, 2009), and N. devanaterra was cultured in acidic (pH 4.5) freshwater medium as described by Lehtovirta-Morley et al. (2011). The media for AOA contained ammonium chloride at concentrations of 1 mM for N. maritimus and 0.5 mM for N. devanaterra. Media were inoculated with 1% or 10% (v/v) of exponential-phase cultures of AOB or AOA, respectively. Bacterial cultures were sampled (1 mL) at intervals of 8 h for 5 days, and archaeal cultures were sampled daily for 10 days. Photoinhibition was investigated in controlled temperature chambers maintained at 26 °C and illuminated by compact fluorescent lights (55 W) and clear strip lights (30 W) (International Lamps Ltd, Hertford, UK) emitting

light with a wavelength spectrum of 400–680 nm with a maximum PLX-4720 order intensity at approximately 580 nm. Ammonia-oxidizing activity of the different cultures was measured under continuous illumination at an intensity of either 15, 60 or 500 μE m−2 s−1 and with diurnal cycles of 8-h light (15 or 60 μE m−2 s−1) and 16-h dark conditions. Control cultures were incubated in the dark in the same incubator. Triplicate cultures were grown for all light treatments and controls. Light intensities were selected

ABT-199 supplier to reflect conditions prevailing in riparian zones of rivers and lakes, with highest light intensity (500 μE m−2 s−1) simulating naturally occurring conditions during a clear summer day in open areas and the lower intensities (60 and 15 μE m−2 s−1) simulating conditions in shaded areas. Ammonia-oxidizing activity was determined by measuring Dichloromethane dehalogenase increases in nitrite () concentration over time for each particular culture and light exposure treatment. Specific growth rate was estimated by linear regression during the linear phase of semi-logarithmic plots of nitrite concentration vs. time, as in previous studies (Powell & Prosser, 1992; Könneke et al., 2005; Lehtovirta-Morley et al., 2011). Estimated specific growth rates in control and illuminated cultures were compared using the Student’s t-test (two-sample

assuming unequal variances). All AOA and AOB strains grew exponentially during incubation in the dark. Initial increases in nitrite concentration were sometimes non-exponential, because of carryover of nitrite with inocula, but subsequent increases in nitrite concentration were exponential. Typical nitrite production kinetics are exemplified in Fig. 1 for cultures of N. multiformis and N. devanaterra under continuous light at 60 μE m−2 s−1 and dark controls. Nitrite production kinetics were analysed prior to limitation by reduction in pH (all strains except N. devanaterra) or high nitrite concentration (N. devanaterra). Continuous illumination at 60 μE m−2 s−1 reduced the specific growth rate of N. multiformis from 1.05 (±0.07) day−1 to 0.62 (±0.01) day−1 and completely inhibited that of N. devanaterra. Effects of illumination and associated statistical analysis are summarized in Fig. 2 and Table 1, respectively. AOA were more sensitive to illumination than AOB.