Lichenized unicellular cyanobacteria fix nitrogen in the light

Diurnal variation in N2-fixation (acetylene reduction) rate was measured in Thyrea girardii (Durieu & Mont.) Bagl. & Carestia and Thyrea confusa Henssen, lichens containing a unicellular cyanobacterial photobiont. In field assays, mean acetylene-reduction rates in the light were 25.8 ± 8.9 (n = 11) and 21.0 ± 5.6 nmol C2H4·g–1·h–1 (n = 13) for T. girardii and T. confusa, respectively, and the respective mean rates in the dark were 8.2 ± 1.8 (n = 26) and 13.5 ± 5.4 (n = 8) nmol C2H4·g–1·h–1. In laboratory assays under relatively isothermal conditions (ca. 19–22 °C), the maximum acetylene reduction rate (52.0 ± 6.0 nmol C2H4·g–1·h–1) was recorded in the light and the minimum rate (20.2 ± 6.0 nmol C2H4·g–1·h–1) in the dark. This diurnal pattern is contrary to expectations for unicellular cyanobacteria. We suggest carbon flow to the fungal symbiont reduces the potential for nitrogenase activity in the dark.

Identification of North American soybean lines that form nitrogen-fixing nodules with Rhizobium fredii USDA257

Rhizobium fredii produces nitrogen-fixing (Fix+) nodules on primitive soybean lines, but most strains do not form such structures with the small number of agronomically advanced lines that have been tested. We systematically evaluated the ability of R. fredii USDA257 to produce Fix+ nodules on 197 soybean lines available in the midwestern United States. Thirty-four of 197 such lines were Fix+. The frequency of this response was positively correlated with increasing maturity group. The acetylene-reduction rate of one advanced cultivar, Davis, was greater than that of the primitive cultivar, Peking. Our data indicate that the capacity to nodulate effectively with USDA257 is widespread in contemporary North American soybean lines.Key words: Acetylene-reduction, nodulation, Rhizobium, soybean

Nodule biomass and acetylene reduction rates of red alder and Sitka alder on Vancouver Island, B.C.

Three comparisons examined acetylene reduction rate and nodule biomass of red alder (Alnusrubra Bong.) and Sitka alder (Alnussinuata (Regel) Rydb.) on Vancouver Island, British Columbia. The first showed that the acetylene reduction rates of Sitka alder on four sites in early July 1979 varied from 8.8 to 22.0 μmol C2H2•g−1 dry nodule per hour; smaller nodules had the higher rates. In the second comparison, red alder and Sitka alder acetylene reduction rates for young plants were similar when assayed on the same day on the same site. Finally, acetylene reduction rates and nodule biomass were determined for both species growing in closed canopy stands on adjacent sites. In this pair of 20-year-old stands, red alder had double the acetylene reduction rate and three times the nodule biomass of the Sitka alder. Combining the nodule biomass estimates with the seasonal average reduction rates yielded current annual nitrogen fixation estimates (using C2H2) of 130 kg•ha−1•year−1 for the red alder plot and 20 kg•ha−1•year−1 for the Sitka alder plot. These estimates are within published ranges for the species. The lower fixation rate of Sitka alder is still substantial, relative to the nitrogen demands of conifers, and its shrub-like growth form makes it an attractive management alternative to red alder for biological nitrogen fixation in mixed conifer–alder plantations.

Nitrogen fixation and methane metabolism in a stream sediment–water system amended with leaf material

The reduction of acetylene and the production and oxidation of methane in a stream sediment–water system amended with either fresh leaves or autumn-shed leaves in the presence and absence of air were studied. Net methane production by the sediment–water system occurred only when leaf material was added, with fresh leaves giving 2.2 times the methane accumulation as autumn-shed leaves. Static incubation in the presence of air had little effect on net methane production, with such production being about the same as (with fresh leaves) or 79% of (with autumn-shed leaves) the anaerobic rates. Acetylene reduction was more affected by the presence of air, with anaerobic rates being at very low levels. The presence of fresh leaves was again stimulatory, with the acetylene reduction rate being 5.5-fold greater than that occurring with autumn-shed leaves.

Nitrogen fixation associated with Scirpus atrovirens and other nonnodulated plants in Massachusetts

Nitrogen fixation was measured using the acetylene reduction technique in soil cores of 13 nonnodulated, herbaceous plant species growing in mesic and wetland habitats. Six species that grew in a well-drained portion of an old field exhibited low rates of N2 fixation (7 g N∙ha−1∙day−1). A bulrush, Scirpus atrovirens, which grew in an area of compacted soil in the old field, showed a moderate rate of N2 fixation (30–100 g N∙ha−1∙day−1). These old field species exhibited a lag of only a few hours before a constant rate of acetylene reduction occurred. The estimated rates for the six wetland species were generally higher, and there was a lag such that the activity was still increasing after 20 h of incubation. N2 fixation in cores of S. atrovirens was primarily associated with the roots and adhering soil. Incubation of the cores in N2 for 24 h caused a fourfold increase in the acetylene reduction rate over that in air, although the rates during the first 4 h were similar.