Photoinhibition of marine nitrifying bacteria. I. Wavelength-dependent response

Axenic cultures of chemolithotrophic nitrifying bacteria (NH,' and NOzoxidizers) 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 the spectrum (400 to 475 nm). Nitrite oxidizers were less sensitive overall to blue llght inhibition than were ammonium oxidizers. Characterstically, the extent of the photoinhibition was species-specific and light response was doseand wavelength-dependent. Photoprotection by higher substrate concentrations was only observed with ammonium oxidizers. Increased cell densities altered the phototolerance of nitrite oxidizers and made these organisms light-susceptible. Similarly, treatment wlth a low light dose for extended periods was more damaging to nitrite oxidizers, when high cell densities were used. Polychromatic irradiations served to confirm the monochromatic results. Cool-white fluorescent light inhibited NH,' oxidizing activity but not NO2oxidizing activity. Exposure to sunlight resulted in inhibition of activity in both types of nitrife r s . These data demonstrate that the effect of light on autotrophic nitrification depends not only on the type of nitrifier (NH,' or N O 2 oxidizer), but also on the conditions of their environment. K E Y WORDS: Nitrification . Photolnhibition . Nitrifying bactena