N2-fixation in cyanobacterial mats from ponds on the McMurdo Ice Shelf, Antarctica

We have investigated the ecological importance of N-2-fixation in cyanobacterial mats, dominated by oscillatorean species, in ponds of the Bratina Island area of the McMurdo Ice Shelf, Antarctica (78 degreesS, 166 degreesE). Nitrogenase activity, estimated as acetylene reducing activity (ARA), was found in all the mats investigated (n = 16). The average ARA was 75.9 mu mol ethylene m(-2) h(-1), ranging from 6 to 201 mu mol ethylene m(-2) h(-1). Nitrogenase activity was positively correlated with dissolved reactive phosphorus concentration in pondwater and the ON ratio of the mat, and was negatively correlated with pondwater NH4+-N concentrations and natural abundance of N-15 in the mats. ARA was restricted to the upper, oxic layer of the mats. Experiments conducted to ascribe ARA to different groups of prokaryotes suggested that ARA was mainly conducted by heterocystous cyanobacteria, since no activity was found in the dark and the activity was inhibited by the photosystem II inhibitor DCMU (3-[3,4-dichlorophenvl]-1,1 -dimethyl urea). In spite of 24 h of daylight, nitrogenase activity showed a diel cycle with maximum activity at midday (10-18 h) and minimal activity at early morning (6-10 h) when pond temperatures were at their minima. Light dependency of nitrogenase activity for three cyanobacterial communities showed that the irradiance required for saturating ARA was low, in every case lower than 100 mu mol photon m(-2)s(-1). Irradiance rarely fell below too mu mol photon m(-2)s(-1) during Antarctic summer days and ARA was likely to be light saturated for much of the time. We estimate that N-2 fixation represented on average a N input into the ponds of over 1 g m(-2)y(-1). This value appears to be the highest N input to this Antarctic ecosystem.