Evidence for NH4+ switch-off regulation of nitrogenase activity by bacteria in salt marsh sediments and roots of the grass Spartina alterniflora

Abstract

The regulatory effect of NH4' on nitrogen fixation in a Sparlina alterniflora salt marsh was examined. Acetylene reduction activity (ARA) measured in situ was only partially inhibited by NH4' in both the light and dark after 2 h. In vitro analysis of bulk sediment divided into sediment particles, live and dead roots, and rhizomes showed that microbes associated with sediment and dead roots have a great potential for anaerobic C2H2 reduction, but only if amended with a carbon source such as mannose. Only live roots had significant rates of ARA without an added carbon source. In sediment, N2-fixing mannose enrichment cultures could be distinguished from those enriched by lactate in that only the latter were rapidly inhibited by NH4'. Ammonia also inhibited ARA in dead and live roots and in surface-sterilized roots. The rate of this inhibition appeared to be too rapid to be attributed to the repression and subsequent dilution of nitrogenase. The kinetic characteristics of this inhibition and its prevention in root-associated microbes by methionine sulfoximine are consistent with the N14' switch-off-switch-on mechanism of nitrogenase regulation.

Long‐term nitrogen and phosphorus fertilization effects on N2 fixation rates and nifH gene community patterns in mangrove sediments

“…However, mangrove plants in our study area are primarily P-limited (Feller et al 2002) and therefore trees may not strongly promote N 2 fixation in N-fertilized areas. The NH þ 4 porewater concentration observed in the N-treatment (81.7 ± 1.7 lmol) is lower than the concentration known to strongly inhibit sediment diazotrophs ($100-500 lm, Yoch & Whiting 1986;Capone 1988), which may help explain the observed active diazotrophic community under N-fertilized conditions in Belizean mangrove sediments. Nevertheless, rates in N-fertilized areas are lower than in controls.…”

Nitrogen Assessments in a Constructed and a Natural Salt Marsh of San Diego Bay

“…In the deep cores this effect of organic matter was possibly masked by the relatively high levels of soil ammonium in the natural marsh. Ammonium has been shown to inhibit nitrogen fixation in salt marsh sediments and in microfiora associated with the rhizosphere , Casselman et al 1981, Yoch and Whiting 1986. The high pore water (Table 1) and extractable ammonium values found in PC (Table 2) support this explanation.…”

Stability of a rhizosphere microbial community exposed to natural and manipulated environmental variability

“…The top 10 cm of each core was sectioned into 5-cm subcores and wet sieved to separate roots and rhizomes from sediment. Live versus dead roots and rhizomes were distinguished on the basis of turgor and general appearance [26]. Live root material was collected and roots and rhizomes separated, dried, and weighed.…”