Pseudomonas mendocina: Metabolic Versatility in Bioremediation and Metal Mobilization

Versatile Carbon Metabolism and Aromatic Compound Degradation. Like other members of its genus, Pseudomonas mendocina exhibits a remarkable aptitude for carbon metabolism, capable of utilizing over 75 different substrates as sole carbon sources [11]. This metabolic flexibility includes the degradation of challenging aromatic and man-made pollutants, such as toluene and chlorophenols. The species employs unique biochemical pathways to process these compounds, making it a subject of significant interest for environmental biotechnology and bioremediation applications. Its ability to thrive on such a diverse menu of carbon sources underscores its genetic adaptability and ecological success.

The degradation of toluene by certain P. mendocina strains occurs via a distinct, oxygen-dependent pathway that culminates in the production of β-carboxy-cis-cis-muconic acid (Figure 3). This pathway is different from two other well-known toluene degradation systems: the TOL pathway, which begins with oxidation of the methyl group to form benzoic acid, and the TOD pathway, which initiates with a ring-opening dioxygenation. The unique output of the P. mendocina pathway is significant because muconic acid is a valuable platform chemical that can be used as a feedstock for producing bioplastics [21, 22]. This positions the bacterium as a potential agent for both cleaning up aromatic hydrocarbon pollution and converting complex polyaromatic hydrocarbons, like plant lignins, into valuable synthetic precursors [23].

Figure 3. Pathway for complete dissimilation of toluene by P. mendocina KR1. The enzymes involved at steps 1,4, and 6 have been characterized, and are toluene-4-monooxygenase,p-hydroxybenzaldehyde dehydrogenase, and protocatechuate-3,4-dioxygenase, respectively. Source: Adapted from Ref. [20]

Furthermore, P. mendocina demonstrates an ability to tackle halogenated aromatic compounds, a particularly resilient class of environmental pollutants. Specific strains have been documented to degrade pentachlorophenol, a toxic and persistent biocide [25-27]. While the precise enzymatic mechanisms in P. mendocina remain to be fully elucidated, this activity places it among other versatile Pseudomonas species known for processing halogenated molecules through diverse metabolic strategies [24]. This capability highlights its potential utility in treating sites contaminated with complex industrial chemicals.

Metal Resistance and Sophisticated Iron Acquisition Strategies. Beyond organic pollutant degradation, P. mendocina exhibits a natural resistance to high levels of transition metals and a pronounced ability to mobilize metals from solid minerals. This is evidenced by strains isolated from metal-contaminated environments, such as S5.2 from a copper-polluted vineyard in France. Genomic sequencing of strain S5.2 confirmed the presence of numerous heavy metal transporters and detoxification enzymes, providing a genetic basis for its observed metal tolerance [16]. These traits are critical for survival in polluted niches and are key for applications in phytoremediation and metal bio-recovery.

A particularly well-studied strain, P. mendocina ymp, was isolated from the Yucca Mountain Project site, a proposed nuclear waste repository. Research focused on this strain aimed to understand how soil bacteria mobilize essential nutritional metals, primarily iron, from mineral sources in arid environments. The concern was that this microbial activity could potentially facilitate the co-transport of radionuclides over long timescales. Consequently, a comprehensive series of studies examined the iron acquisition mechanisms of strain ymp [12, 13, 28-41].

Initial investigations revealed that strain ymp could grow using various ferric oxide minerals—hematite, goethite, and ferrihydrite—as its sole iron source, with growth rates approaching those on soluble iron complexes [38, 40, 41]. This indicated highly effective mechanisms for mineral iron acquisition. Subsequent research documented a multi-faceted strategy: the bacteria adhere to mineral surfaces [40-42], produce non-fluorescent siderophores [13, 34, 35, 38], generate reducing agents [30, 43], and form alginate-rich biofilms that encapsulate both the cells and mineral particles [28]. The production of alginate and related extracellular polysaccharides is a trait observed in other P. mendocina strains as well, contributing to their overall metal mobilization capability [15, 44].

Mechanisms of Mineral Dissolution and Nanomaterial Interactions. The bioavailability of iron to strain ymp was systematically assessed using a wide array of well-characterized natural and synthetic minerals. When normalized for surface area, different iron oxide minerals (hematite, goethite, ferrihydrite) showed varying levels of bioavailability, suggesting that mineral structure, not just solubility, influences bacterial dissolution [32]. Parallel experiments with kaolinite clays demonstrated that iron bioavailability correlated with the clay's accessibility to oxalic acid, implying that the interaction of siderophores or other metabolites with the mineral surface is critical for mobilization [29].

Further research using a siderophore-free mutant of strain ymp grown on hematite clarified the roles of different organic agents. It was concluded that even a tiny concentration (1 pM) of exogenous siderophore could restore normal growth, indicating that the wild-type organism massively overproduces siderophores in response to the mineral iron source [12]. These studies also revealed a synergistic effect between organic acids like oxalate and very low levels (0.1 pM) of siderophores, though the acids had little effect alone. This sophisticated, multi-mechanism approach ensures efficient iron acquisition from diverse sources.

When the particle size of hematite was reduced to the nanoscale, a significant increase in iron bioavailability was observed, even for the siderophore-deficient mutant [36]. This enhanced bioavailability was dependent on direct bacterial access to the mineral surface, requiring adherence, and transmission electron microscopy confirmed the penetration of nanoparticles through the bacterial outer membrane. These findings conclusively demonstrate that P. mendocina ymp employs a versatile suite of strategies, including both siderophore-dependent and siderophore-independent mechanisms, to acquire iron from a vast spectrum of mineral types, from macroscopic crystals to nano-sized particles.