Abstract
Oxygen is essential for life in the sea. Marine waters are supplied with oxygen via air-sea gas exchange with the atmosphere, and photosynthesis by phytoplankton in the photic zone. When oxygen supply is outpaced by the demand of oxygen, this may ultimately result in hypoxia (oxygen <63 μM) or even
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anoxia (oxygen = 0 μM). Depletion of bottom water oxygen in coastal areas is increasing worldwide. This is the compound result of eutrophication linked to increased riverine inputs of nitrogen and phosphorus by human-activities and climate change. De-oxygenation in coastal areas can lead to the development of ‘dead zones’ characterized by mass mortality of marine life, either due to the lack of oxygen or the release of the highly toxic sulphide (euxinia). Recently, a novel type of multicellular filamentous sulphur-oxidising bacteria was discovered. These so-called ‘cable bacteria’ can grow up to several centimetres long, and are a member of the Desulfobulbaceae family. Cable bacteria naturally occur in a broad range of marine environments with contrasting conditions. They are capable of spatially linking sulphide oxidation in deeper sediment layers to oxygen reduction near the sediment-water interface by conducting electrons over centimetre-scale distances. Activity of cable bacteria efficiently removes sulphide and can establish a suboxic zone up to a few centimetres wide. Their activity typically leads to a distinct pH profile, characterised by a relatively high pH (~9) near the sediment-water interface and a low pH (~6.5) in the suboxic zone. The strong pore water acidification in the suboxic zone promotes the dissolution of iron monosulphide and iron, manganese and calcium carbonates. When the iron and manganese ions liberated from the dissolution of these minerals, diffuse upwards, they can precipitate as iron and manganese oxides near the sediment-water interface upon contact with oxygen. Iron and manganese oxides can buffer the benthic release of sulphide and phosphate when bottom water oxygen is low. Since field observations are relatively scarce, it is currently unknown what environmental factors control the prevalence and activity of cable bacteria, and whether the role of cable bacteria in buffering the benthic release of sulphide and phosphate can be generalised to seasonally hypoxic systems. At present, it is largely unknown what impact cable bacteria might have on the permanent burial of phosphorus. This thesis shows that the prevalence of cable bacteria in sediments of the Baltic Sea is governed by three factors: 1) oxygen availability, 2) sulphide supply and 3) the rate of bioturbation. The results demonstrate that cable bacteria are of ecological relevance for seasonally hypoxic brackish coastal systems, through their formation of iron and manganese oxides, which can buffer the benthic release of sulphide and phosphate during peak hypoxia. Their activity does not affect permanent burial of iron, manganese and phosphorus, but rather amplifies their seasonal dynamics. The work further demonstrates that the oxygen demand arising from the accumulation of organic-rich sediments in deep basins of the Baltic Sea over several decades, ‘the legacy of hypoxia’, has significant implications for the biogeochemical response of euxinic basins to re-oxygenation.
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