Anaerobic methane oxidation in an East African great lake (Lake Kivu)

Abstract. This study investigates methane (CH₄) oxidation in the water column of Lake Kivu, a deep meromictic tropical lake containing large quantities of CH₄ in the anoxic deep waters. Depth profiles of dissolved gases (CH₄ and nitrous oxide (N₂O)) and of the different potential electron acceptors for anaerobic methane oxidation (AOM) (nitrate, sulfate, iron and manganese) were determined during six field campaigns between June 2011 and August 2014. Bacterial abundance all along the vertical profiles was also determined by flow cytometry during three field campaigns, and denitrification measurements based on stable isotopes were performed twice. Incubation experiments were performed to quantify CH₄ oxidation and nitrate consumption rates, with a focus on AOM, without and with an inhibitor of sulfate-reducing bacteria activity (molybdate). Nitrate consumption rates were measured in these incubations. Substantial CH₄ oxidation activity was observed in oxic and anoxic waters, and in the upper anoxic waters of Lake Kivu, CH₄ is a major electron donor to sustain anaerobic metabolic processes coupled to AOM. The maximum aerobic and anaerobic CH₄ oxidation rates were estimated to 27 ± 2 and 16 ± 8 µmol L⁻¹ d⁻¹, respectively. We observed a decrease of AOM rates when molybdate was added for half of the measurements, strongly suggesting the occurrence of AOM linked to sulfate reduction, but an increase of AOM rates was observed for the other half. Nitrate reduction rates and dissolved manganese production rates tended to be higher with the addition of molybdate, but the maximum rates of 0.6 ± 0.02 and 11 ± 2 µmol L⁻¹ d⁻¹, respectively, were never high enough to explain AOM rates observed at the same depths. We also put in evidence a difference in relative importance of aerobic and anaerobic CH₄ oxidation between the seasons, with a higher importance of aerobic oxidation when the oxygenated layer was thicker (in dry season).