Abstract
Nitrogen cycling microbes, including nitrite-oxidizing bacteria (NOB), perform critical ecosystem functions that help mitigate anthropogenic stresses and maintain ecosystem health. Activity of these beneficial nitrogen cycling microbes is dictated in part by the microorganisms’ response to physicochemical conditions, such as temperature, pH, and nutrient availability. NOB from the newly described Candidatus Nitrotoga genus have been detected in a wide range of habitats across the globe, yet only a few organisms within the genus have been physiologically characterized. For freshwater systems where NOB are critical for supporting aquatic life, Ca. Nitrotoga have been previously detected but little is known about the physiological potential of these organisms or their response to changing environmental conditions. Here, we determined functional response to environmental change for a representative freshwater species of Ca. Nitrotoga (Ca. Nitrotoga sp. CP45, enriched from a Colorado river). The physiological findings demonstrated that CP45 maintained nitrite oxidation at pH levels of 5–8, at temperatures from 4 to 28°C, and when incubated in the dark. Light exposure and elevated temperature (30°C) completely halted nitrite oxidation. Ca. Nitrotoga sp. CP45 maintained nitrite oxidation upon exposure to four different antibiotics, and potential rates of nitrite oxidation by river sediment communities were also resilient to antibiotic stress. We explored the Ca. Nitrotoga sp. CP45 genome to make predictions about adaptations to enable survival under specific conditions. Overall, these results contribute to our understanding of the versatility of a representative freshwater Ca. Nitrotoga sp. Identifying the specific environmental conditions that maximize NOB metabolic rates may ultimately direct future management decisions aimed at restoring impacted systems.
Highlights
Nitrite-oxidizing bacteria (NOB) play fundamental roles in maintaining the health and resilience of freshwater habitats by regulating nitrogen transformations
CP45 16S rRNA gene sequences were highly conserved with other Ca
CP45 could likely maintain nitrite oxidation at much higher levels of antibiotic pollution in the river. We extended these culture studies to evaluate the impact of antibiotics on nitrite oxidation by naturally occurring microbes in sediments at three sites with varying land-use patterns in the South Platte River Basin (Figure 4): SPCC, influenced primarily by wastewater treatment plants (WWTPs) effluent; CP45, influenced primarily by runoff from animal feeding operations (AFO); and SPKER with a mixed landuse of urban and agriculture
Summary
Nitrite-oxidizing bacteria (NOB) play fundamental roles in maintaining the health and resilience of freshwater habitats by regulating nitrogen transformations. NOB are responsible for the second step of nitrification (oxidizing nitrite to nitrate) and have three key impacts on aquatic habitats. They are responsible for the formation of nitrate, which provides a critical nitrogen source for Freshwater Ca. Nitrotoga Physiology microbial and plant assimilation. They provide a substrate for denitrification, which results in the formation of gaseous dinitrogen for evolution out of the aquatic habitat. They reduce nitrite toxicity to fish and other aquatic organisms (and indirectly reduce ammonia toxicity by consuming the endproduct of microbial ammonia oxidation). If nitrite oxidation rates are reduced or halted due to environmental perturbation, nitrite concentrations will increase and negative effects, such as toxicity, hypoxia, and loss of biodiversity, may propagate through the ecosystem
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