The proposed Asgard superphylum of Archaea comprises the closest archaeal 19 relatives of eukaryotes, whose genomes hold clues pertaining to the nature host cell that 20 acquired the mitochondrion at the origin of eukaryotes 1-4 . Genomes of the Asgard candidate 21 Phylum 'Candidatus Lokiarchaeota' [Lokiarchaeon] suggest an acetogenic H2 -dependent 22 lifestyle 5 and mixotrophic capabilities 6 . However, data on the activity of Lokiarchaeon are 23 currently lacking, and the ecology of the host cell that acquired the mitochondrion is 24 debated 4,7 . Here, we show that in anoxic marine sediments underlying highly productive 25 waters on the Namibian continental shelf Lokiarchaeon gene expression increases with depth 26 below the seafloor, and was significantly different across a redox gradient spanning hypoxic 27 to sulfidic conditions. Notably, Lokiarchaeon increased expression of genes involved in 28 growth, carbohydrate metabolism, and the H2-dependent Wood-Ljungdahl (WLP) carbon 29 fixation pathway under the most reducing (sulfidic) conditions. Quantitative stable isotope 30 probing experiments revealed multiple populations of Lokiarchaeota utilizing both CO2 and 31 diatomaceous extracellular polymeric substances (dEPS) as carbon sources over a 10-day 32 incubation under anoxic conditions. This apparent anaerobic mixotrophic metabolism was 33 consistent with the expression of Lokiarchaeon genes involved in transport and fermentation 34 of sugars and amino acids. Remarkably, several Asgard populations were more enriched 35 with 13 C-dEPS compared to the community average, indicating a preference for dEPS as a 36 growth substrate. The qSIP and gene expression data indicate a metabolism of "Candidatus 37 Lokiarchaeota" similar to that of sugar fermenting homoacetogenic bacteria 8 , namely that 38 Lokiarchaeon can couple fermentative H2 production from organic substrates with electron 39 bifurcation and the autotrophic and H2-dependent WLP. Homoacetogenesis allows to access 40 a wide range of substrates and relatively high ATP gain during acetogenic sugar 41 fermentation 8 thereby providing an ecological advantage for Lokiarchaeon in anoxic, energy 42 limited settings. 43 The Benguela upwelling system is one of the most productive ecosystems of the world's 44 oceans and exhibits an oxygen minimum zone (OMZ) overlying the seafloor on the Namibian 45 continental shelf 9 . Below the Namibian seafloor relatively high amounts of sulfide (H2S) and H2 46 are produced by sulfate reduction and microbial fermentation, respectively 10 . During the Southern 47