M3STRACT Nitrapynn, an lnh~bitor of NH; oxldlz~ng bactena was used to estlmate the a c t~v~t y of NH: oxldlzing bacteria in the bottom 1 to 15 cm of annual sea Ice and in the water column at vanous locations In McMurdo Sound and along the Ross Ice Shelf (RIS) N~t r a p y r~n s~gn~flcantly ~n h~b i t e d dark ' 4 C -H C 0 3 uptake in virtually all sea-ice samples, indlcahng the presence of NH: oxld~zlng bacteria Inorganic carbon futahon by sea ice NH: o x~d~z e r s was only a small fract~on of that futed by sea-lce photoautotrophs, both on an hourly and annual b a s~s Despite their relative lack of ~mportance to lnorganlc carbon fixation NH: oxidizing bactena may have an important role in the N d y n a m~c s w~t h l n the b~o g e n i c layer of annual sea Ice both In terms of N H~ utlllzatlon and eventual NO; product~on Inorgamc carbon futatlon in the water column beneath sea ice was generally not i n h~b~t e d s~gnif~cantly by nitrapynn NH: oxldizer actlvlty was also not detectable in deep water flow~ng beneath (southward) or from under (northward) the RIS N 2 0 (a by-product of NH: oxidation) levels in p e l a g~c samples were always near 100 % of saturation w t h respect to the alr above the sea surface corroborating the low levels of NH:ox~dizer activity found In the water column

The response of photosynthesis, and of the uptake of NO3-. NH4+ and serine, to irradiance was measured in diatom-dominated sea ice microbial assemblages from bottom ice and surface ice of McMurdo Sound, Antarctica. Uptake responses for dissolved inorganic nitrogen (DIN; N o 3 -and NH4+) could be fitted to a standard model used for photosynthesis after the addtion of a dark uptake parameter; serine uptake showed no dependence on irradiance. The derived uptake models were used to predict the patterns of photosynthesis and DIN uptake over die1 irradiance cycles. According to model predictions, uptake rates in the bottom ice assemblage were always limited by irradiance; neither light saturation nor photoinhibition regulated photosynthesis or DIN utilization in this assemblage. Conversely, photosynthesis in the surface ice assemblage was nearly always light-saturated, whereas DIN uptake was photo-inhibited near midday and saturated at the minimum irradance. Integrated daily C : DIN uptake ratios (g:g) in the bottom ice and surface ice assemblages were 8.6 and 9.7, respectively, corresponding to particulate C:N ratios (g:g) of 8.1 and 5.8 for these respective diatom-dominated communities. Our results indicate that information on d e l patterns of photosynthesis and N uptake is required to evaluate accurately the stoichiometric balance of essential elements in sea ice microalgae.