N 2 ‐fixing pseudomonads and related soil bacteria

Pseudomonas ‐like organisms form a highly heterogeneous and ubiquitous group of bacteria. Recent identification of an authentic Pseudomonas genus should help to decrease difficulties which arose from taxonomic uncertainties. Further difficulties in recognising N 2 ‐fixing Pseudomonas species lie in the confirmation of diazotrophy. Optimised conditions are often needed for the detection of nitrogenase activity since it is controlled by specific environmental factors and physiological requirements. However, genetically constructed N 2 ‐fixing strains from authentic Pseudomonas species have demonstrated that at least some members of the genus possess mechanisms to accommodate and express nif (N 2 fixation) genes from a well‐studied diazotroph, Klebsiella pneumoniae . Renowned for their catabolic versatility, pseudomonads can use a wide range of carbon and energy substrates. Hence, potential N 2 ‐fixing pseudomonads are conceivably less limited by carbon and energy sources available in the environment compared to other N 2 ‐fixing organisms. Pseudomonas species dominate in the rhizosphere of some plants from which isolates have been shown to be diazotrophic. Several strains are also chemolithotrophs, capable of using H 2 as energy and electron source and CO 2 as carbon source. Besides assays for N 2 ‐fixing activity, DNA hybridisation to the well conserved molybdo‐nitrogenase structural gene probe is an indicator of diazotrophy. However, absence of hybridisation to this probe, despite N 2 fixation activity, has been reported. Although the genetics of N 2 fixation in pseudomonads have hardly been studied, some nif genes have been shown to be plasmid‐borne. Pseudomonas species are also predominant soil denitrifiers, reducing nitrate and nitrite to gaseous forms of nitrogen during anaerobic respiration. Hence, they play an important role in the global biological nitrogen cycle. Several diazotrophic species including a few pseudomonads can also denitrify. The potential contribution by N 2 ‐fixing pseudomonads to the sinks and sources of soil nitrogen is considered small in the short term but essentially remains unclear in the absence of experimental data. Reliable rapid methods for their specific enumeration are indispensable for assessing their population dynamics and ascertaining their ecological significance.