Critical assessment of protozoa contamination and control measures in mass culture of the diatom Phaeodactylum tricornutum

•First critical assessment of protozoa contamination in P. tricornutum mass culture.•Seven protozoan strains were identified in P. tricornutum mass culture.•The heterolobosean amoeba was the most destructive predator.•400 mg L-1 NH4HCO3 inhibits amoeba without affecting algal growth.•Halting culture mixing at night prevents amoeba proliferation.