Role of H₂o₂ in Single‐cell Growth of the Blue‐green Alga, Anacystis Nidulans¹

SUMMARY Inhibition of growth of single cells of Anacystis nidulans on the surface of or in plates was due to H 2 O 2 . This H 2 O 2 arises from reaction of citrate and Mn 2+ in a defined medium, C, by heat (autoclaving) or light (especially 365 mμ). The simplest way to avoid H 2 O 2 formation was to lower or omit Mn 2+ from the medium. Thus medium C minus Mn 2+ routinely gave excellent quantitative growth of single cells of Anacystis under simple incubation conditions. The H 2 O 2 sensitivity of Anacystis was unexpectedly high; ≈ 50 μg/liter inhibited single cells on the surface of agar plates. However, other blue‐greens tested had not this exaggerated sensitivity. Seawater contained small quantities of apparently free H 2 O 2 . H 2 O 2 in seawater perhaps may have an ecological role.