Abstract
The conditions present in both in vitro and in vivo ecosystems determine the microbial population harbouring it. One commonly accepted theory is that a species with a high substrate affinity and low growth rate (k-strategist) will win the competition against a second species with a lower substrate affinity and higher growth rate (r-strategist) if both species are subjected to low substrate concentrations. In this study two nitrite oxidizing bacteria (NOB), Nitrospira defluvii (k-strategist) and Nitrobacter vulgaris (r-strategist), were cultivated in a continuous reactor systems. The minimal hydraulic retention time (HRT) required for maintaining the slower growing Nitrospira was first determined. A reactor containing Nitrobacter was set to the same HRT and Nitrospira was injected to evaluate the effect of the dilution rate on the competition between both species. By following the microbial population dynamics with qPCR analysis, it was shown that not only the substrate affinity drives the competition between k- and r-strategists but also the dilution rate. Experimental data and numerical simulations both revealed that the washout of Nitrobacter was significantly delayed at dilution rates close to the μmax of Nitrospira. The competition could be even reverted towards Nitrobacter (r-strategist) despite of low nitrite concentrations and dilution rates lower than the μmax of Nitrospira.
Highlights
In microbiology the competition of two species for one substrate is driven by their substrate affinity and their doubling time [1, 2]
Since the knowledge of maximum growth rate of Nitrospira defluvii was highly important for this experiment, its value was estimated by determining the minimal hydraulic retention time before washout occurred, by gradually increasing the flow rate (S5 Table)
The results presented here demonstrate that the dilution rate is of crucial importance regarding the competition between k and r strategists (Figs 1 and 2)
Summary
In microbiology the competition of two species for one substrate is driven by their substrate affinity and their doubling time [1, 2]. An example is given by two common genera of nitrite oxidizing bacteria (NOB), namely Nitrospira (k-strategist) and Nitrobacter (r-strategist). Nitrospira possess a low maximum specific growth rate, but is well-adapted to low nitrite concentrations, whereas Nitrobacter is known to be a relatively fast-growing NOB with low affinities to nitrite [3, 4]. Since Nitrospira spp. and Nitrobacter spp. both use nitrite as substrate for growth, only the most adapted species will win the competition for nitrite and will outcompete the other one [5, 6]. NOB are vital organisms in the global nitrogen cycle [7] and play.
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