The antimicrobial activity of lipophilic extracts of mat-forming Phormidium-like cyanobacterium isolated from Egypt was investigated under different phosphate concentrations. The antimicrobial profile changed with different phosphate levels indicating metabolic changes under stressful conditions. The fractions that resulted in highest antimicrobial activity from the three different phosphate concentrations were chosen for further analyses. The bioactive compounds were identified using chromatographic and spectroscopic techniques including UV, FTIR, GC-MS and proton-NMR. The chemical analyses indicated that the compound at standard phosphate concentration was eugenol whereas the bioactive compound at half phosphate concentration was 4-tert-butylcyclohexanol. The third bioactive compound at quarter phosphate concentration was octadecanoic acid. The eugenol is known for its antimicrobial as well as pain relief properties and can be used in many pharmaceutical preparations whereas the octadecanoic acid and cyclohexanol derivative are used in some antimicrobial pharmaceuticals. The study highlights the change in antimicrobial profile of bioactive compounds derived from cyanobacteria through manipulating the concentration of a key nutrient in growth medium. This strategy can be employed for mass production of these compounds and others for future biotechnological applications.