Production of a Microbial Biofilm and Its Application on Tomato Seeds to Improve Crop Development in a Lead-Contaminated Substrate

The plant growth-promoting bacterium, B. subtilis subsp. Spizizizenii, has been proven to develop a biofilm under certain culture conditions, which can be applied as an efficient bioinoculant. Biofilm can be produced cost-effectively using biodiesel byproduct glycerol as a carbon source. Soils from urban peripheries may contain very high lead (Pb) levels. The main aim of this study was to assess the impact of biofilm seed inoculation on plant development and fruit quality of tomatoes growing on a Pb-contaminated substrate. Also, effects of excess Pb on biofilm production, stability, and seed germination were analyzed. B. subtilis biofilm was produced with Pb concentrations ranging from 0 to 300 ppm. Biofilm stability was tested at 4 °C and 25 °C. The impacts of Pb and inoculation on seed germination were evaluated in laboratory conditions, while the impacts on plant agronomic parameters were assessed via a greenhouse assay. Adding Pb to the culture medium increased biofilm production by about 20%. Regardless of Pb level, biofilms were more stable at 4 °C than at 25 °C. Beneficial effects of biofilm on germination were greater on seeds exposed to 200 and 300 ppm Pb. Excess Pb significantly reduced plant biomass and tomato yield. However, biofilm inoculation significantly increased plant aboveground and root biomass, plant height, leaf area, fruit number, and fruit size, regardless of substrate Pb excess. Tomato fruits of plants grown in the metal-contaminated substrate showed no significant increases in Pb concentration with respect to the control. In summary, the biofilm produced by B. subtilis subsp. spizizenii proved to be an effective bioinoculant to counteract the negative effects of substrate excess Pb on tomato germination, growth, and production.