Description
Water security is an urgent global issue, especially because many regions of the world are experiencing, or are predicted to experience, water shortage conditions: More than one in six people globally do not have access to safe drinking water (United Nations, 2006)[1]. Seawater comprises ninety-seven percent of the Earth’s water resource; consequently, several efficient methods have been developed to generate freshwater from the ocean, among which reverse osmosis has been used for desalination in a large scale. However, the high energy consumption of these technologies has limited their application greatly.
Here we introduce a new method of desalination called biodesalination, which means to absorb sodium chloride from saltwater through biological membranes of photosynthetic organisms. [1]The energy source of biodesalination is sunlight, which makes it a sustainable, energy-efficient and environment-friendly process. Cyanobacteria possess salt-tolerance mechanisms which allow them to live in environments with different and changing salt concentrations.[2] Together with salt-tolerance, the following characteristics make them an ideal organism for biodesalination: fast-growing, photoautotrophic, amenable to genetic transformation.
Halorhodopsin is a light-driven inward-directed chloride pump from halobacteria and its electrophysiological properties have been characterized in Xenopus laevis oocytes.[3] We use it as our biodesalination driver which confers cyanobacteria the ability to absorb chloride to a significant degree. And we propose that the negative membrane potential generated by halorhodopsin would drive the influx of cation through sodium ion channels. Additionally, cyanobacteria can actively export sodium ion which requires H+ gradient or ATP as the energy source. Therefore, the functional expression of halorhodopsin and depletion of ATP reserves in cyanobacteria could be regarded as the keys to the success of biodesalination.
To achieve precisely timed expression of our biodesalition driver, halorhodopsin, we use green-light induced PcpcG2 and darkness-induced Pdark as our biodesalination controller. According to the absorption spectrum of chlorophyll, green light cannot provide energy for the cyanobacteria, which indicates that green light, like darkness, will lead to depletion of ATP. As a result, PcpcG2 and Pdark are both compatible with our three-stage biodesalination process. The cultivation of engineered cyanobacteria comprises three phases: growth phase, expression phase and desalination phase. After cyanobacteria reaching a high density, we induce the expression of halorhodopsin, and subsequently we move the cyanobacteria into white-light condition afterwards, which allows the halorhodopsin to work.
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