Knowledge of taxis (directed swimming) in the Archaea is currently expanding through identification of novel receptors, effectors, and proteins involved in signal transduction to the flagellar motor. Although the ability for biological cells to sense and swim toward hydrogen gas has been hypothesized for many years, this capacity has yet to be observed and demonstrated. Here we show that the average swimming velocity increases in the direction of a source of hydrogen gas for the methanogen, Methanococcus maripaludis using a capillary assay with anoxic gas-phase control and time-lapse microscopy. The results indicate that a methanogen couples motility to hydrogen concentration sensing and is the first direct observation of hydrogenotaxis in any domain of life. Hydrogenotaxis represents a strategy that would impart a competitive advantage to motile microorganisms that compete for hydrogen gas and would impact the C, S and N cycles. H ydrogen gas (H 2 ) is a crucial substrate for methanogens as well as a common source of energy for other organisms in both anaerobic and aerobic environments, including acetogens, sulfate-and sulfur-reducers, and hydrogen-oxidizers [1][2][3][4] . Biological methane (CH 4 ) production from H 2 and carbon dioxide (CO 2 ) contributes to greenhouse gas emissions and is possibly one of the oldest microbial metabolisms 5,6 . Understanding the ecological strategies of methanogens is not only important for our knowledge of early earth processes and present-day anaerobic environments, but also for determining potential roles in human health conditions (e.g., colon cancer and periodontal disease), where positive correlations have been made with incidence of disease and occurrence of methanogens 7,8 . Methanococcus maripaludis is an anaerobic archaeum that can use H 2 or formate as electron donor to reduce CO 2 to CH 4 and is considered a model mesophilic methanogen. Recently, the swimming behavior of M. maripaludis was described 9 , but chemotactic responses have not been shown. Chemotaxis has been demonstrated for Archaea, including methanogens 10,11 , but taxis to hydrogen has not been shown for any domain of life. The chemotaxis signal transduction system in Archaea is similar to the well-studied system in Bacteria; however, the flagellar switch is different and none of the archaeal flagellar proteins have homologs to bacterial flagellar proteins [12][13][14][15] . Chemotaxis has been the subject of many mathematical models and the majority have concentrated on reproducing the population-level observation of migrating bands of high cell concentration in swarm plates and capillary experiments 16 . Pioneering work in modeling chemotaxis behavior by Keller and Segel in 1971 has been the basis of the most common mathematical models 17 . In one dimension, with x being the spatial variable, the Keller-Segel model can be described as a flux, J, such thatwhere m is the cell diffusion coefficient that takes random, non-directed, movement of cells into account. b is the microbial population density, s...
