Transient bottom water oxygenation creates a niche for cable bacteria in long-term anoxic sediments of the Eastern Gotland Basin

Cable bacteria have been reported in sediments from marine and freshwater locations, but the environmental factors that regulate their growth in natural settings are not well understood. Most prominently, the physiological limit of cable bacteria in terms of oxygen availability remains poorly constrained. In this study, we investigated the presence, activity and diversity of cable bacteria in relation to a natural gradient in bottom water oxygenation in a depth transect of the Eastern Gotland Basin (Baltic Sea). Cable bacteria were identified by FISH at the oxic and transiently oxic sites, but not at the permanently anoxic site. Three species of the candidate genus Electrothrix, i.e. marina, aarhusiensis and communis were found coexisting within one site. The highest filament density (33 m cm(-2)) was associated with a 6.3 mm wide zone depleted in both oxygen and free sulphide, and the presence of an electric field resulting from the electrogenic sulphur oxidizing metabolism of cable bacteria. However, the measured filament densities and metabolic activities remained low overall, suggesting a limited impact of cable bacteria at the basin level. The observed bottom water oxygen levels (< 5 M) are the lowest so far reported for cable bacteria, thus expanding their known environmental distribution.