Biofilm Mediated Mineral Weathering in Response to Nutrient Stress.

Deficiency of nutrient cations is a fundamental environmental stress limiting primary production in many ecosystems. In nature, the development of rhizospheric plant-microbe associations is thought to be an adaptation to such stress by putting in place the machinery of “plant-driven chemical weathering”. This is the principal means by which ecosystems extract mineral-derived nutrients and build nutrient capital over the course of succession. To investigate how a bacterial biofilm responds to nutrient stress, a strain of Pseudomonas putida was isolated from the rhizosphere of a white pine (Pinus strobus) seedling and assayed for weathering ability. The bacterium was grown in a biofilm drip-flow reactor on biotite or glass coupons with media either deficient or replete with Fe. The hypothesis was that Fe-deficiency, defined as concentrations below 10 µM, would enhance biofilm weathering of the biotite which contains the nutritive cations Fe, Mg and K. Scanning electron microscopy (SEM) revealed that after biofilm growth the small, easy-to-access mineral particles on biotite surfaces were dissolved away, and sharp edges of score marks on the mineral were softened. Biofilms were acid digested after growing on biotite or glass and cation concentrations of Fe, Mg, and K were measured. Higher concentrations of each cation were observed when biofilms were grown on biotite as compared to those formed on glass. High-resolution Magic Angle Spinning proton nuclear magnetic resonance (HRMAS ¹H NMR) spectroscopy revealed nearly twice as much polysaccharide in the biofilm matrix grown under Fe-deficient conditions when grown on biotite. Confocal laser scanning microscopy (CLSM) and attenuated total reflectance Fourier transformed infrared (ATR FT-IR) spectroscopy revealed the presence of alginate in the matrix of biofilms grown on biotite with or without Fe supplementation. However, increased amounts of alginate and alginate capsules were present in biofilms formed under Fe depletion. When grown on biotite without Fe supplementation, compared to Fe-replete conditions, the biofilm became a more effective cation sink for all three weathered nutrients. Alginate has a high concentration of carboxylic acid functional groups and when present in greater quantities, it confers increased cation holding capacity in the biofilm matrix. Thus the present study provides mechanistic clues into how the nutrient-stress response of common rhizospheric associates could alter the nutrient-extractive and -storage properties of biofilms associated with plant root systems.