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

Abstract. The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification has developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and it controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Some individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and from soil bacteria (adenosylhopane) through allochthonous input after the Littorina transgression, whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a positive correlation of BHP abundances with Corg and δ15N.

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Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

Biogeosciences ◽

10.5194/bg-9-4969-2012 ◽

2012 ◽

Vol 9 (12) ◽

pp. 4969-4977 ◽

Cited By ~ 26

Author(s):

O. Schmale ◽

M. Blumenberg ◽

K. Kießlich ◽

G. Jakobs ◽

C. Berndmeyer ◽

...

Keyword(s):

Surface Water ◽

Baltic Sea ◽

Water Column ◽

Methane Oxidation ◽

Stable Carbon Isotope ◽

Carbon Isotope Ratio ◽

Depth Interval ◽

Type I ◽

Gotland Deep ◽

Redox Zone

Abstract. Water column samples taken in summer 2008 from the stratified Gotland Deep (central Baltic Sea) showed a strong gradient in dissolved methane concentrations from high values in the saline deep water (max. 504 nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steep methane-gradient (between 115 and 135 m water depth) within the redox-zone, which separates the anoxic deep part from the oxygenated surface water (oxygen concentration 0–0.8 mL L−1), implies a methane consumption rate of 0.28 nM d−1. The process of microbial methane oxidation within this zone was evident by a shift of the stable carbon isotope ratio of methane between the bottom water (δ13C CH4 = −82.4‰ and the redox-zone (δ13C CH4 = −38.7‰. Water column samples between 80 and 119 m were studied to identify the microorganisms responsible for the methane turnover in that depth interval. Notably, methane monooxygenase gene expression analyses for water depths covering the whole redox-zone demonstrated that accordant methanotrophic activity was probably due to only one phylotype of the aerobic type I methanotrophic bacteria. An imprint of these organisms on the particular organic matter was revealed by distinctive lipid biomarkers showing bacteriohopanepolyols and lipid fatty acids characteristic for aerobic type I methanotrophs (e.g., 35-aminobacteriohopane-30,31,32,33,34-pentol), corroborating their role in aerobic methane oxidation in the redox-zone of the central Baltic Sea.

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Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?

Biogeosciences ◽

10.5194/bg-12-4875-2015 ◽

2015 ◽

Vol 12 (16) ◽

pp. 4875-4894 ◽

Cited By ~ 30

Author(s):

C. Lenz ◽

T. Jilbert ◽

D.J. Conley ◽

M. Wolthers ◽

C.P. Slomp

Keyword(s):

Baltic Sea ◽

Water Column ◽

North Sea ◽

The Baltic Sea ◽

The Past ◽

Gotland Deep ◽

Carbonate Formation ◽

The Baltic ◽

Mn Oxides ◽

Deep Area

Abstract. Expanding hypoxia in the Baltic Sea over the past century has led to the development of anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the potential consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of dated sediment cores from eight locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic, hypoxic and euxinic sites are consistent with an active release of Mn from these areas. Although the present-day fluxes are significant (ranging up to ca. 240 μmol m−2 d−1), comparison to published water column data suggests that the current benthic release of Mn is small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate and Mn sulfide layers appear to form independently of inflow events, and are possibly related to the much larger and continuous input of Mn oxides linked to sediment focusing. Whereas Mn-enriched sediments continue to accumulate in the Landsort Deep, this does not hold for the Gotland Deep area. Here, a recent increase in euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), coincides with a decline in sediment Mn content. Sediment analyses also reveal that recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. Our data suggest that eutrophication has not only led to a recent rise in sulfate reduction rates but also to a decline in reactive Fe input to these basins. We hypothesize that these factors have jointly led to higher sulfide availability near the sediment–water interface after inflow events. As a consequence, the Mn oxides may be reductively dissolved more rapidly than in the past and Mn carbonates may no longer form. Using a simple diagenetic model for Mn dynamics in the surface sediment, we demonstrate that an enhancement of the rate of reduction of Mn oxides is consistent with such a scenario. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.

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Bacteriohopanepolyols record stratification, nitrogen fixation and other biogeochemical perturbations in Holocene sediments of the Central Baltic Sea

Biogeosciences Discussions ◽

10.5194/bgd-9-17139-2012 ◽

2012 ◽

Vol 9 (12) ◽

pp. 17139-17165

Author(s):

M. Blumenberg ◽

C. Berndmeyer ◽

M. Moros ◽

M. Muschalla ◽

O. Schmale ◽

...

Keyword(s):

Baltic Sea ◽

Water Column ◽

Stable Stratification ◽

The Baltic Sea ◽

Allochthonous Input ◽

The North ◽

Water Column Stratification ◽

Gotland Deep ◽

History Of ◽

The Baltic

Abstract. The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the Central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and, through allochthonous input after the Littorina transgression, from soil bacteria (adenosylhopane), whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a good correlation of BHP abundances with Corg and δ15N.

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Microbial methane oxidation at the redoxcline of the Gotland Deep (Central Baltic Sea)

Biogeosciences Discussions ◽

10.5194/bgd-9-8783-2012 ◽

2012 ◽

Vol 9 (7) ◽

pp. 8783-8805 ◽

Cited By ~ 1

Author(s):

O. Schmale ◽

M. Blumenberg ◽

K. Kießlich ◽

G. Jakobs ◽

C. Berndmeyer ◽

...

Keyword(s):

Surface Water ◽

Baltic Sea ◽

Water Column ◽

Methane Oxidation ◽

Water Depth ◽

Stable Carbon Isotope ◽

Depth Interval ◽

Type I ◽

Gotland Deep ◽

Key Species

Abstract. Methane concentrations in the stratified water column of the Gotland Deep (Central Baltic Sea) show a strong gradient from high values in the saline deep water (max. 504nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steepest gradient is present within the redoxcline (between 115 and 135 m water depth) that separates the anoxic deep part from the oxygenated surface water, implying a methane consumption rate of 0.28 nM d−1. The process of microbial methane oxidation within the redoxcline is mirrored by a shift of the stable carbon isotope ratio of methane between the bottom water (δ13C CH4 = −82.4‰) and the suboxic depth interval (δ13C CH4 = −38.7‰). A water column sample from 100 m water depth was studied to identify the microorganisms responsible for the methane turnover at the redoxcline. Notably, methane monoxygenase gene expression analyses for the specific water depth demonstrated that accordant methanotrophic activity was due to only one microbial phylotype. An imprint of these organisms on the particular organic matter was revealed by distinctive lipid biomarkers showing bacteriohopanepolyols and lipid fatty acids characteristic for aerobic type I methanotrophic bacteria (e.g. 35-aminobacteriohopane-30,31,32,33,34-pentol). In conjunction with earlier findings, our results support the idea that biogeochemical cycles in Central Baltic Sea redoxclines are mainly driven by only a few microbial key species.

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Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?

Biogeosciences Discussions ◽

10.5194/bgd-11-9889-2014 ◽

2014 ◽

Vol 11 (6) ◽

pp. 9889-9918

Author(s):

C. Lenz ◽

T. Jilbert ◽

D. J. Conley ◽

M. Wolthers ◽

C. P. Slomp

Keyword(s):

Baltic Sea ◽

Water Column ◽

Pore Water ◽

North Sea ◽

The Baltic Sea ◽

Gotland Deep ◽

Carbonate Formation ◽

The Baltic ◽

Mn Oxides ◽

Deep Area

Abstract. Expanding hypoxia in the Baltic Sea over the past century has led to anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of well-dated sediment cores from 8 locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic and hypoxic sites are in line with an active release of Mn from these areas. However, this flux of Mn is only small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate and Mn sulfide layers form independent of inflow events, with pore water Mn produced in deeper layers of the sediment acting as a key Mn source. While formation of Mn enrichments in the Landsort Deep continues to the present, this does not hold for the Gotland Deep area. Here, increased euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), goes hand in hand with a decline in sediment Mn and recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. We postulate that the reduction of Mn oxides by hydrogen sulfide following inflows has become so rapid that Mn2+ is released to the water column before Mn carbonates can form. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.

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