Abstract: Phosphate-regulated expression of recombinant glycoprotein antigens and other recombinant proteins in diatoms is described herein. More specifically, described herein is the expression and purification of glycosylated, immunogenic, and serologically active receptor-binding domain (RBD) of the SARS-CoV-2 spike protein, as well as SARS-CoV-2 nucleocapsid protein, in the marine pennate diatom Phaeodactylum tricornutum, as well as a functional lateral flow assay-based diagnostic device based on the produced recombinant RBD and nucleocapsid protein. Also described herein is the use of phosphate/iron levels in culture media to regulate expression/secretion of recombinant proteins under control of an HASP1 promoter in P. tricornutum or other suitable host cells. Also described herein is a method for increasing the expression/secretion of a recombinant protein by engineering the recombinant protein to lack a Tobacco Etch Virus (TEV) protease cleavage site.