Presence and succession of cable bacteria at Hydrate Ridge, NE Pacific Ocean

Cable bacteria and Beggiatoa are two filamentous microorganisms that oxidize sulfide for energy, which can result in a seasonal competitive succession of microbial metabolism and growth, preventing the release of free sulfide (euxinia) in coastal marine environments. Hydrate Ridge (HR) is a seafloor environment where areas of surface sediments are saturated by methane gas rising from deeper reservoirs due to tectonic activity along the Cascadia convergent margin and where hydrogen sulfide is enriched as a consequence of sulfate reduction. At ~775 m water depth, extensive areas of HR are also located in the Northeast Pacific oxygen minimum zone which is permanently hypoxic. In these areas of HR, Beggiatoa mats are a natural occurrence while traces of cable bacteria have never been identified. This study attempts to understand if the growth of cable bacteria in HR sediment is limited by the scarcity of dissolved oxygen at the sediment-water interface and whether a population of cable bacteria could emerge under higher oxygen concentrations. After exposing HR sediment to air-saturated seawater for 3 weeks in the laboratory, cable bacteria were found using microscopic labeling and imaging methods although Beggiatoa were still visibly dominant. Microsensor profiling methods indicated a zone of overlapping oxygen and sulfide persisted near the sediment-water interface. These results suggest that the elevation of bottom water oxygen concentrations can stimulate the growth of cable bacteria, though it may not be the only environmental factor affecting the competition between Beggiatoa and cable bacteria. Through the continuation of oxygen exposure, we hope to give the cable bacteria additional time to replace Beggiatoa and to alter the geochemistry of incubated HR sediment. The discovery of cable bacteria in HR sediment does suggest that they are widespread throughout ocean sediments.