Environmental changes affect the microbial release of hydrogen sulfide and methane from sediments at Boknis Eck (SW Baltic Sea)

Abstract

Anthropogenic activities are modifying the oceanic environment rapidly and are causing ocean warming and deoxygenation, affecting biodiversity, productivity, and biogeochemical cycling. In coastal sediments, anaerobic organic matter degradation essentially fuels the production of hydrogen sulfide and methane. The release of these compounds from sediments is detrimental for the (local) environment and entails socio-economic consequences. Therefore, it is vital to understand which microbes catalyze the re-oxidation of these compounds under environmental dynamics, thereby mitigating their release to the water column. Here we use the seasonally dynamic Boknis Eck study site (SW Baltic Sea), where bottom waters annually fall hypoxic or anoxic after the summer months, to extrapolate how the microbial community and its activity reflects rising temperatures and deoxygenation. During October 2018, hallmarked by warmer bottom water and following a hypoxic event, modeled sulfide and methane production and consumption rates are higher than in March at lower temperatures and under fully oxic bottom water conditions. The microbial populations catalyzing sulfide and methane metabolisms are found in shallower sediment zones in October 2018 than in March 2019. DNA-and RNA profiling of sediments indicate a shift from primarily organotrophic to (autotrophic) sulfide oxidizing Bacteria, respectively. Previous studies using data collected over decades demonstrate rising temperatures, decreasing eutrophication, lower primary production and thus less fresh organic matter transported to the Boknis Eck sediments. Elevated temperatures are known to stimulate methanogenesis, anaerobic oxidation of methane, sulfate reduction and essentially microbial sulfide consumption, likely explaining the shift to a phylogenetically more diverse sulfide oxidizing community based on RNA.

Disentangling artificial and natural benthic weathering in organic rich Baltic Sea sediments

“…The ratio of F TA /F Ca of 1.45 (Table 3) in the calcite treated cores is lower than the theoretical value of 2 (Equation 2), which points toward a loss of alkalinity which is not associated with Ca. We suggest that TA derived from CaCO 3 dissolution was diminished by the oxidation of reduced species in the oxic surface sediment (Wallmann et al, 2008;Dale et al, 2011Dale et al, , 2021Rassmann et al, 2020;Perner et al, 2022). The F TA / F Ca values which represent the ratio of enhanced fluxes compared to the control experiment, though, are close to 2 (1.99), which would be the ratio expected for calcite dissolution.…”

“…For studying the composition of the microbial community, a 16S amplicon library compatible with the Illumina MiSeq System was prepared as described previously (Böhnke et al, 2019;Perner et al, 2022). Briefly, RNA was extracted from around 300 mg sediment sample using the NucleoBond RNA Soil Mini kit (Macherey-Nagel, Düren, Germany) followed by subsequent DNaseI digestion using the RapidOut DNA Removal kit (Thermo Fisher Scientific GmbH, Waltham, Massachusetts, USA).…”

“…Additionally, bioturbation might entrain added alkaline substrates into the surface sediments where microbial activity can lead to very low pH and subsequently low Ω Cal values, which may enhance dissolution of both olivine and calcite (Meysman et al, 2015;Meysman and Montserrat, 2017;Montserrat et al, 2017). However, the shallow sedimentary depocenters in the Baltic Sea display strong biogeochemical and physical seasonality, which leads to a highly complex chemical system governed by variable oxygen availability, bottom water temperatures, microbial activities, and organic matter degradation rates (Dale et al, 2011;Melzner et al, 2013;Perner et al, 2022). Moreover, the large benthic background fluxes induced by natural CaCO 3 input via continental erosion and diatom dissolution (Gasiunaite et al, 2005;Borawska et al, 2022;Wallmann et al, 2022) constitute further challenges for disentangling natural and enhanced weathering, as they might overprint potential fluxes induced by enhanced benthic weathering.…”

Benthic-pelagic coupling and isotopic fractionation of barium in Kiel Bight, SW Baltic Sea

“…We do not observe a correlation between pore water Ba and the major nutrients PO 4 3− , SiO 4 4− and NH 4 + . However, such a correlation is not expected given that below the surface sediments (>1 -2 cm), nutrient and Ba profiles are governed by different processes: PO 4 3− , NH 4 + and SiO 4 4− concentrations continue to increase (Figures 3E-G) due to remineralization of refractory organic matter (Dale et al, 2013;Perner et al, 2022) and opal dissolution, respectively. In contrast, dissolved Ba concentrations decrease and approach equilibrium with respect to barite (Figures 3H, I).…”

“…This observation indicates that both non-lithogenic Ba and fresh organic material are present within the uppermost sediment layer. Furthermore, intense remineralization of fresh organic material also occurs within the uppermost 1 -2 cm of the sediment (Dale et al, 2013;Perner et al, 2022), which can explain Ba release into the pore water (Figure 4).…”