Impact of cable bacteria on biogeochemical cycling in sediments of a seasonally hypoxic marine basin

Oxygen is a key element for life on earth. It can be taken up by ocean waters via air-sea gas exchange, but is also formed through photosynthesis by phytoplankton in the photic zone. In seawater, oxygen can be consumed through aerobic respiration, where it is used as an electron acceptor in the breakdown of organic matter, in a process known as remineralisation. Dissolved oxygen is necessary for the respiration and metabolism of many marine organisms, several types of which are sensitive to concentrations below thresholds as high as 91 µM O2; yet an oxic water body is defined as containing at least 62.5 µM (2 mg L-1) oxygen. An increased oxygen demand or reduction in oxygen supply can result in oxygen-deficient or hypoxic conditions (< 62.5 µM) and may eventually lead to oxygen depletion or anoxia. Hypoxia and anoxia in coastal waters can occur naturally, mainly affecting water bodies in which water exchange and circulation are restricted. However, low oxygen conditions are becoming increasingly prevalent in the bottom waters of many coastal systems, as a direct consequence of eutrophication, caused by human land-use practices. Increased nutrient input from agricultural run-off and anthropogenic waste disposal can lead to increased primary production in surface waters, where a consequent increase in oxygen demand upon sinking of this organic matter into deeper waters can eventually exceed the supply of dissolved oxygen. Oxygen is the most energetically-favourable electron acceptor for microbial respiration in sediments, but in its absence, a cascade of alternative electron acceptors is available for anaerobic respiration. Anaerobic mineralisation dominates in most coastal sediments due to the high organic matter supply. Iron and manganese-(oxyhyr)oxides can serve as electron acceptors for microbial respiration in the absence of oxygen. Many trace metals can be associated with iron and manganese minerals. Variations in trace metal enrichments in coastal surface sediments can be used as indicators of hypoxic and anoxic conditions in these environments. Iron and manganese cycling also plays a pivotal role in nutrient cycling in coastal environments. Increased availability of nutrients like phosphorus, for autotrophic metabolism, can lead to enhanced primary productivity in coastal surface waters. Under low oxygen concentrations, phosphorus is released from surface sediments, liberated from iron- and manganese-(oxyhydr)oxides into overlying waters. Sedimentary phosphorus cycling is very redox-sensitive and phosphorus recycling can be pivotal to the maintenance of low oxygen conditions in coastal systems, by fuelling primary production and organic matter supply to bottom waters. Furthermore, sulphur-oxidising cable bacteria can influence redox conditions, as well as elemental recycling and burial in coastal sediments. Cable bacteria have been observed to link sulphide oxidation to the reduction of oxygen over centimetre-long distances in the sediment via electrogenic sulphur oxidation (e-SOx). Cable bacteria induce a range of secondary biogeochemical reactions including dissolution and precipitation of minerals. For example, proton generation associated with anodic sulphide oxidation can lead to dissolution of sulphide and carbonate minerals and mobilisation of calcium, iron and sulphate ions to the pore water. Cable bacteria have been detected in many aquatic systems worldwide and are now known to occur in environments ranging from hot vents, freshwater and marine sediments, to mangroves and even aquifers

Impact of cable bacteria on biogeochemical cycling in sediments of a seasonally hypoxic marine basin

Oxygen is a key element for life on earth. It can be taken up by ocean waters via air-sea gas exchange, but is also formed through photosynthesis by phytoplankton in the photic zone. In seawater, oxygen can be consumed through aerobic respiration, where it is used as an electron acceptor in the breakdown of organic matter, in a process known as remineralisation. Dissolved oxygen is necessary for the respiration and metabolism of many marine organisms, several types of which are sensitive to concentrations below thresholds as high as 91 µM O2; yet an oxic water body is defined as containing at least 62.5 µM (2 mg L-1) oxygen. An increased oxygen demand or reduction in oxygen supply can result in oxygen-deficient or hypoxic conditions (< 62.5 µM) and may eventually lead to oxygen depletion or anoxia. Hypoxia and anoxia in coastal waters can occur naturally, mainly affecting water bodies in which water exchange and circulation are restricted. However, low oxygen conditions are becoming increasingly prevalent in the bottom waters of many coastal systems, as a direct consequence of eutrophication, caused by human land-use practices. Increased nutrient input from agricultural run-off and anthropogenic waste disposal can lead to increased primary production in surface waters, where a consequent increase in oxygen demand upon sinking of this organic matter into deeper waters can eventually exceed the supply of dissolved oxygen. Oxygen is the most energetically-favourable electron acceptor for microbial respiration in sediments, but in its absence, a cascade of alternative electron acceptors is available for anaerobic respiration. Anaerobic mineralisation dominates in most coastal sediments due to the high organic matter supply. Iron and manganese-(oxyhyr)oxides can serve as electron acceptors for microbial respiration in the absence of oxygen. Many trace metals can be associated with iron and manganese minerals. Variations in trace metal enrichments in coastal surface sediments can be used as indicators of hypoxic and anoxic conditions in these environments. Iron and manganese cycling also plays a pivotal role in nutrient cycling in coastal environments. Increased availability of nutrients like phosphorus, for autotrophic metabolism, can lead to enhanced primary productivity in coastal surface waters. Under low oxygen concentrations, phosphorus is released from surface sediments, liberated from iron- and manganese-(oxyhydr)oxides into overlying waters. Sedimentary phosphorus cycling is very redox-sensitive and phosphorus recycling can be pivotal to the maintenance of low oxygen conditions in coastal systems, by fuelling primary production and organic matter supply to bottom waters. Furthermore, sulphur-oxidising cable bacteria can influence redox conditions, as well as elemental recycling and burial in coastal sediments. Cable bacteria have been observed to link sulphide oxidation to the reduction of oxygen over centimetre-long distances in the sediment via electrogenic sulphur oxidation (e-SOx). Cable bacteria induce a range of secondary biogeochemical reactions including dissolution and precipitation of minerals. For example, proton generation associated with anodic sulphide oxidation can lead to dissolution of sulphide and carbonate minerals and mobilisation of calcium, iron and sulphate ions to the pore water. Cable bacteria have been detected in many aquatic systems worldwide and are now known to occur in environments ranging from hot vents, freshwater and marine sediments, to mangroves and even aquifers

Coupled dynamics of CH4-S-FeP in Black Sea sediments

Surface sediments in the deep basin of the Black Sea are underlain by extensive deposits of iron (Fe) oxide-rich lake sediments that were deposited prior to the inflow of marine Mediterranean Sea waters ca. 9000 years ago. The ongoing downward diffusion of marine sulfate into the methane (CH4)-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ). While the cycles of sulfur (S), CH4 and Fe in the SMTZ have been extensively studied, relatively little is known about their impact on sedimentary phosphorus (P) and the biogeochemical processes occuring below the SMTZ. In this study, we combine detailed geochemical analyses with multicomponent diagenetic modeling to demonstrate that sulfate-mediated anaerobic oxidation of CH4 substantially enhances the downward sulfidization of the lake deposits. This drives the release of Fe oxide bound P to the pore water and subsequent formation of authigenic Fe(II)-P minerals below the sulfidization front. We further show that downward migrating sulfide becomes partly re-oxidized to sulfate by reaction with oxidized Fe minerals, fueling a cryptic S cycle with slow rates of sulfate reduction in the deep limnic deposits. However, our results reveal that cryptic S cycling is unlikely to explain the observed release of dissolved Fe2+ below the SMTZ. Instead, we suggest that CH4 oxidation coupled to the reduction of Fe oxides may provide a possible mechanism for the apparent Fe oxide reduction at depth in the sediment. The coupled CH4-S-Fe-P dynamics described here may strongly overprint burial records of Fe, S and P in depositional marine systems subject to changes in organic matter loading or water column salinity. Such diagenetic alterations should not be interpreted as primary sedimentary signals