Synergistic Plant Biostimulatory Effects of an Inter-Kingdom Interaction: Chlorella sp. and Kocuria rhizophila Algal–Bacterial Co-Culture for Sustainable Crop Production

Plant biostimulatory effects of the green alga <i>Chlorella</i> sp. MACC-360, the <i>Kocuria rhizophila</i> FSP120 bacterial strain, and the combined inter-kingdom co-culture of the alga and bacterium were investigated using <i>Solanum lycopersicum</i> as a model plant grown under controlled greenhouse conditions. The application of algal-bacterial co-cultures using the soil drench method significantly improved plant growth parameters, vegetative biomass yield, fruit yield, and photosynthetic performance of the tomato plants. The combined treatment resulted in a 43.7% increase in mean fruit yield, while individual applications of <i>K. rhizophila</i> FSP120 and <i>Chlorella</i> sp. MACC-360 enhanced yields by 30.85% and 19.44%, respectively. Although total yield increases did not reach statistical significance due to high intra-group variability, the treatment's efficacy was statistically confirmed through key yield parameters including significantly higher fruit weight and fruit diameter (<i>p</i> < 0.05). The enhanced specific biostimulatory effects of the combined treatment could be at least partly attributed to the increased level of algal extracellular polymeric substances (EPS), which was a specific effect of algal co-cultivation with a <i>Kocuria rhizophila</i> bacterium. Detailed analysis of plant phenotypic alterations, biomass yield, fruit and flowering parameters, as well as microbial community analysis of the rhizosphere, were conducted and compared among the various treatments. Our results indicate that an appropriately chosen combination and application of biostimulatory microbes can significantly enhance crop production, which might contribute to more sustainable agriculture.