The present invention relates to fermentation processes for the production of cyanophycin in a microorganism whereby a plant-derived nitrogen source is converted by the microorganism into cyanophycin. The plant-derived nitrogen source preferably is a process stream being obtained in the processing of agricultural crops such as e.g., a by-product in the processing of starch from agricultural crops like corn, potato or cassave. The invention further relates to processes for the conversion of cyanophycin into a variety of compounds including e.g., ornithine, 1,4-butanediamine, n-alkyl amino alcohols, acrylonitrile, as well as cyanophycin derived functionalised poly(aspartic acid)s wherein the arginine residues have been functionalised to ornithine, (N-L-arginino)succinate, N-phospho-L-arginine or agmantine and the lysine residues have been functionalised to N6-hydroxy-L-lysine, 2,5-diaminohexanoate, N6-(L-1,3-dicarboxypropyl), pentanediamine, 5-aminopentanamide or N6-acetyl-L-lysine. These functionalised groups can be further subjected to subsequent chemical and/or enzymatic modifications.