1) Motivation

　Protein interactions, enzymatic reactions, folding of RNA, riboswitches... nearly all of these biological reactions are greatly affected by salt concentation. Salt concentration is also important for most biological devices designed by synthetic biologists. Synthetic biologist try to develope new devices that provide solutions to our daily problems in the world. However, the situations and circumstances vary greatly among each problem, often resulting in undesired salt concentrations. If the salt concentration is insufficient in the surrounding environment for a developing device, adding salt to the culture solution may solve a problem.
　 But how about when salt concentrations are too high? This is a difficult problem to solve. Once devices that absorb salt from environment are developed, we can greatly support various functions of other devices, for example, sensing substances in the environment or bioremediation. Besides, if we can apply salt-absorbing device to the situation outside the laboratory, there is a possibility that it will be useful for purifying water in various scenes such as industrial wastewater and salt damage.
　Although this has a great possibility, the question of how feasible is this environmental desalination using synthetic biology principles has never been explored thoroughly. Therefore, we addressed this problem this year.

2) Let's create a system that absorbs salt from a liquid using genetically modified yeast

　 Some salt-tolerant plants have developed systems that sequester Na+ from the cytoplasm to vacuoles in order to protect themselves from Na+ flowing in. In addition, some plants have gained Na+ tolerance system by producing compatible solutes. In the process of searching papers, we noticed that many genes involved in Na+ sequestration were also shown to function in budding yeast and give them salt tolerance. The budding yeast itself also has a mechanism for exporting Na+ out of the cell. It is also known that budding yeasts, in which Na+-exporting genes have been destroyed and the mechanism are failed, are very sensitive to the high salt concentration in the medium, suggesting that these genes contribute to salt-tolerance of budding yeast. Hence, we assume that if these mechanisms can be combined, it is possible to develope a "biological desalination" device that stores Na+ in the vacuoles of budding yeast and lowers the salt concentration of the environment.
　 We further investigated and noticed that team Aachen 2017, tried to create a similar budding yeast. We contacted the team Aachen and heard a detailed story. Here we proposed a question to ourselves. Can we combine genes of salt-tolerant plants and genes of salt-resistant and build a desalination system which team Aachen did not develop? We started to create budding yeast by expressing various genes and set "creating the device that minimizes the salt concentration of medium in the test tube" as the primary goal.

3) Aggregation system for increasing the efficiency of recollection and biosafety

　 As mentioned above, we initially aimed to create a yeast that removes salt in "test tubes". As a result, we assumed to promote the usage of the biosensor, which detects trace elements contained in test samples such as blood and waste liquid, without being inhibited their function by salt in solution. However, from the activities at Human Practice, we have found a new goal that this system can also be used for social problems such as to stop the problem of salt damage. Assuming such a case, we have to incorporate a mechanism that prevents genetically modified yeast from diffusing into the environment. For this purpose, we tried introducing a new aggregation system as a system for collecting yeast more efficiently.

4) Calculating the amount of yeast necessary to lower the concentration through modeling

　The advantage of a salt removal system with artificially placed defined components is that you can operate the amount and combination of these components freely. You can use different factors depending on the situation, such as the difference in initial salt concentration of an extracellular fluid, a difference in target salt concentration, a difference in time required for salt concentration manipulation, a difference in a required amount of yeast and more. In order to enable this flexible application, it is necessary that the system to be used is sufficiently understood, and modeling is done mathematically. By using the results of this modeling it is possible to estimate the optimal initial yeast input when using the device of this study.