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

Carlos Manuel Martínez-Reyes,Ma Remedios Mendoza-López,Alejandro Alarcón,Nathalie Cabirol,Salvador Rodríguez-Zaragoza

doi:10.1007/s00248-021-01813-9

Carlos Manuel Martínez-Reyes, Ma Remedios Mendoza-López + Show 3 more

Open Access

https://doi.org/10.1007/s00248-021-01813-9

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Abstract

Biological nitrogen fixation is limited to several groups of prokaryotes, some of them reduce nitrogen as free-living nitrogen-fixing bacteria. Protozoa predation on these latter releases sequestered nitrogen that may enhance the formation of new bacterial biomass and possibly increase nitrogen fixation within soil microbial communities. We aim to evaluate the predation effect of Colpoda sp. on two nitrogen fixers: Azospirillum lipoferum and Stenotrophomonas sp. during their lag, early exponential, and exponential phases. The kinetics of bacterial population growth was determined in the predators' presence or absence and the effect of predation on the rate of N fixation 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 and both species responded by increasing their specific growth rate. Likewise, predation promoted greater nitrogen fixation rate by CFU during the lag phase in A. lipoferum (0.20nM/CFU with predation vs 0.09nM/CFU without predation) and Stenotrophomonas sp. (0.22nM/CFU vs 0.09nM/CFU respectively). During early exponential phase (29h), the rate diminished to 0.13 and 0.05nM/CFU in A. lipoferum and to 0.09nM/CFU and 0.05nM/CFU in Stenotrophomonas sp. Finally, during the exponential phase (52h), only A. lipoferum without predation produced 0.003nM/CFU of ethylene. Thus, the nitrogenase activity was higher in the lag and the early exponential phases when predator activity was involved.

Highlights

Biological nitrogen fixation (BNF) is the process by which N2 is reduced to ammonia (NH3) through the enzyme nitrogenase, a crucial process to assure nitrogen availability in terrestrial ecosystems [1]
Colpoda sp doubled its population after 30 h of contact with the two bacteria separately and in the mixture of both (Figure 3)
Over A. lipoferum, as revealed by the difference in the number of trophozoites and cysts found in the respective wells

Summary

Introduction

Biological nitrogen fixation (BNF) is the process by which N2 is reduced to ammonia (NH3) through the enzyme nitrogenase, a crucial process to assure nitrogen availability in terrestrial ecosystems [1]. BNF can be endosymbiotic (i.e., with nodule formation) or reduced by bacteria in a free-living stage, defined as N fixation, which occurs without the formal endosymbiotic process between microorganisms and plants [2]. Soil productivity is based on a continuous mobilization of organic nitrogen, which can be a result of cell lysis, predatory debris, microbial biomass and the decomposition of nitrogenous components trapped in litter. All these activities may lead to N mineralization [10], while biological fixation means a net input of reduced N allowing the sustainability of the soil system [11]. Once the trophic networks of the soil ecosystem have been established, BFNVL provide the necessary nitrogen to cover losses due to volatilization and denitrification [11]

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