Linking microbes to in situ methane oxidation rates in a eutrophic freshwater lake

Jennifer A Baily^1,2Zachary W Hudspeth³Joshua L Morningstar³Howard P Mendlovitz³Christopher S Martens³Karen G Lloyd^1,2*

• ¹Department of Microbiology, The University of Tennessee, Knoxville, Knoxville, United States
• ²Department of Earth Sciences, University of Southern California, Los Angeles, United States
• ³Department of Earth, Marine and Environmental Sciences, University of North Carolina at Chapel Hill, Chapel Hill, United States

Aerobic methanotrophs and nonmethanotrophic methylotrophs drive methane cycling in oxic freshwater lakes. Most knowledge about biological aerobic methane oxidation (MOx) comes from ex situ rate experiments, laboratory cultures, and static measurements of natural abundances. We investigated the link between MOx rate constants measured with a novel in situ incubation device and the microbial community in Jordan Lake, a methane-rich freshwater lake in NC, USA. We coupled relative abundances of 16S rRNA genes and quantitative PCR of particulate methane monooxygenase subunit A (pmoA) to methane, oxygen, temperature, and in situ MOx rate constants, all collected using the novel iBag in situ incubation system. In 16 incubations spread across 13 months, Methylococcaceae, whose cultured members are obligate aerobic methanotrophs, strongly and inversely correlate with naturally-varying oxygen but not with methane. Nonmethanotrophic methylotrophs and facultative aerobic methanotrophs are more abundant (up to 15.4% of amplicons), but do not correlate with either dissolved gas. Methylococcaceae correlate better than all other families in the methane-oxidizing community with the first-order MOx rate constants obtained from the in situ incubation data. Changes in the methane-oxidizing community across incubations were inconsistent between experiments but replicable within parallel incubations. The lack of response of the methanotrophic community to ammonium and organic carbon additions suggest these are not limiting. Our results suggest Methylococcaceae primarily drive MOx in Jordan lake, despite often not being the most abundant methanotrophic group, and that high oxygen concentrations may suppress this group independently of their association with lower methane concentrations.