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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. Character~stically, 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 nitrif~e 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

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