Axenic cultures of chemolithotrophic nitrifying bacteria (NH, ' and NOz-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 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 dose-and 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 NO2-oxidizing 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.
Nitrifying bacteria (NH4+ and NO2-oxidizers) are capable of recovery from photoinhibitlon in the dark. After short-term (2 to 4 h) irradiations, significant differences were found between the 2 groups. NH,' oxidizers subjected to longer wavelengths (>400 nm; 25 W m-2) or polychromatic light (15 W m-2) regained activity faster (0.5 to 1 h) than if exposed to shorter wavelengths (<400 nm; 25 W m-') or sunlight (360 to 400 W m"). In contrast, NOz-oxidizers only failed to recuperate activity after sunlight and near-UV (300 to 375 nm) treatment. Artificial light (5 to 25 W did not affect nitrite oxidation. Thus, recovery of NH,+ and NO2' oxidizing activities exhibited both dose and wavelength dependencies. These distinct recovery responses imply that nitrogen turnover in aquatic ecosystems depends on a number of factors among which light transmission properties of different water types (i.e. from lakes, rivers, estuaries, coastal marine and oceans) and physiological differences between nitrifying bacteria play significant roles.
Spectroscopic analysis of nitrifying bacteria revealed the presence of a porphyrin-like pigment with an absorption maximum at 408 nm. The photoresponsive pigment accumulated during the late exponential phase of growth. The photoreceptor was found at higher concentrations in NH4+ oxidizers than in NO2-oxldizers. When absorbance scans and action spectra of the nitrifiers were compared, it was found that the regression between the degree of photoinhibition and higher absorbances at 408 nm was significant (r2 = 0.7). Reversible light-induced absorbance changes were observed in vivo and in vitro. Absorbance changes were maximally elicited by light in the 400 nm region for both types of nitrifiers, but the change was only significant (p < 0.05) for NH,' oxldizers. This spectral sensitivity of the NH,' oxidizing process suggests that the absorbance change observed is related to the blue light sensitivity of NH,+ oxidizers.
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