Polyunsaturated aldehydes are oxylipins that are formed from lipids (specifically the fatty acid portion of lipids) when diatoms are exposed to environmental stresses. Stresses can include nutrient limitations, grazing by predators, and wounding.
In particular, damage to diatom cells as a result of grazing by zooplankton invokes a chemical defense mechanism that produces PUA’s as secondary metabolites from fatty acids. The production mechanism is as follows:
# Grazing by predators results in diatom cell membrane disruption. 
# Enzymes (lipases) are produced in response to the damaged membranes. These enzymes make contact with newly freed phospholipids (from cell membranes) and catalyze the formation of polyunsaturated fatty acids.
# The enzyme lipoxygenase then catalyzes the reaction of fatty acids to polyunsaturated aldehydes, which are then directly exposed to the grazing zooplankton.
Thalassiosira rotula represents the most well-studied diatom species in terms of polyunsaturated aldehyde production. Wichard et al. determined that only 30% of PUA precursor molecules remain in T. rotula within minutes of cell membrane wounding, indicating a fast rate of response by diatoms to zooplankton grazing.