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

## TL;DR

This study links specific microbes to methane oxidation rates in a freshwater lake using a new in situ incubation method.

## Contribution

A novel in situ incubation device (iBag) was used to measure methane oxidation rates and link them to microbial communities.

## Key findings

- Methylococcaceae strongly correlate with in situ methane oxidation rates despite not always being the most abundant group.
- Non-methanotrophic methylotrophs and facultative methanotrophs are more abundant but do not correlate with methane or oxygen.
- High oxygen levels may suppress Methylococcaceae activity independently of methane concentration.

## Abstract

Aerobic methanotrophs and non-methanotrophic 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. Non-methanotrophic 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.