Title: Growth of Anaerobic Methane Oxidizing Archaea and Sulfate Reducing Bacteria in a High 1 Pressure Membrane-capsule Bioreactor 2 3 Running Title: High Pressure Activity and Growth of Anme and Srb 4 5

22 Anaerobic methane oxidizing communities of archaea (ANME) and sulfate reducing bacteria 23 (SRB) grow slowly, which limits physiological studies. High methane partial pressure was 24 previously successfully applied to stimulate growth, but it is not clear how different ANME 25 subtypes and associated sulfate reducing bacteria (SRB) are affected by it. Here, we report 26 growth of ANME/SRB in a membrane-capsule bioreactor inoculated with Eckernförde Bay 27 sediment that combines high pressure incubation (10.1 MPa methane) and thorough mixing 28 (100 rpm) with complete cell retention by a 0.2 µm membrane. Results were compared to 29 previously obtained data from an ambient-pressure (0.101 MPa methane) bioreactor 30 inoculated with the same sediment. Labelled-methane oxidation rates were not higher at 10.1 31 MPa, likely because measurements were done at ambient pressure. The subtype ANME-2a/b 32 was abundant in both reactors, but subtype ANME-2c was only enriched at 10.1 MPa. SRB at 33 10.1 MPa mainly belonged to the SEEP-SRB2, Eel-1 group and Desulforomonadales and not 34 to the typically found SEEP-SRB1. Increase of ANME-2a/b occurred in parallel with increase 35 of SEEP-SRB2 which was previously only found associated with ANME-2c. Our results 36 imply that the syntrophic association is flexible and that methane pressure and sulfide 37 concentration influence growth of different ANME-SRB consortia. 38 We also studied the effect of elevated methane pressure on methane production and oxidation 39 by a mixture of methanogenic and sulfate-reducing sludge. Here, methane oxidation rates 40 decreased and were not coupled to sulfide production, indicating trace methane oxidation 41 during net methanogenesis and not anaerobic methane oxidation, even at high methane partial 42 pressure.