Protozoa-bacteria interactions enhance nitrogen and phosphorus release in a simulated liquid culture: A proof-of-concept study with Colpoda cf. grandis and Priestia aryabhattai

Black soils in Northeast China are increasingly challenged by reduced nutrient bioavailability due to long-term continuous cropping and excessive chemical fertilization. Protozoa-bacteria interactions are recognized as key drivers of soil nutrient cycling, yet quantitative data on the simultaneous release of nitrogen and phosphorus by specific protozoan-bacterial pairs remain scarce. Here, we isolated a ciliate ( Colpoda cf. grandis ) and a bacterium ( Priestia aryabhattai ) from sugar beet rhizosphere and established a simulated liquid co-culture system to investigate their interactions over 96 h. Co-culture increased ammonium nitrogen and total phosphorus by 882% and 468% relative to initial levels, and by 37.8% and 455% compared to the bacterium-only treatment, respectively. The apparent grazing rate of Colpoda cf. grandis on Priestia aryabhattai was estimated at 2588 bacterial cells per protozoan per hour. An unexplained nitrate increase was observed in the protozoa-only treatment at 96 h; however, its source (biotic vs. abiotic) could not be resolved in this study. Co-culture significantly elevated intracellular activities of glutaminase, glutamate dehydrogenase, acid phosphatase, and alkaline phosphatase in Colpoda cf. grandis , and increased the fluorescence intensities of tryptophan-like and humic-like substances. Phosphatase gene expression did not parallel enzyme activity trends, suggesting possible post-transcriptional or environmental modulation. The Colpoda cf. grandis - Priestia aryabhattai interaction promotes the release of inorganic nitrogen and phosphorus in liquid culture. These findings provide quantitative mechanistic insights into protozoan-bacterial nutrient dynamics and identify a candidate consortium for further evaluation as a soil bio-amendment. However, field validation in real soil systems is necessary before application for black soil management can be considered.