of chitosan oligosaccharides mainly include physical and chemical methods, which usually involve high energy consumption and complex reaction conditions, and the yield and purity are difficult to guarantee. In contrast, the enzymatic method, with its mild reaction conditions and high specificity, can effectively convert chitosan into chitosan oligosaccharides, produce less waste, and have better economic benefits and environmental friendliness. Therefore, our team hopes to obtain chitooligosaccharides from shrimp and crab shells by an enzymatic method.
Our Solution
Production of chitooligosaccharides from shrimp and crab shells using an enzymatic method
In order to obtain chitooligosaccharides of a specific degree of polymerisation, we chose the wild-type chitosanase CsnB from Bacilius sp. BY01 for modification in the hope that its degradation product would be changed from a mixture of chitobiose and chitotriose to a single degree of polymerisation chitobiose or chitotriose. Thereby enhancing their economic value and promoting recycling. This approach offers a viable solution to the environmental challenges posed by shell disposal.
References
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[9] ZHENG J, YUAN X, CHENG G, et al. Chitosan oligosaccharides improve the disturbance in glucose metabolism and reverse the dysbiosis of gut microbiota in diabetic mice [J]. Carbohydrate Polymers, 2018, 190: 77-86.