Investigation of the microalga Nannochloropsis oceanica as a platform for protein secretion

The demand for recombinant proteins (RPs) is continuously increasing due to their extensive variety of applications, from improving human health to solving industrial and environmental problems. Currently, RPs are mainly expressed in microorganisms such as bacteria and yeasts, while other microbial platforms, like microalgae, are still largely unexplored. These organisms instead can offer a sustainable cultivation with other advantages such as safety for human applications. In order to develop these applications, however, there is the need to develop methods and technologies for protein production that are clearly less developed than in other microorganisms. One important tool would be the possibility of secreting RP thus avoiding all costs connected with purification. This requires the elucidation of mechanisms involved in protein secretion in industrially relevant microalgae, such as Nannochloropsis oceanica, that is particularly challenging due to its complex cellular architecture as a secondary endosymbiont. This project investigates the protein secretion ability of three transgenic lines of the seawater microalga N. oceanica, overexpressing YFP fusion proteins containing Nterminal signal peptides (SPs) that putatively can direct proteins for secretion. The effective secretion of the proteins, driven by these peptides was validated and shown to be significantly enhanced upon exposure to excess CO2. In the case of the most promising peptide (SP3-YFP) it was possible to describe a possible mechanism for YFP protein secretion of this microalga. We hypothesize that this process might include an intermediate step through the periplastid compartment (PPC), which is the reduced cytoplasm of the red alga symbiont. This pathway might make use of vesicular network to move YFP from the PPC to the extracellular environment.