Biomimetic Silicification Of Microalgae And Its Application

Energy crisis and environmental destruction have become the two major global problems facing today’s human development. Since entering the industrial society, the utilization of fossil fuels not only brought human society a huge development, but also posed serious environmental problems to the Earth. With the continuously depletion of fossil fuels and the increasingly serious environmental pollution, developing renewable energy to improve energy structure has become an important component of the global strategy of sustainable development. Biofuels are currently the only clean renewable energy which can be a large-scale alternative to fossil fuels, and microalgae are considered to be one of the most promising feedstocks for biofuels. Its role has increasingly attracted people’s attention. Microalgae can produce biofuels along with fixing carbon dioxide and reducing greenhouse gases, but the large proliferation of algae can also bring pollution damage to the environment, such as water bloom. Utilization and control of microalgae lies in the nature of microalgae, therefore, how to modify the microalgae play an important role in utilization and control of microalgae.In the process of biological evolution, organisms gradually learned to form organic-inorganic composite material with hierarchical structure by biomineralization, and using the properties of the materials to achieve a variety of different functions for adapting to different living environment. For example diatoms can form silica shells with sophisticated nanostructures through biosilicification, and the silica shell can not only provide mechanical protection for the diatoms, but also provide a function of photonic crystals to improving photosynthetic efficiency. Biomineralization is the process of inorganic mineral generation regulated by organisms with biological macromolecules. Inspired by biomineralization phenomenon, modifying organisms by means of biomimetic mineralization has gradually become a research direction in recent years. Therefore, we envisage to achieve the modification of microalgae cells by means of biomimetic mineralization, and explore the application of functional modification in microalgal utilization and prevention.The thesis consists of five chapters:Chapter 1:we mainly introduced the background of microalgae and biomimetic mineralization, and summarized the research route of the thesis. At first, we described the main species of microalgae, then highlighted the microalgae biofeuls and the interaction between microalgae and environment, displayed the recent main strategies of microalgal modification. Secondly, we introduced the strategy of biomimetic mineralization:introduction of biomineralization from the form of biomineralization, the classification of biominerals and the control strategies of biomineralization; the fundmental principles of biomimetic mineralization; mini-review of the application in bio-modification. At last, we proposed the research route and aim of the thesis.Chapter 2:we used cyanobacterial cells as model system to study the effect of biomimetic mineralization of cell surface on photosynthesis. We conferred silica shell on cyanobacterial cell surface through layer by layer and biomimetic silicification. We investigated the effect of silica shell on cyanobacterial photosynthetic activity, and focused on the effect of the silica shell on alleviation of high-light-induced photoinhibition in cyanobacteria. The results showed that cyanobacteria with silica shell performed better photosynthesis under high light condition, which would benefit for enhancing the photosynthetic biomass.Chapter 3:we changed our research object from cyanobacteria to green algae, and our research target also transformed to improving the ability of photobiological H2 production by green algae. We chose commercialized Chlorella pyrenoidosa as experimental subject, and induced cell aggregation of Chlorella cells through biomimetic silicification. With suitable aggregation, spatial-functional differentiation of photosynthetic O2 evolution and H2 production between Chlorella cells can be achieved, so that Chlorella aggregates can continuously produce H2 under aerobic conditions. This study realized sustainable photobiological H2 production under naturally aerobic conditions for the first time, which can provide new insights to photobiological H2 production and microalgal biofuels production. More generally, the similar chemical-material-based modification of cells may be extended to other microorganisms to induce a designed functional transformation.Chapter 4:we changed our research object from green algae to cyanobacteria, but our research target transformed to controlling the proliferation of cyanobacterial cells. Microcystis spp., the dominant specie of cyanobacterial blooms in Taihu Lake, was selected as experimental subject. Inspired by diatoms, we imitated the strategy of biomimetic mineralization, and used silica nanoparticles and cationic polyelectrolyte with positively charged quaternary amines to induce the quick combination of cell and silica nanoparticles. Microcystis cells quickly aggregated after the combination with nanosilica, and could sedimentate to the water bottom in a short time. The sedimentation can not only restrain the photoautotrophic growth of Microcystis cells, but also inhibit the release of the microcystin. The study provides a safe, effective and low cost strategy for preventing cyanobacterial blooms.Chapter 5:we briefly summarized the studies in this dissertation. Not only how to deepen and boost the current studies by improving of material technique was discussed, but also great prospects of cell (especially microalgal cell) functional modification based on biomimetic mineralization were clearly displayed.