Anaerobic Oxidation of Methane in Sediments of Two Boreal Lakes

In this study, potential for Anaerobic oxidation of methane (AOM) coupled with metal reduction was studied in boreal lake sediments. Slurries of sediment samples collected from two sites in southeastern Finland, i.e. from Lake Orivesi, Heposelkä, an vegetated littoral site, dominated by Phragmites australis (Sample Sa, sediment layer 0 – 25 cm) and from the profundal zone of a mesotrophic Lake Ätäskö (Aa, 0 – 10 cm; Ab, 10 – 30 cm; Ac, 90 - 130 cm), were incubated in laboratory in anaerobic conditions at in situ temperatures for up to 5 months. The samples were amended either 1) with ¹³CH₄, 2) ¹³CH₄ + manganese(IV) oxide (MnO₂) or 3) ¹³CH₄ + iron(III) hydroxide (Fe(OH)₃), and the processes were measured by following the ¹³C transfer to the carbon dioxide (CO₂) pool and by concentration measurements of CH₄ and CO₂. Changes in microbial communities were studied from DNA extracted from sediment samples before and after incubation period by next-generation sequencing (Ion Torrent) of polymerase chain reaction (PCR) - amplified bacterial and archaeal 16S rRNA and methyl coenzyme-M reductase gene (mcrA) amplicons.

AOM took place in every sample except in deepest sample (Ac) with potential rates up to 2.1 nmol CH₄ d^-1 g^-1_wetsedim (~2 nmol d^-1 cm^-3) which are considerably lower than previously reported metal-driven AOM in marine sediments (10-40 nmol d^-1 cm^-3) but within a range of NO₃^- -driven AOM in an oligotrophic lake (0.6-3.6 nmol d^-1 cm^-3). AOM took place without metal additions but addition of Mn⁴⁺ increased the potential rates and this increase was especially high in 10-30 cm layer (Ab) of the profundal site (5-fold increase). The structure of the bacterial and archaeal communities changed considerably during incubation. Communities incubated with Mn⁴⁺ were especially different from those incubated with Fe³⁺ or without metals which were more similar with each other. Surprisingly, anaerobic methanotrophic archaea detected, ANME-2D and AOM-associated archaea (AAA), were almost absent in the littoral site and constituted only up to 3.5% and 5% of archaeal 16S rRNA and mcrA gene sequences, respectively, in the profundal site. Mn⁴⁺ induced an increase in the abundance of ANME-2D and AAA in the profundal site which indicates that they are involved in Mn⁴⁺ - dependent AOM.