PHOTOINHIBITION OF MARINE NITRIFYING BACTERIA .1. WAVELENGTH-DEPENDENT RESPONSE

Axenic cultures of chemolithotrophic nitrifying bacteria (NH4+ and NO2 - oxidizers) respond to light in distinct manners. After short-term (2 to 4 h) monochromatic irradiations both types of nitrifying bacteria demonstrated a widespread photosensitivity in the near-UV region (300 to 375 nm) and a differential photosensitivity in the blue region of t he spectrum (400 to 475 nm). Nitrite oxidizers were less sensitive ove rall to blue Light inhibition than were ammonium oxidizers. Characteri stically, the extent of the photoinhibition was species-specific and L ight response was dose- and wavelength-dependent. Photoprotection by h igher substrate concentrations was only observed with ammonium oxidize rs. Increased cell densities altered the phototolerance of nitrite oxi dizers and made these organisms light-susceptible. Similarly, treatmen t with a low light dose for extended periods was more damaging to nitr ite oxidizers, when high cell densities were used. polychromatic irrad iations served to confirm the monochromatic results. Cool-white fluore scent light inhibited NH4+ oxidizing activity but not NO2- oxidizing a ctivity. Exposure to sunlight resulted in inhibition of activity in bo th types of nitrifiers. These data demonstrate that the effect of Ligh t on autotrophic nitrification depends not only on the type of nitrifi er (NH4+ or NO2- oxidizer), but also on the conditions of their enviro nment.