Effect of Predation by Colpoda sp. in Nitrogen Fixation Rate of Two Free-Living Bacteria

Abstract Biological nitrogen fixation (BNF) is limited to several groups of prokaryotes, which can reduce nitrogen through complex endosymbiotic relationships or as free-living nitrogen-fixing bacteria (FLNFB). Predation of FLNFB by protozoa releases reduced nitrogen, enhancing the formation of plant and bacterial biomass as well as nitrogen (N) mineralization within soil microbial communities. We aim to evaluate the predation effect of Colpoda sp. on two FLNFB Azospirillum lipoferum and Stenotrophomonas sp. during their exponential and lag phase. The likelihood of Colpoda sp. to feed on the former species was needed to ensure there is a predation effect. The kinetics of bacterial population growth was determined in the predators’ presence or absence and the effect of predation on the biological fixation of N was evaluated through the reduction of acetylene to ethylene technique. Colpoda sp. showed a non-significant difference in preferences between the two species offered as prey. Consequently, the abundance of A. lipoferum and Stenotrophomonas sp. decreased significantly due to predator’s pressure. However, it had a higher positive effect on the formation of new bacterial biomass on Stenotrophomonas sp.as revealed by the increase of its specific growth rate. Likewise, predation promoted greater nitrogen fixation in A. lipoferum and Stenotrophomonas sp. during the lag phase (0.34 nM and 0.38 nM) than in the exponential phase (0.27 nM and 0.17 nM). We concluded that predation by Colpodasp stimulates the rate of nitrogen fixation of A. lipoferum and Stenotrophomonas sp.