Abstract
The aim of this study was to compare nitrogen fixation rates by cyanobacteria measured by acetylene reduction and 15N2, fixation. The effect of environmental stress on the theoretical conversion factor 4 (mol C2H2 per mol N2 reduced) between the methods was studied experimentally in cultures and in field samples from the open Baltic Sea. Axenic and non-axenic laboratory cultures of Nodularia spumigena Mertens (strain BY1). Nodularia sphaerocarpa Bornet et Flahault (strain HKVV) and Aphanizomenon cf. flos-aquae Ralfs ex Bornet et Flahault (strain TR183) isolated from the Baltic Sea were used to test the effect of salinity and NH4+ concentration. The conversion factor was 0.6–4.8 for Nodularia spumigena. 2.6–11.0 for Nodulorio sphaerocarpa and 1.0–15.5 for Aphanizomenon. Changes in salinity and NH4+ concentrations did not affect the conversion factors. but the conversion factor varied among cyanobacterial strains and growth phases.The effect of sample concentration and light level on the conversion factor was tested during a field study of 11 days at the entrance to the Gulf of Finland. A decrease in nitrogenase activity from 0.08 to 0.01 nmol N2 (μg particulate organic nitrogen) 1 h−1 (unconcentrated sample) (15N2 method) occurred when the heterocyst frequency of Nodularia spumigena and Aphanizomenon cf. flos-aquae decreased. indicating that the bloom was decaying. The conversion factor between methods was 1.0–31.8 during the field study, and there was an increase in the factor during the study period. A higher conversion factor in nature than in culture samples may be due to methodological problems during nitrogen fixation measurements. to variation in the conversion factor. or to both.
Published Version
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