(620a) Biotechnology of Phototrophs - from Metabolic Understanding to Scalable and Energy-Efficient Photobioreactors

Thanks to their sustainable capacity for solar energy conversion, phototrophic organisms like (micro/macro)algae and cyanobacteria represent almost one third of the total primary biomass that is being generated on the scale of our planet. At the same time they are by far the most important carbon dioxide consumers in the biosphere. Hence they are actively considered as potential bioeconomy actors for biofuels and high-value ingredients.  The biotechnology of phototrophs is an emerging area with high potential to offer a paradigm shift in bioprocessing. This presentation summarizes our recent work ranging from metabolic understanding and exploitation of novel extremophilic microalgal strains to the rational design of scalable and energy-efficient photobioreactor systems. Photoheterotrophy conditions with glycerol addition to the new microalgal strain Dactylococcus dissociatus MT1 can simultaneously increase biomass production and reduce or eliminate the need for gas sparging. This behaviour is modelled mechanistically by linking the oxygen exchange between the cells and the culture medium as a function of glycerol concentration, cellular chlorophyll content, and photosynthetic efficiency to biomass productivity. The latter is greatest when the photosynthetic and respiratory pathways are nearly balanced. This implies that an intracellular recycling of O₂ and CO₂ increases biomass production efficiency and that a process set-point can be reached which eliminates the need to provide supplementary O₂ or CO₂. In the same strain, environmental stressors can induce the production of the valuable carotenoids canthaxanthin, adonixanthin and astaxanthin. Light intensity has a positive influence on the accumulation of the major carotenoid, canthaxanthin, similarly to salinity stress, while nitrate deprivation has more of an effect on lipid production. Nitrate depletion and salinity stress in combination increase both lipid and carotenoid accumulation. The growth and carotenogenesis phases can be initiated, reversed and repeated at regular intervals, thus opening the way to a scalable and sustainable bioprocess. The future establishment of a sustainable microalgal industry necessitates not only efficient and robust phototroph strains but large-scale, inexpensive and dependable photobioreactor systems. In addition to gaining further basic understanding on these organisms’ metabolism, more technoeconomic interventions such as cutting down the construction and operational costs might emerge as the primary principle that should guide the development of future industrial photobioreactor systems. In this vein, we have designed a production platform which can generate algal biomass in an energy efficient and cost-effective manner while at the same time controlling the photonic input to the cell culture and maintaining aseptic conditions. This is a novel and controllable, yet simple, annular-plate airlift photobioreactor configuration with internal illumination and a high conversion efficiency of energy input to biomass.