Phenazines

Phenazines are redox-active metabolites that are produced by certain soil bacteria and play an important role in the chemistry and ecology of the pedosphere. The compounds have been found across habitats and are especially abundant in root zones. Their redox activity makes them very reactive, and although they are found in relatively low concentrations, they have a disproportionate impact on the microbial ecology of the rhizosphere. Producer bacteria use phenazines to effectively compete with other soil microbes and acquire inorganic nutrients, which are processes that could be harnessed for sustainable agricultural practices in the future.

Biosynthesis and Chemistry

Redox-active metabolites (RAMs) can be generally defined as ‘natural byproducts’ or ‘secondary metabolites’ of soil bacteria. Although they are produced intracellularly and support cellular redox balancing, they are also secreted and may react extracellularly in the soil. They accept and donate electrons to various soil constituents, hence why they are called ‘redox-active.'

Phenazines, in particular, are heterotricyclic nitrogen-containing metabolites. They are produced by Actinobacteria and Proteobacteria, including the genera Pseudomonas spp., Streptomyces spp., and Pantoea agglomerans. All producer genomes contain the core biosynthesis genes phzA/BCDEFG that synthesize one of the two main phenazines which are precursors for all other derivatives: phenazine-1-carboxylic acid (PCA) in pseudomonads and phenazine-1,6-dicarboxylic acid (PDC) in most other species. . Auxiliary genes modify the core structure of these parent compounds to create different daughter phenazines with varying redox potentials and solubilities, ultimately affecting their beneficial function and toxicity. The variety of functional groups gives rise to a full spectrum of colors as well. For instance, 5-methyl-7-amino-1-carboxyphenazinium betaine (aeruginosin A) is deep red, PCA is lemon yellow, 1-hydroxy-5-methylphenazine (pyocyanin, PYO) is bright blue, 2-hydroxyphenazine-1-carboxylic acid (2-OHPCA) is bright orange, and 1-hydroxyphenazine (1-OHPHZ) is bright green.

The chemistry and toxicity of phenazines is dictated by the chemical microenvironment in the soil, namely oxygen, pH, and moisture. For instance, at low pH, PCA is uncharged and more cell permeable, ultimately making it more toxic. At high oxygen levels, PCA reacts with oxygen to create toxic reactive oxygen species (ROS). In anoxic environments, PCA reacts with ferric iron minerals and produces ferrous iron through the process of reductive dissolution which may help solubilize mineral-adsorbed phosphorous.