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.

The effects of CO and light on the ammonium oxidizing bacterium Nitrosomonas cryotolerans and the nitrite oxidizing bacterium N~trobacter sp Nb297 were investigated. Ammonium oxldahon was inhibited by C O concentrations ranging from 2 n M to 11.4 FM. CO inhibition was cell density dependent and increased if the cells had been either previously starved or deprived of iron. Ammonium oxidation was inhibited by 63 % at intensities a s low a s 5 W m-2 of artificial light. Nitrobactersp. Nb297, on the other hand, was able to tolerate C O concentrations as high as 400 PM. Artificial light (25 W m-*) slightly reduced nitrite oxidation while sunlight (628 W m-') decreased activity to ca 20 % of its original value. In addition, C O and CH, oxidation by N. cryotolerans and natural assemblages of bacteria were inhibited by light. It is apparent from these studies that the mechanisms of light and C O inhibition are different for the NH: and NO; oxidizers examined. Characterization of light and C O responses by marine nitrifying bacteria is critical in any attempt to explain the formation of the subsurface primary nitrite maximum.

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.

Recovery of activity by 2 marine nitrifying bacteria, Nitrosomonas cryotolerans and Nitrobacter sp. Nb297, was monitored after exposure to light and/or carbon monoxide. N. cryotolerans recovered more rapidly after exposure to artificial light (25 W m-') than CO (11.4 PM). The adhtion of 100 yM NH,+ during the time of light or CO exposure allowed the cells to recover faster and decreased the difference observed between the inhibitory effects of light and CO. Cells exposed to sunlight recovered much illore slowly than cells exposed to either CO or artificial hght. The ammonium oxidizer N. cryotolerans when exposed to sunlight (2 h) recovered up to 35 "/o of its original ammonium oxidizing activity. Recovery increased to 57 % in the presence of additional substrate (100 pM NH,'). The nitrite oxidizer Nitrobactersp. Nb297 did not show a noticeable recovery from sunlight whether substrate was added or not.