20 Geobacter sulfurreducens was originally considered a strict anaerobe. However, this 21 bacterium was later shown to not only tolerate exposure to oxygen but also to use it as 22 terminal electron acceptor. Research performed has so far only revealed the general ability of 23 G. sulfurreducens to reduce oxygen, but the oxygen uptake rate has not been quantified yet, 24 nor has evidence been provided as to how the bacterium achieves oxygen reduction.25 Therefore, microaerobic growth of G. sulfurreducens was investigated here with better 26 defined operating conditions as previously performed and a transcriptome analysis was 27 performed to elucidate possible metabolic mechanisms important for oxygen reduction in G.28 sulfurreducens. The investigations revealed that cell growth with oxygen is possible to the 29 same extent as with fumarate if the maximum specific oxygen uptake rate (sOUR) of 30 95 mg O2 g CDW -1 h -1 is not surpassed. Hereby, the entire amount of introduced oxygen is 31 reduced. When oxygen concentrations are too high, cell growth is completely inhibited and 32 there is no partial oxygen consumption. Transcriptome analysis suggests a menaquinol 33 oxidase to be the enzyme responsible for oxygen reduction. Transcriptome analysis has 34 further revealed three different survival strategies, depending on the oxygen concentration 35 present. When prompted with small amounts of oxygen, G. sulfurreducens will try to escape 36 the microaerobic area; if oxygen concentrations are higher, cells will focus on rapid and 37 complete oxygen reduction coupled to cell growth; and ultimately cells will form protective 38 layers if a complete reduction becomes impossible. The results presented here have important 39 implications for understanding how G. sulfurreducens survives exposure to oxygen. 40 41 42 43 Introduction 44 Geobacter sulfurreducens, a δ-proteobacterium found in subsurface environments, was 45 originally reported to be a strict anaerobe (1). However, it was later shown that this bacterium 46 can grow with oxygen as terminal electron acceptor when 5 % of oxygen or less are added to 47 the headspace of cultivation flasks (2). But this study did not investigate dissolved oxygen 48 concentrations reached in the liquid phase under these conditions (2). Further, the analysis of 49 the genome of G. sulfurreducens revealed several enzymes that could be involved in the 50 reduction of oxygen (3). The expression of many of the corresponding genes is regulated by 51 the RpoS regulon in G. sulfurreducens, which was shown to be inevitable for cell growth with 52 oxygen (4). Part of the RpoS regulon are genes for the cytochrome c oxidase, which is 53 thought to be most likely responsible for cell growth with oxygen, but genes for a cytochrome 54 d ubiquinol oxidase and a rubredoxin-oxygen oxidoreductase have also been found to be 55 regulated by RpoS and encode for enzymes that may play a role in oxygen reduction (3,5).56 The gene for a cytochrome d ubiquinol oxidase has also been found upregulated in Geobacter 5...