Pelagic Redoxclines Research Articles

Mid- to late Holocene sea-level rise recorded in Hells Bells 234U/238U ratio and geochemical composition

Hells Bells are underwater secondary carbonates discovered in sinkholes (cenotes) southeast of Cancun on the north-eastern Yucatán peninsula, Mexico. These authigenic calcite precipitates, reaching a length of up to 4 m, most likely grow in the pelagic redoxcline. Here we report on detailed 230Th/U-dating and in-depth geochemical and stable isotope analyses of specimens from cenotes El Zapote, Maravilla and Tortugas. Hells Bells developed since at least ~ 8000 years ago, with active growth until present day. Initial (234U/238U) activity ratios (δ234U0) in Hells Bells calcite decreas from 55 to 15‰ as sea level converges toward its present state. The temporal evolution of the geochemistry and isotope composition of Hells Bells calcites thus appears to be closely linked to sea-level rise and reflects changing hydrological conditions (desalinization) of the aquifer. We suggest that decelerated leaching of excess 234U from the previously unsaturated bedrock traces Holocene relative sea-level rise. Considering this proxy, the resulting mean sea-level reconstruction contains half as much scatter, i.e. improves by a factor of two, when compared to previously published work for the period between 8 and 4 ky BP.

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Cover Caption: The cover image is based on the Research Article Microbially promoted calcite precipitation in the pelagic redoxcline: Elucidating the formation of the turbid layer by Kerstin M. Leberecht, et al., https://doi.org/10.1111/gbi.12492

Microbially promoted calcite precipitation in the pelagic redoxcline: Elucidating the formation of the turbid layer.

Large bell-shaped calcite formations called "Hells Bells" were discovered underwater in the stratified cenote El Zapote on the Yucatán Peninsula, Mexico. Together with these extraordinary speleothems, divers found a white, cloudy turbid layer into which some Hells Bells partially extend. Here, we address the central question if the formation of the turbid layer could be based on microbial activity, more specifically, on microbially induced calcite precipitation. Metagenomic and metatranscriptomic profiling of the microbial community in the turbid layer, which overlaps with the pelagic redoxcline in the cenote, revealed chemolithoautotrophic Hydrogenophilales and unclassified β-Proteobacteria as the metabolic key players. Bioinformatic and hydrogeochemical data suggest chemolithoautotrophic oxidation of sulfide to zero-valent sulfur catalyzed by denitrifying organisms due to oxygen deficiency. Incomplete sulfide oxidation via nitrate reduction and chemolithoautotrophy are both proton-consuming processes, which increase the pH in the redoxcline favoring authigenic calcite precipitation and may contribute to Hells Bells growth. The observed mechanism of microbially induced calcite precipitation is potentially applicable to many other stagnant sulfate-rich water bodies.

Biogeochemical and mineralogical effects of Fe-P-S dynamics in sediments of continental shelf sea: Impact of salinity, oxygen conditions, and catchment area characteristics

In this study, we investigate how salinity, oxygen concentration and catchment area characteristics impact the dynamics of Fe-P-S cycling in the continental shelf sea sediments. Samples were collected from three sites representing different environmental conditions: Gdańsk Deep (southern Baltic Sea), Gotland Deep (central Baltic Sea) and Bothnian Sea (northern Baltic Sea). Sediments were analysed for their mineral composition and speciation of iron and phosphorus. The main groups of Prokaryota involved in Fe-P-S cycling in sediments were indicated. Concentrations of sulphate, hydrogen sulphide, alkalinity, chloride, calcium, phosphate and iron were measured in pore waters. We demonstrated that in the eutrophicated southern region with moderate salinity and oxygen deficit in bottom water, sediments had high potential for retaining Fe and releasing P as indicated by high concentrations of pyrite and labile forms of phosphorus, respectively. Strong salinity stratification and intermittent pelagic redoxcline in the central Baltic Sea resulted in a clearly higher rate of pyrite deposition. Sediment was enriched with Mn due to the formation of Ca-Mn carbonates driven by intensive Mn redox cycling and sulphate reduction. Because of high availability of Mn oxides connected with episodic inflows of oxic seawater from the North Sea, sulphate was present in the entire profile of the studied sediments in the Gotland Deep. Sediments in the well-oxygenated, virtually fresh and rich in land-derived iron northern Baltic Sea retained significant amounts of P in authigenic minerals. Organic matter mineralisation in the surface sediment of this area was dominated by iron reduction. The variability of environmental conditions and consequent availability of electron acceptors were the cause of regional differences in the composition of Prokaryota communities – the number of sulphate reducers in the Gdańsk and Gotland Deeps was greater than in the Bothnian Sea, where there were more Fe reducers and bacteria that oxidise Fe and S.

Candidatus Sulfurimonas marisnigri sp. nov. and Candidatus Sulfurimonas baltica sp. nov., thiotrophic manganese oxide reducing chemolithoautotrophs of the class Campylobacteria isolated from the pelagic redoxclines of the Black Sea and the Baltic Sea

Species of the genus Sulfurimonas are reported and isolated from terrestrial habitats and marine sediments and water columns with steep redox gradients. Here we report on the isolation of strains SoZ1 and GD2 from the pelagic redoxcline of the Black Sea and the Baltic Sea, respectively. Both strains are gram-stain-negative and appear as short and slightly curved motile rods. The autecological preferences for growth of strain SoZ1 were 0–25°C (optimum 20°C), pH 6.5–9.0 (optimum pH 7.5–8.0) and salinity 10–40gL−1 (optimum 25gL−1). Preferences for growth of strain GD2 were 0–20°C (optimum 15°C), pH 7.0–8.0 (optimum pH 7.0–7.5) and salinity 5–40gL−1 (optimum 21gL−1). Strain SoZ1 grew chemolithoautotrophically, while strain GD2 also showed heterotrophic growth with short chained fatty acids as carbon source. Both species utilized hydrogen (H2), sulfide (H2S here taken as the sum of H2S, HS− and S2−), elemental sulfur (S0) and thiosulfate (S2O32−) as electron donors and nitrate (NO3−), oxygen (O2) and particulate manganese oxide (MnO2) as electron acceptors. Based on 16S rRNA gene sequence similarity, both strains cluster within the genus Sulfurimonas with Sulfurimonas gotlandica GD1T as the closest cultured relative species with a sequence similarity of 96.74% and 96.41% for strain SoZ1 and strain GD2, respectively. Strains SoZ1 and GD2 share a ribosomal 16S sequence similarity of 99.27% and were demarcated based on average nucleotide identity and average amino acid identity of the whole genome sequence. These calculations have been applied to the whole genus. We propose the names Candidatus Sulfurimonas marisnigri sp. nov. and Candidatus Sulfurimonas baltica sp. nov. for the thiotrophic manganese reducing culture isolates from the Black Sea and Baltic Sea, respectively.

Subaqueous speleothems (Hells Bells) formed by the interplay of pelagic redoxcline biogeochemistry and specific hydraulic conditions in the El Zapote sinkhole, Yucatán Peninsula, Mexico

Abstract. Unique bell-shaped underwater speleothems were recently reported from the deep (∼ 55 m) meromictic El Zapote sinkhole (cenote) on the Yucatán Peninsula, Mexico. The local diving community has termed these speleothems as Hells Bells because of their shape and appearance in a dark environment in ∼ 28–38 m water depth above a sulfidic halocline. It was also suggested that Hells Bells form under water, yet the mystery of their formation remained unresolved. Therefore, we conducted detailed hydrogeochemical and geochemical analyses of the water column and Hells Bells speleothems including stable carbon isotopes. Based on the comprehensive results presented in this study we deduce that both biogeochemical processes in the pelagic redoxcline and a dynamic halocline elevation of El Zapote cenote are essential for Hells Bells formation. Hells Bells most likely form in the redoxcline, a narrow 1–2 m thick water layer immediately above the halocline where a pelagic chemolithoautotrophic microbial community thrives from the upward diffusion of reduced carbon, nitrogen and sulfur species released from organic matter degradation in organic-rich debris. We hypothesize that chemolithoautotrophy, in particular proton-consuming nitrate-driven anaerobic sulfide oxidation, favors calcite precipitation in the redoxcline and hence Hells Bells formation. A dynamic elevation of the halocline as a hydraulic response to droughts, annual tidal variability and recharge events is further discussed, which might explain the shape of Hells Bells as well as their occurrence over a range of 10 m water depth. Finally, we infer that highly stagnant conditions, i.e., a thick halocline, a low-light environment and sufficient input of organic material into a deep meromictic cenote are apparent prerequisites for Hells Bells formation. This might explain their exclusivity to only a few cenotes in a restricted area of the northeastern Yucatán Peninsula.

Effect of fluctuating oxygen concentration on iron oxidation at the pelagic ferrocline of a meromictic lake

Environmental context The cycling of iron plays an important role in pelagic boundary zones such as the oxic–anoxic interface where physical and chemical gradients occur. The turnover of iron in this zone depends on oxygen fluctuation and the duration of the fluctuation event. This study increases the understanding of biogeochemical iron transformation in such hotspots. Abstract In stratified iron-rich lakes, the interface between oxic and anoxic water bodies, the oxycline, is accompanied by a steep gradient of dissolved iron, the ferrocline. It is a hotspot of biogeochemical transformations, namely the cycling of iron (Fe). The rate of iron oxidation, both chemical and microbial, depends on pH, iron and oxygen concentration, and microbial activity. We investigated the ferrocline of the meromictic Lake Waldsee to find out how the ferrocline is influenced by fluctuating oxygen concentrations. We measured diurnal fluctuations of Fe2+, O2 and pH along vertical profiles during two campaigns in July and September 2011 as well as rates of iron oxidation in laboratory incubations. The oxygen content of the water column varied both between the campaigns and diurnally. We observed a diurnal intrusion of O2 into the ferrocline. The diurnal signal was visible in the iron profile in July but not in September. Iron oxidation rates determined in the laboratory demonstrate the importance of microbial iron reduction and the strong pH dependency. We related the reaction timescales for iron oxidation to the characteristic timescale of oxygen fluctuations by calculating non-dimensional numbers. This analysis showed that an oxygenation event had to last at least 10h in order to affect the depth and vertical extent of the ferrocline, which was the case in July but not in September. Our results show that the duration of events can be an important parameter regulating biogeochemical interactions in pelagic redoxclines.

N and O isotope fractionation in nitrate during chemolithoautotrophic denitrification by Sulfurimonas gotlandica.

Chemolithoautotrophic denitrification is an important mechanism of nitrogen loss in the water column of euxinic basins, but its isotope fractionation factor is not known. Sulfurimonas gotlandica GD1(T), a recently isolated bacterial key player in Baltic Sea pelagic redoxcline processes, was used to determine the isotope fractionation of nitrogen and oxygen in nitrate during denitrification. Under anoxic conditions, nitrate reduction was accompanied by nitrogen and oxygen isotope fractionation of 23.8 ± 2.5‰ and 11.7 ± 1.1‰, respectively. The isotope effect for nitrogen was in the range determined for heterotrophic denitrification, with only the absence of stirring resulting in a significant decrease of the fractionation factor. The relative increase in δ(18)ONO3 to δ(15)NNO3 did not follow the 1:1 relationship characteristic of heterotrophic, marine denitrification. Instead, δ(18)ONO3 increased slower than δ(15)NNO3, with a conserved ratio of 0.5:1. This result suggests that the periplasmic nitrate reductase (Nap) of S. gotlandica strain GD1(T) fractionates the N and O in nitrate differently than the membrane-bound nitrate reductase (Nar), which is generally prevalent among heterotrophic denitrifiers and is considered as the dominant driver for the observed isotope fractionation. Hence in the Baltic Sea redoxcline, other, as yet-unidentified factors likely explain the low apparent fractionation.

Close coupling of N‐cycling processes expressed in stable isotope data at the redoxcline of the Baltic Sea

AbstractOver the past decades, the hypoxic state of the central Baltic Sea has deteriorated because of eutrophication, but little is known about the extent to which related factors such as nitrogen removal have been altered. The Baltic Sea is a stratified semi‐enclosed basin with a large, anoxic bottom water mass in its central Gotland Basin and highly active microbial nitrogen transformation processes at the redoxcline, the interface between oxic and anoxic waters. In this study, we identified and quantified the dominant transformation processes of reactive nitrogen by exploiting fine resolution profiles of δ15NNO3, δ18ONO3, and δ15NNH4 through the pelagic redoxcline between 60 and 140 m. Our results showed increasing δ15NNO3 and δ18ONO3 values with decreasing nitrate concentrations, but the associated low apparent isotope effect (ε = ~5‰), as inferred from a closed system Rayleigh model, was not consistent with the high ε (~25‰) characteristic of denitrification in the water column. These findings could be explained by substrate limitation. The observed δ18ONO3:δ15NNO3 ratio of 1.38:1 rather than the usual 1:1 ratio typical for denitrification‐dominated systems could be explained by the occurrence of both nitrification and denitrification. We then developed a numeric reaction‐diffusion model, according to which a realistic denitrification rate of 14 nmol N L−1 d−1 was estimated, and a nitrification rate of 6.6 nmol N L−1 d−1 confirmed. Our study not only demonstrates the value of stable isotope data for investigating nitrogen transformation processes but also highlights that care is needed in interpreting systems with closely coupled processes such as those at ocean redoxclines.

Morphological and molecular approaches reveal highly stratified protist communities along Baltic Sea pelagic redox gradients

The deep basins of the central Baltic Sea are characterized by anoxic and sulfidic bottom water and steep vertical pelagic redox gradients. The highly active prokaryotic assem- blages of this and other redox transition zones have been intensely studied, while knowledge on the protistan communities remains fragmentary. Thus, we conducted a multi-annual microscopy- based study, combined in one year with 18S rRNA gene and transcript-based DGGE fingerprints to identify the dominant protist taxa and to assess their vertical distribution. Both approaches, applied in high vertical resolution, demonstrated strong stratifications of the protist community composition along the redox gradient. The suboxic zone was dominated by dinoflagellates and oligotrichous ciliates related to Strombidium, whereas the interface and upper sulfidic zone were dominated by ciliates of the genera Mesodinium and Metacystis. Several flagellate taxa within the jakobids, euglenozoans and choanoflagellates occurred exclusively in sulfidic water. Our morpho- logical approach indicates that the pelagic redoxclines of the central Baltic Sea are inhabited by a stable and characteristic protist community. Incongruously, certain taxa (e.g. Mesodinium and Metacystis sp.) which remained undetected by the molecular fingerprinting technique could be identified and enumerated by microscopic observations, whereas small and virtually amorphous protists (especially flagellates) were detected only by sequencing DGGE bands. Fine-scaled assessment of dominant protists in distinct redox strata is a crucial step in understanding their impact and interactions with the prokaryotic world and the biogeochemical processes they medi- ate in these zones.

Stationary sinking velocity of authigenic manganese oxides at pelagic redoxclines

The redox-sensitive trace metal manganese (Mn) is an important electron donor and acceptor in aquatic environments. In stratified basins, the transformation between dissolved Mn2+, 3+ and particulate Mn(III/IV) oxides constitutes the “manganese pump” at the redoxcline, which separates the oxic surface from anoxic bottom waters. In addition to fluxes of dissolved components and reaction rates, accurate estimates of the sinking velocities of authigenic Mn oxides (MnOx) are an important prerequisite to balance and model such euxinic systems. However, models describing biogeochemical cycles suffer from the lack of measured particle sinking velocities and instead must rely on values calculated according to Stokes' law or on estimates derived from budget calculations. Here we present the first stationary sinking velocities of authigenic MnOx particles from the pelagic redoxclines of the currently anoxic Landsort and Gotland Deeps (Baltic Sea). Sinking velocities were determined on board ship by using high-resolution backlit imaging (shadowgraphy) and manual particle tracking. Within the typical size range of MnOx particles (2–20μm), the average sinking velocity of 1972 particles was determined to be 0.76md−1, distinctly lower than the values calculated according to Stokes' law and those used in various models. This difference could be attributed to the observed star-shaped structure of the MnOx particles, which was far away from a spherical shape, and to the embedment of the particles in a complex organic matrix, which significantly modified their density and consequently their sinking velocity.

Pyruvate utilization by a chemolithoautotrophic epsilonproteobacterial key player of pelagic Baltic Sea redoxclines

Pelagic redoxclines of the central Baltic Sea are dominated by the epsilonproteobacterial group Sulfurimonas GD17, considered to be the major driver of chemolithoautotrophic denitrification in this habitat. Autecological investigations of a recently isolated representative of this environmental group, Sulfurimonas gotlandica str. GD1(T), demonstrated that the bacterium grows best under sulfur-oxidizing, denitrifying conditions. However, in the presence of bicarbonate, this strain is able to use pyruvate as both an additional carbon source and an alternative electron donor. These observations suggested that the environmental group GD17 actively metabolizes organic substrates in situ. To examine this possibility, we used RNA-based stable isotope probing (RNA-SIP) on a natural redoxcline community provided with ¹³C-labeled pyruvate. While in this experiment, we were able to identify putative heterotrophic microorganisms, the uptake of ¹³C-pyruvate in GD17 nucleic acids could not be established. To resolve these contradictory findings, combined incorporation experiments with ¹⁴C- and ¹³C-labeled pyruvate were carried out in cells of strain GD1(T) cultivated under chemolithoautotrophic conditions, which favor pyruvate uptake rather than oxidation. An analysis of the labeled biomolecules revealed that pyruvate was mostly incorporated in cellular components such as amino acids, whose synthesis requires only minimal transformation. Carbon transfer into nucleic acids was not observed, explaining the inability of RNA-SIP to detect pyruvate incorporation by strain GD1(T) and the environmental group GD17. Together, these findings suggest that by integrating organic compounds such as pyruvate into cellular components S. gotlandica GD1(T) is able to replenish chemolithoautotrophic growth and thus ensure its survival in nutrient-limited habitats such as marine pelagic redoxclines.

Acetate-utilizing bacteria at an oxic-anoxic interface in the Baltic Sea

Pelagic redoxclines represent chemical gradients of elevated microbial activities. While chemolithoautotrophic microorganisms in these systems are well known as catalysts of major biogeochemical cycles, comparable knowledge on heterotrophic organisms is scarce. Thus, in this study, identity and biogeochemical involvement of active heterotrophs were investigated in stimulation experiments and activity measurements based on samples collected from pelagic redoxclines of the central Baltic Sea in 2005 and 2009. In the 2009 samples, (13)C-acetate 16S rRNA stable isotope probing (16S rRNA-SIP) identified gammaproteobacteria affiliated with Colwellia sp. and Neptunomonas sp. in addition to epsilonproteobacteria related to Arcobacter spp. as active heterotrophs at the oxic-anoxic interface layer. Incubations from sulfidic waters were dominated by two phylogenetic subgroups of Arcobacter. In the 2005 samples, organics, manganese(IV), and iron(III) were added to the sulfidic waters, followed by the determination of metal reduction and identification of the stimulated organisms. Here, the same Arcobacter and Colwellia subgroups were stimulated as in 2009, with Arcobacter predominating in samples, in which manganese(IV) reduction was highest. Our results offer new insights into the heterotrophic bacterial assemblage of Baltic Sea pelagic redoxclines and suggest Arcobacter spp. as a heterotroph with presumed relevance also for manganese cycling.

SUP05 Dominates the Gammaproteobacterial Sulfur Oxidizer Assemblages in Pelagic Redoxclines of the Central Baltic and Black Seas

Gammaproteobacterial sulfur oxidizers (GSOs), particularly SUP05-related sequences, have been found worldwide in numerous oxygen-deficient marine environments. However, knowledge regarding their abundance, distribution, and ecological role is scarce. In this study, on the basis of phylogenetic analyses of 16S rRNA gene sequences originating from a Baltic Sea pelagic redoxcline, the in situ abundances of different GSO subgroups were quantified by CARD-FISH (catalyzed reporter fluorescence in situ hybridization) with oligonucleotide probes developed specifically for this purpose. Additionally, ribulose bisphosphate carboxylase/oxygenase form II (cbbM) gene transcript clone libraries were used to detect potential active chemolithoautotrophic GSOs in the Baltic Sea. Taken together, the results obtained by these two approaches demonstrated the existence of two major phylogenetic subclusters embedded within the GSO, one of them affiliated with sequences of the previously described SUP05 subgroup. CARD-FISH analyses revealed that only SUP05 occurred in relatively high numbers, reaching 10 to 30% of the total prokaryotes around the oxic-anoxic interface, where oxygen and sulfide concentrations are minimal. The applicability of the oligonucleotide probes was confirmed with samples from the Black Sea redoxcline, in which the SUP05 subgroup accounted for 10 to 13% of the total prokaryotic abundance. The cbbM transcripts presumably originating from SUP05 cells support previous evidence for the chemolithoautotrophic activity of this phylogenetic group. Our findings on the vertical distribution and high abundance of SUP05 suggest that this group plays an important role in marine redoxcline biogeochemistry, probably as anaerobic or aerobic sulfur oxidizers.

Impact of protist grazing on a key bacterial group for biogeochemical cycling in Baltic Sea pelagic oxic/anoxic interfaces

Barrier zones between oxic and anoxic water masses (redoxclines) host highly active prokaryotic communities with important roles in biogeochemical cycling. In Baltic Sea pelagic redoxclines, Epsilonproteobacteria of the genus Sulfurimonas (subgroup GD17) have been shown to dominate chemoautotrophic denitrification. However, little is known on the loss processes affecting this prokaryotic group. In the present study, the protist grazing impact on the Sulfurimonas subgroup GD17 was determined for suboxic and oxygen/hydrogen sulphide interface depths of Baltic Sea redoxclines, using predator exclusion assays and bacterial amendment with the cultured representative 'Sulfurimonas gotlandica' strain GD1. Additionally, the principal bacterivores were identified by RNA-Stable Isotope Probing (RNA-SIP). The natural Sulfurimonas subgroup GD17 population grew strongly under oxygen/hydrogen sulphide interface conditions (doubling time: 1-1.5 days), but protist grazing could consume the complete new cell production per day. In suboxic samples, little or no growth of Sulfurimonas subgroup GD17 was observed. RNA-SIP identified five active grazers, belonging to typical redoxcline ciliates (Oligohymenophorea, Prostomatea) and globally widespread marine flagellate groups (MAST-4, Chrysophyta, Cercozoa). Overall, we demonstrate for the first time that protist grazing can control the growth, and potentially the vertical distribution, of a chemolithoautotrophic key-player of oxic/anoxic interfaces.

Partaking of Archaea to biogeochemical cycling in oxygen‐deficient zones of meromictic saline Lake Faro (Messina, Italy)

We used a combination of molecular and microbiological approaches to determine the activity, abundance and diversity of archaeal populations inhabiting meromictic saline Lake Faro (Messina, Italy). Analysis of archaeal 16S rRNA, amoA, accA and hbd genes and transcripts revealed that sub- and anoxic layers of Lake Faro are primarily inhabited by the organisms related to the clusters of Marine Group I.1a of Thaumarchaeota frequently recovered from oxygen-depleted marine ecosystems. These organisms dominated the metabolically active archaea down to the bottom of the lake, indicating their adaptation to recurrent changes in the levels of water column hypoxia. The upper microaerobic layer of Lake Faro redoxcline has the maximal rates of dark primary production much lower than those of other previously studied pelagic redoxclines, but comparable to the values of meso- and bathypelagic areas of Mediterranean Sea. Application of bacterial inhibitors, especially azide, significantly declined the CO2 fixation rates in the low interface and monimolimnion, whereas archaea-specific inhibitor had effect only in upper part of the redoxcline. Based on these findings, we hypothesize that dark bicarbonate fixation in suboxic zone of Lake Faro results mainly from archaeal activity which is affected by the predicted lack in oxygen in lower layers.

Biomarkers for aerobic methanotrophy in the water column of the stratified Gotland Deep (Baltic Sea)

Filter samples from the oxic zone and suboxic zone of the physically stratified water column and sediment samples of the Gotland Deep, Baltic Sea, were analyzed for bacteriohopanepolyol (BHP) and phospholipid fatty acid (PLFA) concentrations. In total, eight BHPs were identified, with the greatest diversity in the suboxic zone. There, 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol) and 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), whose concentrations decreased concurrently from the lower to the upper suboxic zone, indicated type I methanotrophic bacteria and thus aerobic oxidation of methane. The presence and activity of type I aerobic methanotrophic bacteria was further supported by the presence of 13C-depleted PLFAs, specifically 16:1ω8c and 16:1ω5c (δ13C as low as −41.2‰). However, the relative amount of methanotroph-specific compounds was low (aminopentol, <0.2% of total BHPs; 16:1ω8c, ca. 0.5% of total PLFAs), suggesting a minor contribution of aerobic methanotrophic bacteria to the particulate organic matter. The distinctive BHP pattern in the suboxic zone, including aerobic methanotroph biomarkers and a tentative marker for a pelagic redoxcline [putative 22S isomer of the ubiquitous 22R-bacteriohopanetetrol (BHT)], was mirrored in the sediment samples. Our data indicate that a major portion of the sedimentary hopanoids of the Gotland Deep is sourced from the suboxic part of the water column via an effective but unknown transport mechanism.

Protist grazing and viral lysis as prokaryotic mortality factors at Baltic Sea oxic−anoxic interfaces

Interface zones between oxic and anoxic water masses (pelagic redoxclines) host highly active prokaryotic communities, mediating important biogeo- chemical transformations. However, for marine pelagic redoxclines almost no knowledge exists on the magni- tude of the loss processes affecting these prokaryotic communities. We assessed the importance of protist grazing and viral lysis as prokaryotic mortality factors for 2 central Baltic Sea redoxclines using a combination of microscopy and experimental community manipula- tion techniques. Our results demonstrate that protist grazing dominated prokaryotic mortality at suboxic (<30 µmol l !1 oxygen) and oxygen!hydrogen sulphide interface depths, with 50 to 100% of prokaryotic stand- ing stocks grazed daily, compared to 2 to 20% of virally infected cells. Grazing was mediated by 2 different pro- tist associations, viz. dinoflagellates and Strombidium- like ciliates in the suboxic zone, and larger ciliates (cf. Mesodinium, Metacystis spp., cf. Coleps and un - identified morphotypes) at the oxygen!hydrogen sul- phide interface. In contrast, heterotrophic nanoflagel- lates (HNF) played a minor role, generally grazing <5% daily of prokaryotic standing stocks. Thus, these redox zones show major differences in microbial food web structure when compared to surface waters, with ciliates and dinoflagellates constituting the major bacterivores instead of HNF. At sulphidic depths, grazing was below the detection limit, and the frequency of virally infected cells decreased, leaving the identity of the major pro - karyotic mortality factor at these depths unresolved.

Dissolved reactive manganese at pelagic redoxclines (part II): Hydrodynamic conditions for accumulation

Dissolved reactive Mn (dMnreact) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle, Germany). This dMnreact fraction is rapidly oxidised under oxygen atmosphere and is assumed to consist predominantly of Mn(III). There is a distinct increase of dMnreact from the outer regions towards the central part of the Landsort Deep. Although the presence of MnOx particles in the Gotland Basin is evidence of ongoing oxidation of reduced Mn species, almost no dMnreact was detected. Since completely different processes of Mn oxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents and intrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessary for accumulation of dMnreact. Such perturbations supply trace amounts of O2 and H2S, causing either immediate oxidation/reduction of dMnreact or deterioration of its stabilising ligands. dMnreact has also been determined in Lake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification of this lake. In contrast, H2S appearance throughout the entire water column and a pH <5 prevent accumulation of dMnreact in Lake Fuchskuhle.

Dissolved reactive manganese at pelagic redoxclines (part I): A method for determination based on field experiments

Experiments with water samples from the redoxclines of the Black Sea and the Baltic Sea identified a fraction of dissolved Mn which is completely oxidised to solid MnOx within less than 48h at laboratory conditions. Disproportionation of this dissolved reactive Mn (dMnreact) into Mn (II) and Mn (IV) did not occur. Our data suggest that bacteria using oxygen are responsible for the fast oxidation of dMnreact. The operational definition of dMnreact is a Mn phase that passes a 0.45μm filter, but can be separated from remaining dissolved Mn (II) by filtration 48h after exposure to atmospheric oxygen. The application of this method to water samples from the redoxcline of the Black Sea reveals dMnreact profiles comparable to published Mn (III) profiles analysed by polarography thus identifying Mn (III) as the dominating constituent of dMnreact. As the degree of autocatalytic oxidation of dissolved Mn (II) by readily produced MnOx and microbial Mn (II) oxidation within the applied oxidation period is unknown, dMnreact is at least a semi-quantitative measure of dissolved Mn (III). Furthermore, the present method helps to assess the full potential for oxidation of dissolved Mn within aquatic ecosystems. This method has the advantage that sample preparation can be easily done on site, followed by analysis of dissolved Mn by conventional methods.