Pattern Of N2 Fixation Research Articles

Regulation of nitrogen fixation by different nitrogen sources in the filamentous non-heterocystous cyanobacterium Microcoleus sp.

The pattern of N2 fixation, the synthesis and activity of nitrogenase under different nitrogen sources was studied in the filamentous, non-heterocystous cyanobacterium Microcoleus sp. grown under defined culture conditions. Cells grown under a 10 h light/14 h dark (10L/14D) cycle with N2 as an inorganic nitrogen source showed highest nitrogenase activity (acetylene reduction) at the end of the light phase and then a decrease after entering the dark phase. Nitrogenase synthesis was neither suppressed after 7 days of growth with 2 mM NaNO3 or 0.2 mM (NH4)2SO4 or 0.3 mM urea nor with 20 mM NaNO3 or 3 mM (NH4)2SO4 or 4 mM urea under the 10L/14D cycle. Western immunoblots tested with polyclonal antisera against the Fe-protein revealed the following: (1) the Fe-protein was synthesized in cells grown with N2 as well as in cells grown with NaNO3 or (NH4)2SO4 under the 10L/14D cycle; (2) the Fe-protein was found in cells grown with urea under the 10L/14D cycle, but not in the darkness; (3) only one protein band, corresponding to the Fe-protein, was found in cells harvested during the light phase of the 10L/14D cycle under the tested conditions. No nitrogenase activity was observed when chloramphenicol was added to the cultures 4 h before the onset of the light period. This observation suggest de novo synthesis of nitrogenase in Microcoleus sp.

Environmental and Physiological Controls on Diel Patterns of N2 Fixation in Epiphytic Cyanobacterial Communities

Epiphytic communities dominated by cyanobacteria on standing, dead Spartina alterniflora stems in a North Carolina salt marsh were examined. Community composition changed over an annual cycle, with heterocystous cyanobacteria (Calothrix spp., Nostoc spp.) dominant in the spring, and nonheterocystous cyanobacteria (Lyngbya spp., Phormidium spp.) dominant in the fall. Diel patterns of N2 fixation were investigated with acetylene reduction (AR) assays. Incubation conditions were varied in AR assays to determine physiological controls on N2 fixation rates. AR assays conducted under aerial conditions exhibited significant daytime maxima only in the spring. In contrast, N2 fixation measured in submerged AR assays exhibited daytime maxima through October, although the diel pattern was more pronounced in the spring and summer. Anoxic (purged with N2) incubations always exhibited daytime maxima in AR. Nighttime AR assays conducted under submerged or anoxic conditions exhibited lower rates of AR than nighttime aerial incubations. These data suggest that the N2-fixing microorganisms in the epiphytic community rely on aerobic metabolism for nighttime N2 fixation, presumably in order to provide energy and reductant to nitrogenase. In situ rates of nighttime N2 fixation are, thus, partially controlled by tidal stage, as submergence promotes the development of anoxic conditions. AR rates were extremely low in epiphytic films held continuously in the dark, pointing to the dependence on photosynthate by N2-fixing microorganisms.

Seasonal nitrogen fixation dynamics in a marine microbial mat: Potential roles of cyanobacteria and microheterotrophs

Diel rates of nitrogen (N2) fixation (acetylene reduction) and primary production (14CO2 fixation) were examined seasonally on a North Carolina Atlantic coastal, intertidal, benthic microbial mat community dominated by the filamentous, nonheterocystous cyanobacterial genera Microcoleus and Lyngbya. Highest hourly and daily rates of N2 and CO2 fixation were observed during spring through fall. During this period, an inverse temporal relationship was noted between these processes, with CO2 fixation closely tracking irradiance and N2 fixation rates remaining low during daylight and becoming maximal at night. Under the influence of the photosynthetic (PS 2) inhibitor 3‐(3,4 dichlorophenyl)‐1,1 dimethylurea (DCMU), daytime N2 fixation was enhanced, indicating in situ O2 inhibition of N2 fixation. The most pronounced DCMU stimulation of daytime N2 fixation was in spring‐fall. Both N2 and CO2 fixation rates were lower in winter. Winter patterns of diel N2 fixation were the reverse of those in summer, with maximum rates at midday. The reversal was related to seasonal changes in daily and hourly photosynthetic rates, leading to differential O2 suppression of N2 fixation. Seasonal changes in cyanobacterial community composition and bacterial diazotrophy may have played additional roles in determining diel rates and patterns of N2 fixation and mat production. Results indicate a more important role for bacteria in the dynamics of mat N2 fixation than has been previously recognized.

Landscape-scale estimates of dinitrogen fixation by Pisum sativum by nitrogen-15 natural abundance and enriched isotope dilution

Topography and slope position influence the soil and environmental factors that affect N2 fixation by legumes. The present study was conducted to (1) estimate N2 fixation by field peas in a gently rolling farm field using the natural 15N abundance and the 15N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N2 fixation at the landscape scale. Whereas soil available water capacity, available NHinf4sup+, total crop yield, and percent N derived from N2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NOinf3sup-levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NHinf4sup+but not with available soil water, pH, electrical conductivity, NOinf3sup-, or particle size. Estimates of the % Ndfa using enriched 15N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the δ 15N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched 15N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched 15N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N2 fixation, large variations in N2 fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N2 fixation.

Metabolic Changes Associated with the Diurnal Pattern of N2 Fixation in Gloeothece

When grown in alternating cycles of light and darkness, non-synchronous cultures of Gloeothece fixed N2 mainly in the dark phase. This diurnal pattern of N2 fixation was independent of the doubling time (69 ± 16 h) of the organism and, since cell division was asynchronous, N2 fixation does not seem to be confined to a specific phase in the cell cycle. Fluctuations in the rate of N2 fixation coincided with similar fluctuations in the rate of nitrogenase synthesis. Diurnal fluctuations also occurred in the utilization of glucan and acid-soluble polyphosphate and in the ADP/ATP ratio. Based on these observations, it is proposed that specific metabolic changes are involved in the regulation of the diurnal pattern of N2 fixation by Gloeothece.

In situ H2 production and utilization by natural populations of N2-fixing blue-green algae

Contemporaneous in situ acetylene-reduction, 15N2-fixation, and 3H2-exchange assays reveal parallel patterns of N2 fixation and H2 utilization in natural populations of the blue-green algae Anabaena and Aphanizomenon. As spring and summer blooms progress, increasing ratios of acetylene reduction versus 15N2 fixation closely follow elevated rates of cellular H2 utilization. Both acetylene-reduction and H2-utilization rates were largely attributable to blue-green algae as opposed to associated bacteria and other phytoplankton. It is concluded that elevated H2 utilization reflects increased H2 production via nitrogenase. This can be substantiated by monitoring rising acetylene-reduction versus 15N2-fixation ratios during bloom development. Simultaneous deployment of the above techniques provides evidence for (a) in situ H2 production and (b) seasonal trends in rates of H2 production among natural blue-green algal populations.