Nitrogenase activity (acetylene reduction activity) and diversity of six soil Nostoc strains

Abstract

Morphology and nitrogenase activity were determined in six different soil Nostoc strains, identified taxonomically according to traditional characters. Two strains of N. muscorum and N. calcicola agg. and one strain of N. ellipsosporum and N. edaphicum agg. were studied. Frequency of heterocytes and their position in filaments, macrostructure of colony and changes in time (life cycle) were particularly evaluated. These characteristics were stable for each strain under our cultivation conditions and very similar in strains of the same type. Thus they provide good information for description of distinct natural morphotypes (traditional species). However, differences in size of vegetative cells, akinctes and heterocytes were variable and not significant between all strains. Namely, N. ellipsosporum substantially differed from the others. Nitrogenase activity (NA) was determined as an acetylene reduction activity in 6, 21, 42 days old cultures und standard culture condition. Although there were some differences in NA between the strains, they were mostly insignificant due to the enormous variability between individual samples. Mean values of NA in six days old cultures ranged from 33.0 to 44.3 μmol ethylene.g -1 .h -1 , in 21 days old cultures from 2.1 to 22.3 μmol ethylene.g -1 .h -1 and from 1.2 to 6.4 μmol ethylene.g -1 .h -1 in 42 days old cultures. High values of NA were connected with low dry mass of colony and low values of NA with high dry mass of colony. We suppose that massive mucilaginous envelopes of filaments could affect diffusion of gases and penetration of light into bigger (older) colonies. This resulted in a lower nitrogenase activity in inner parts of colony. The specific nitrogenase activity expressed per one gram of dry mass decreased with increasing dry mass of the colony. We suggest the total nitrogen fixation of Nostoc colony depends much more on total dry weight of colony (i.e. on colony size) and actual physiological status (phase of life cycle etc), then on mean frequency of heterocytes.