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Impact of cable bacteria on the biogeochemical cycling in a seasonally hypoxic coastal basin
Seitaj, D. (2016). Impact of cable bacteria on the biogeochemical cycling in a seasonally hypoxic coastal basin. PhD Thesis. Vrije Universiteit Brussel: [s.l.]. 217 pp.

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Abstract
    Coastal hypoxia refers to the oxygen depletion that occurs in summer in the bottom waters of semi-enclosed and stratified coastal systems. There is evidence for a global increase in the frequency, extent, intensity and duration of coastal hypoxia, which has been linked to an increased anthropogenic input of nutrients into the coastal ocean in combination with climate change. Bottom water hypoxia has major consequences for the functioning of coastal ecosystems, as it has profound effects on the biogeochemical cycling, and on the survival and behavior of marine organisms in coastal systems. Hypoxia reaches a particularly harmful stage when sulfide, is released to the bottom water where it can give rise to the establishment of euxinia (i.e. sulfidic bottom waters). As sulfide is highly toxic for marine life, the occurrence of euxinia can have devastating ecosystem consequences. Although coastal hypoxia is relatively common, reports of euxinia are less frequent, and so the question remains why euxinia is so uncommon.This thesis documents that electricity-producing cable bacteria, living in the sediments of seasonal hyopoxic basins, can prevent, or substantially delay, the development of euxinia. Cable bacteria induce the formation of a large pool of sedimentary iron oxides before the onset of summer hypoxia. This pool of iron oxides acts as a ‘firewall’ against the release of sulfide to the bottom water in early summer, and likely prevents the development of bottom water euxinia. Overall, cable bacteria appear to be key drivers of iron and phosphorous cycling in seasonal hypoxic basins at the ecosystem scale, which reveals that the biogeochemical impact of sedimentary microbes may extend far beyond the sediment-water interface.Electrogenic sulfur oxidation by cable bacteria is able to connect oxygen and sulfide in distinct sediment horizons, and so it enables aerobic sulfide oxidation by means of long-distance electron transport. The electrogenic metabolism of cable bacteria provides an efficient way of deep sulfide removal in marine sediments. This way cable bacteria provide another solution to the “aerobic sulfide oxidation paradox”. This paradox states that most sulfide in sediments is removed through oxidation with oxygen, though oxygen and sulfide are never into contact, as they are separated by a wide suboxic zone. These insights provide a better understanding and guidance as to the mechanisms of sulfur cycling in the seafloor.

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