Mapping the pathway for protein secretion in the secondary endosymbiotic alga Nannochloropsis oceanica

Microalgae are key primary producers in marine ecosystems, and their interactions with the surrounding environment rely on the secretion of intracellular metabolites and macromolecules, particularly proteins, supporting essential functions such as nutrient acquisition, environmental sensing and biotic interactions. Most abundant and ecologically relevant seawater algae are secondary endosymbionts, where multiple endosymbiotic events extensively reshaped plastids and intracellular membrane systems, requiring adaptation of protein trafficking mechanisms. This study presents the identification of signal peptides that direct protein secretion in the seawater microalga Nannochloropsis oceanica. Their expression in frame with a fluorescent tag enabled to reconstruct the protein secretion pathway in this organism. Proteins channelled for export are first targeted to the periplastidial compartment, an exclusive structure of secondary endosymbiotic algae, that acts as hub for protein trafficking. Subsequently, vesicle-mediated transport directs proteins through the endoplasmic reticulum into the periplasmic space between the cell membrane and the cell wall, from where they are released upon cell division. These findings reveal an evolutionarily remodeled protein secretion pathway, in which host- and endosymbiont-derived trafficking mechanisms merged into an integrated functional system. Significance StatementThe most abundant and ecologically relevant marine algae are secondary endosymbionts whose evolution required extensive re-adaptation of multiple cellular processes. Among them, protein secretion is essential for the interaction with external environment, and required specific re-shaping to the increased cellular complexity associated with endosymbiosis. This work uncovers protein secretory pathway in the secondary endosymbiont seawater alga Nannochloropsis oceanica showing that is does not follow a direct route, but proteins are first accumulated in the periplastidial compartment, a unique structure derived from its endosymbiotic history, before being directed for secretion. The final pathway integrated components derived from both the host and endosymbiont, highlighting how evolution was able to merge different biological modules to build an integrated and functional system.