Seasonal oxygen depletion (hypoxia) in coastal bottom waters can lead to the release and persistence of free sulfide (euxinia), which is highly detrimental to marine life. Although coastal hypoxia is relatively common, reports of euxinia are less frequent, which suggests that certain environmental controls can delay the onset of euxinia. However, these controls and their prevalence are poorly understood. Here we present field observations from a seasonally hypoxic marine basin (Grevelingen, The Netherlands), which suggest that the activity of cable bacteria, a recently discovered group of sulfur-oxidizing microorganisms inducing long-distance electron transport, can delay the onset of euxinia in coastal waters. Our results reveal a remarkable seasonal succession of sulfur cycling pathways, which was observed over multiple years. Cable bacteria dominate the sediment geochemistry in winter, whereas, after the summer hypoxia, Beggiatoaceae mats colonize the sediment. The specific electrogenic metabolism of cable bacteria generates a large buffer of sedimentary iron oxides before the onset of summer hypoxia, which captures free sulfide in the surface sediment, thus likely preventing the development of bottom water euxinia. As cable bacteria are present in many seasonally hypoxic systems, this euxiniapreventing firewall mechanism could be widely active, and may explain why euxinia is relatively infrequently observed in the coastal ocean. sediment biogeochemistry | cable bacteria | coastal hypoxia | sulfur cycling | microbial competition

Phosphorus is an essential nutrient for life. The release of phosphorus from sediments is critical in sustaining phytoplankton growth in many aquatic systems and is pivotal to eutrophication and the development of bottom water hypoxia. Conventionally, sediment phosphorus release is thought to be controlled by changes in iron oxide reduction driven by variations in external environmental factors, such as organic matter input and bottom water oxygen. Here, we show that internal shifts in microbial communities, and specifically the population dynamics of cable bacteria, can also induce strong seasonality in sedimentary iron-phosphorus dynamics. Field observations in a seasonally hypoxic coastal basin demonstrate that the long-range electrogenic metabolism of cable bacteria leads to a dissolution of iron sulfides in winter and spring. Subsequent oxidation of the mobilized ferrous iron with manganese oxides results in a large stock of iron-oxide-bound phosphorus below the oxic zone. In summer, when bottom water hypoxia develops and cable bacteria are undetectable, the phosphorus associated with these iron oxides is released, strongly increasing phosphorus availability in the water column. Future research should elucidate whether formation of iron-oxide-bound phosphorus driven by cable bacteria, as observed in this study, contributes to the seasonality in iron-phosphorus cycling in aquatic sediments worldwide.

The infamous garbage patches on the surface of subtropical oceanic gyres are proof that plastic is polluting the ocean on an unprecedented scale. The fate of floating plastic debris 'trapped' in these gyres, however, remains largely unknown. Here, we provide the first evidence for the vertical transfer of plastic debris from the North Pacific Garbage Patch (NPGP) into the underlying deep sea. The numerical and mass concentrations of plastic fragments (500 µm to 5 cm in size) suspended in the water column below the NPGP follow a power law decline with water depth, reaching values <0.001 pieces/m 3 and <0.1 µg/m 3 in the deep sea. The plastic particles in the NPGP water column are mostly in the size range of particles that are apparently missing from the ocean surface and the polymer composition of plastic in the NPGP water column is similar to that of floating debris circulating in its surface waters (i.e. dominated by polyethylene and polypropylene). Our results further reveal a positive correlation between the amount of plastic debris at the sea surface and the depth-integrated concentrations of plastic fragments in the water column. We therefore conclude that the presence of plastics in the water column below the NPGP is the result of 'fallout' of small plastic fragments from its surface waters.

Abstract. The surface sediments in 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 subsequent downward diffusion of marine sulfate into the methane-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ), including anaerobic oxidation of methane (AOM) with sulfate. While the sedimentary cycles of sulfur (S), methane and Fe in the SMTZ have been extensively studied, relatively little is known about the diagenetic alterations of the sediment record occurring below the SMTZ.Here we combine detailed geochemical analyses of the sediment and porewater with multicomponent diagenetic modeling to study the diagenetic alterations below the SMTZ at two sites in the western Black Sea. We focus on the dynamics of Fe, S and phosphorus (P), and demonstrate that diagenesis has strongly overprinted the sedimentary burial records of these elements. In line with previous studies in the Black Sea, we show that sulfate-mediated AOM substantially enhances the downward diffusive flux of sulfide into the deep limnic deposits. During this downward sulfidization, Fe oxides, Fe carbonates and Fe phosphates (e.g., vivianite) are converted to sulfide phases, leading to an enrichment in solid-phase S and the release of phosphate to the porewater. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe2+) lead to sequestration of downward-diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front.Our model results further demonstrate that downward-migrating sulfide becomes partly re-oxidized to sulfate due to reactions with oxidized Fe minerals, fueling a cryptic S cycle and thus stimulating slow rates of sulfate-driven AOM ( ∼  1–100 pmol cm−3 d−1) in the sulfate-depleted limnic deposits. However, this process is unlikely to explain the observed release of dissolved Fe2+ below the SMTZ. Instead, we suggest that besides organoclastic Fe oxide reduction and reactivation of less reactive Fe oxides by methanogens, AOM coupled to the reduction of Fe oxides may also provide a possible mechanism for the high concentrations of Fe2+ in the porewater at depth. Our results reveal that methane plays a key role in the diagenetic alterations of Fe, S and P records in Black Sea sediments. The downward sulfidization into the limnic deposits is enhanced through sulfate-driven AOM with sulfate, and AOM with Fe oxides may provide a deep source of dissolved Fe2+ that drives the sequestration of P in vivianite below the sulfidization front.

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20 Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence 21 of sulphide near the sediment-water interface and can thereby record historic bottom-water 22 oxygen depletion. Here, we assess the impact of temporal changes in manganese (Mn) cycling 23 and bottom-water oxygen on sedimentary Mo dynamics in a seasonally-hypoxic coastal marine 24 basin (Lake Grevelingen, the Netherlands). High resolution line scans obtained with LA-ICP-25 MS and discrete sample analyses reveal distinct oscillations in Mo with depth in the sediment. 26 These oscillations and high sediment Mo concentrations (up to ~130 ppm) are attributed to 27 deposition of Mo-bearing Mn-oxide-rich particles from the overlying water, the release of 28 molybdate (MoO4 2-) to the pore water upon reduction of these Mn-oxides, and subsequent 29 sequestration of Mo. The latter process only occurs in summer when sulphide concentrations 30 near the sediment-water interface are elevated. We hypothesise that cable bacteria enhance the 31 seasonality in sediment Mo records by contributing to remobilisation of Mo as MoO4 2during 32 oxic periods and by enhancing the pool of Mn-oxides in the system by dissolving Mn-33 carbonates. A sediment record that spans the past ~45 years indicates that sediment Mo 34 concentrations have increased over the past decades, despite less frequent occurrences of anoxia 35in the bottom waters based on oxygen measurements from water column monitoring. We 36 suggest that the elevated Mo in recent sediments reflects both enhanced rates of sulphate 37 reduction and sulphide production in the surface sediment as a result of increased input of 38 organic matter into the basin from the adjacent North Sea since 1999, and an associated 39 enhanced "Mn refluxing" in the marine lake in summer. 40 93 refluxing" is thought to contribute to the high Mo burial fluxes in environments with weakly 94 sulphidic bottom waters (Algeo and Lyons, 2006). To our knowledge, there are no detailed 95 seasonal studies of the dynamics of Mo and Mn in both pore waters and sediments of hypoxic 96 systems to confirm the suggested seasonality in coupled Mn-Mo cycling. 97 Recently, it was discovered that sulphide-oxidising cable bacteria (Nielsen et al., 2010; 98 Pfeffer et al., 2012) may dissolve Fe-sulphides and Mn-carbonates in surface sediments of 99 seasonally-hypoxic systems. Consequently, these bacteria actively contribute to the formation 100 of an oxidised, Fe-and Mn-oxide rich surface layer in winter and spring (Seitaj et al., 2015; 101 Sulu-Gambari et al., 2016b; Sulu-Gambari et al., 2016a). In contrast, sulphur oxidising 102 Beggiatoaceae, present in autumn, had a more limited effect on the formation of Fe-and Mn-103 oxides in the surface sediment (Seitaj et al., 2015; Sulu-Gambari et al., 2016b; Sulu-Gambari 104 et al., 2016a). Due to the coupling of Mn, Fe, S and Mo cycles in hypoxic systems, we 105 hypothesise that the activity of cable bacteria may also be of relevance to the sedimentary 106 dynamics of Mo. More specifical...

Seasonal hypoxia refers to the oxygen depletion that occurs in summer in the bottom water of stratified systems, and is increasingly observed in coastal areas worldwide. The process induces a seasonal cycle on the biogeochemistry of the underlying sediments, which remains poorly quantified. Here, we investigated the sedimentary oxygen consumption within Lake Grevelingen (The Netherlands), a saline coastal reservoir that is impacted by yearly recurrent bottom water hypoxia. Monthly sampling campaigns were conducted throughout 2012 at three sites along a depth gradient. Macrofauna sampling and sediment profile imaging demonstrated how summer hypoxia strongly impacts the benthic communities below 15 m of water depth. Benthic fluxes of oxygen, dissolved inorganic carbon, total alkalinity, and ammonium were determined by closed core incubations, while oxygen depth profiles were recorded by microsensor profiling of sediment cores. Our results reveal a pronounced seasonality in the sedimentary oxygen consumption. Low uptake rates in summer were caused by oxygen limitation, and resulted in the build-up of an "oxygen debt" through the accumulation of reduced iron sulfides. Highest oxygen uptake rates were recorded in fall, linked to the reoxidation of the pool of iron sulfides in the top layer. However, uptake rates remained unexpectedly high during winter and early spring, likely associated with the oxidation of iron sulfides down to centimeters depth due to the electrogenic sulfur oxidation by cable bacteria. Overall, our results suggest that the sedimentary oxygen dynamic in seasonally hypoxic coastal systems is characterized by a strongly amplified "oxygen debt" dynamics induced by cable bacteria.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; The surface sediments in 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 subsequent downward diffusion of marine sulfate into the methane-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ), including anaerobic oxidation of methane (AOM) with sulfate. While the sedimentary cycles of sulfur (S), methane and Fe in the SMTZ have been extensively studied, relatively little is known about the diagenetic alterations of the sediment record occurring below the SMTZ. Here we combine detailed geochemical analyses of the sediment and pore water with multicomponent diagenetic modeling to study the diagenetic alterations below the SMTZ at two sites in the western Black Sea. We focus on the dynamics of Fe, S and phosphorus (P) and demonstrate that diagenesis has strongly overprinted the sedimentary burial records of these elements. Our results show that sulfate-mediated AOM substantially enhances the downward diffusive flux of sulfide into the deep limnic deposits. During this downward sulfidization, Fe oxides, Fe carbonates and Fe phosphates (e.g. vivianite) are converted to sulfide phases, leading to an enrichment in solid phase S and the release of phosphate to the pore water. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe&lt;sup&gt;2+&lt;/sup&gt;) lead to sequestration of downward diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front. &lt;br&gt;&lt;br&gt; Our model results further demonstrate that downward migrating sulfide becomes partly re-oxidized to sulfate due to reactions with oxidized Fe minerals, fueling a cryptic S cycle and thus stimulating slow rates of sulfate-driven AOM (~ 1&amp;#8211;100 pmol cm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; d&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) in the sulfate-depleted limnic deposits. However, this process is unlikely to explain the observed release of dissolved Fe&lt;sup&gt;2+&lt;/sup&gt; below the SMTZ. Instead, we suggest that besides organoclastic Fe oxide reduction, AOM coupled to the reduction of Fe oxides may also provide a possible mechanism for the high concentrations of Fe&lt;sup&gt;2+&lt;/sup&gt; in the pore water at depth. Our results reveal that methane plays a key role in the diagenetic alterations of Fe, S and P records in Black Sea sediments. The downward sulfidization into the limnic deposits is enhanced through sulfate-driven AOM with sulfate and AOM with Fe oxides may provide a deep source of dissolved Fe&lt;sup&gt;2+&lt;/sup&gt; that drives the sequestration of P in vivianite below the sulfidization front.&lt;/p&gt;