N2-Fixation in Cyanobacterial Mats from Ponds on the McMurdo Ice Shelf,Antarctica |
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Authors: | Fernández-Valiente E Quesada A Howard-Williams C Hawes I |
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Institution: | (1) Dpto. Biología, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco 28049-Madrid, Spain, ES;(2) National Institute of Water and Atmospheric Research, P.O. Box 8602, Christchurch, New Zealand, NZ |
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Abstract: | We have investigated the ecological importance of N2-fixation in cyanobacterial mats, dominated by oscillatorean species,
in ponds of the Bratina Island area of the McMurdo Ice Shelf, Antarctica (78°S, 166°E). Nitrogenase activity, estimated as
acetylene reducing activity (ARA), was found in all the mats investigated (n = 16). The average ARA was 75.9 mmol ethylene
m-2 h-1, ranging from 6 to 201 mmol ethylene m-2 h-1. Nitrogenase activity was positively correlated with dissolved reactive
phosphorus concentration in pondwater and the C/N ratio of the mat, and was negatively correlated with pondwater NH4+-N concentrations
and natural abundance of 15N 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-dichlorophenyl]-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 mmol photon m-2s-1. Irradiance rarely fell below 100 mmol photon m-2s-1 during Antarctic summer days and ARA was likely
to be light saturated for much of the time. We estimate that N2 fixation represented on average a N input into the ponds of
over 1 g m-2y-1. This value appears to be the highest N input to this Antarctic ecosystem. |
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