Methane Oxidation and the Competition for Oxygen in the Rice Rhizosphere

A mechanistic approach is presented to describe oxidation of the greenhouse gas methane in the rice rhizosphere of flooded paddies by obligate methanotrophic bacteria. In flooded rice paddies these methanotrophs compete for available O2 with other types of bacteria. Soil incubation studies and most-probablenumber (MPN) counts of oxygen consumers show that microbial oxygen consumption rates were dominated by heterotrophic and methanotrophic respiration. MPN counts of methanotrophs showed large spatial and temporal variability. The most abundant methanotrophs (a Methylocystis sp.) and heterotrophs (a Pseudomonas sp. and a Rhodococcus sp.) were isolated and characterized. Growth dynamics of these bacteria under carbon and oxygen limitations are presented. Theoretical calculations based on measured growth dynamics show that methanotrophs were only able to outcompete heterotrophs at low oxygen concentrations (frequently <5 mM). The oxygen concentration at which methanotrophs won the competition from heterotrophs did not depend on methane concentration, but it was highly affected by organic carbon concentrations in the paddy soil. Methane oxidation was severely inhibited at high acetate concentrations. This is in accordance with competition experiments between Pseudomonas spp. and Methylocystis spp. carried out at different oxygen and carbon concentrations. Likely, methane oxidation mainly occurs at microaerophilic and low-acetate conditions and thus not directly at the root surface. Acetate and oxygen concentrations in the rice rhizosphere are in the critical range for methane oxidation, and a high variability in methane oxidation rates is thus expected. Rice paddies are an important source of the greenhouse gas methane (33). The magnitude of methane emission from rice paddies reflects the balance between methanogenesis and methanotrophy. Methane oxidation occurs at anaerobic-aerobic interfaces with available oxygen and methane: the soilwater interface and the rice rhizosphere. At the soil-water interface, methane oxidation efficiencies are fairly constant at 70 to 95% of the transported methane (21, 25). Estimates of methane oxidation in the rice rhizosphere are much more variable. They range from 7 to 90% of the transported methane (17, 21, 25, 32) and still vary from 7 to 52% if only data obtained from specific inhibitor studies are included. A better mechanistic understanding of rice rhizospheric methane oxidation is important to be able predict methane emissions from rice paddies. Obligate methanotrophic microorganisms carry out methane oxidation. In freshwater wetlands, high-affinity methanotrophy (3) does not have to be considered due to the high methane concentrations (53) and neither does anaerobic methane oxidation (60). Nitrifiers are also of little importance to methane oxidation in the rice rhizosphere (7). Methanotrophic activity is thus determined by oxygen and methane concentrations. Methanotrophs in the rice rhizosphere do not have to compete for methane with microbial or chemical competitors, although there is a strong sink of methane by methane transport. However, intensive competition for oxygen occurs. To understand the importance of methane oxidation in the rice rhizosphere, the competition for oxygen needs to be quantified. Important oxygen sinks are plant respiration, chemical oxidation, and microbial oxidation. This paper gradually addresses more-detailed questions. After oxygen sinks in rice paddies are quantified, the isolation and characterization of the most abundant microbial oxygen consumers in this system are described. Their growth kinetics in relation to oxygen and carbon substrate concentrations is studied. Finally, experiments concerning the competition for oxygen between these organisms and theoretical considerations of competition for oxygen are presented.