| Abstract ENG | Successfulimplementation of green chemistry processes and sustainable alternative fuel production represent crucial challenges for the modern society, as highlighted by directive strategiesof the European Union Commission (H2020-BB-2016-2017, H2020-LCE-2016-2017).In this respect, one of the most attractive strategy relies on the use of aquatic photosynthetic unicellular organisms, particularly ofthe prokaryotic cyanobacteria,because they are generally well characterised, have plain nutritional requirements and posses large geneticdiversity. This makes them ideal candidates for the sustainable and biological production of valuable products, such as carotenoids, which are well known to act as effective antioxidant and that, in mammals, have to be introduced by dietary sources. Therefore a wide range of carotenoids have significant industrial interest, ranging from dietary supplements, cosmetics and food colouring. For instance, one of the main cost in salmon and rainbow trout aquaculture is the dietary supplement of the carotene astaxanthin(Asx), which gives the characteristic pink-orangecolour to the fish flesh. This is because the production of Asxfrom natural sources, generally preferred to increase the quality and value of the product, is at present cumbersome: marine and freshwater algal species able to naturally produce this carotene are difficult to culture. Cyanobacterial species which are most commonly employed for biotechnological applications, such as SynechocystisPCC6803 and SynechococcusPCC7942, do not naturally accumulate Asx. Their oxygenated carotenoidsbiosynthetic pathway leads to the accumulation of zeaxanthin, which is however the main metabolic precursor of Asx. Therefore, by introducing a single metabolic step, the production of Asxcan be promoted in these strains, which are easily cultured and harvested. In turn, this is expected to significantly lower the costs of Asxproduction from natural sources.In this context is however worth noticing that the overall productivity of photobioreactors(PBR)is nowadaysaffected bythe steep decrease of the light intensity through the culture, a process known as “self-shading”, which is common to all photosynthetic microorganisms. This process is one of the main sources limiting biomass accumulation in PBRs. Moreover, culture self-shading results in a great enrichmentin near-infrared and far-red lightsin the inner bioreactor layers. These portions of the solar spectrum are poorly absorbed by the vast majority of photosynthetic organisms. In order to limit the inhomogeneityof light penetration across the PBR layersdue to self-shading, the cultures are often thoroughly mixed, which constitutes an increase in the energy input required to run the plant. However, geometrical architecture aimed at reducing or avoiding mixingto improve the overall cost efficiency have been discussed. Recent studies demonstrated that, in aquatic environments, either marine or freshwater, somecyanobacterial species synthesiseunconventionalchlorophylls (known as Chldand f)havingred-shifted absorption compared to the otherwise ubiquitous Chl apigment. Thesenatural species inhabit ecological niches resemblingthe dimmed and far-red enriched conditions which occurin artificial
bioreactors. We therefore propose to expand and extendthe light-harvesting potential of cyanobacterialcultures by controlling the synthesis of the unconventional Chl d, improving therebytheirgrowth rate and fitness under PBRrelevant conditions. We expect this to represent a major breakthrough in increasing the productivity of cyanobacteria therebyimprovingtheir biotechnological application potential.Therefore in this project, CYAnobacteria platform Optimised for bioproduction (CYAO), we propose to engineer the model cyanobacteria species SynechocystisPCC6803 and SynechococcusPCC7942i) to improve their growth yield in PBRs, and ii) to efficiently accumulate Asx, as this bioproduct has a potential economic return. The biotechnological potential of the CYAO strains will be evaluated in test trials of rainbow trout farming, which is a significant economicsector in our local territory.Principal aims of the project:1.Development of cyanobacterial strains having improved growth yield and biomass accumulation by the engineering of their light-harvesting capacity.2.Development of cyanobacterial strains producing and accumulating high levels of Asx.3.Evaluation of the feasibility of rainbow trout aquaculturein which thebioproduct Asx,extracted from the engineered cyanobacterial strains, is used as feed supplement. |
