| dc.description.abstract | Cable bacteria are filamentous chemo-litho-trophic bacteria that live in diffusion-dominated seasonally, anoxic sediments. They become active after transition to oxic overlying waters has taken place. Cable bacteria oxidize sulfide with oxygen and thereby deplete the upper sediment layer of porewater sulfide. This creates a gap (10-30 mm) between the oxic and euxinic zone. In between, there is a suboxic zone, in which no sulfide can be measured, because sulfide, produced from sulfate reduction and released by FeS-dissolution, is efficiently consumed by cable bacteria in a cryptic sulfur cycle. Electrons produced in the euxinic and suboxic zone are conducted by the cable bacteria filaments to the oxic zone, where oxygen reduction takes place. Within coarse lines, the metabolism of cable bacteria has been depicted. But multiple questions still remain regarding the extent of sulfide oxidation cable bacteria are able to perform, and, also, the carbon metabolism of cable bacteria has been so far under investigated. To investigate sulfide metabolism, a new experimental method was investigated that would allow to measure sulfide consumption rate within the suboxic zone. The method relies on breaking the suboxic cryptic sulfur cycle by exerting anoxic conditions. This eliminates sulfide consumption by cable bacteria, leading to sulfide-release within the porewater. This could then be measured using micro-sensors. Using this method, a suboxic consumption rate of 1.3 mmol*m-2*d-1, was determined to occur within oxically incubated Rattekaai sediment. Validation of the new method still has to be performed to ascertain accuracy of the obtained values. To investigate carbon metabolism, 13C-DIC and 15N-NH4 stable-isotope-enrichment was performed of cable bacteria, residing in oxically-incubated Rattekaai sediment. The biomass 13C and 15N fractional abundances were subsequently analysed using NanoSIMS. The main observation made in the investigation of carbon metabolism was that 13C-DIC was exclusively taken up by cells that resided within the suboxic zone. Thus, the oxic zone cells seemed to have a differential function compared to the suboxic zone cells. This suggested that oxic cells, merely served to reduce the oxygen using the electrons conducted by the filament. These observations, serve as the first example of cells having a differential function within a cable bacteria filament. Also, a surprisingly homogenous uptake was observed within a filament, suggestive of a mechanism that actively or passively allows for sharing of available reduction potential. Based on these findings, cells might truly function within the context of the filament rather than being electron-conducting links in the chain. In the future, using the new sulfide consumption rate measurement method, as well as stable-isotope labelling with organic carbon and DIC, a highly complete picture of combined carbon and sulfide metabolism can be established. | |
