Dr. Oliver Schmale

PICASSO - Process Insights into the sourCes And SinkS of methane in the upwelling region of COncepcíon, DFG SCHM 2503/10-1

From a biogeochemical perspective, upwelling regions and the associated oxygen minimum zones are characterized by the complexity of their oceanographic conditions and the diversity of the biogeochemical transformations that take place along pronounced oxygen gradients. The biomass transportation and subsequent degradation in the sediment under anoxic conditions promotes the canonical microbial methanogenisis. In addition, aerobic methane (CH4) production from bacterial cleavage of methylated compounds (like methylphosphonate, MPn) contributes to CH4 supersaturation in the surface mixed layer. Recent studies have shown that coastal upwelling associated with oxygen minimum zones are considered as the largest pelagic CH4 reservoirs in the ocean, making them marine hotspots for greenhouse gases. However, it is not yet clear whether climate change has the potential to cause an increase in greenhouse gas emissions from upwelling regions. In order to better predict the extent to which greenhouse gas sources and sinks in these areas will be affected by climate change, it is essential to (i) improve the quantification of the individual sources and sinks of relevant greenhouse gases and (ii) gain a mechanistic understanding of the underlying physical and microbial processes to allow better predictions of how a changing environment will affect the source strength of these gases. Based on our own studies in upwelling regions and other oxygen minimum zones, we hypothesize that (i) pelagic microbial CH4 oxidation within the oxic/anoxic transition zone efficiently reduces CH4 flux to the sea surface but is influenced by the intensity of vertical mixing between oxic surface water and CH4-enriched deep water, and that (ii) degradation of MPn in surface waters contributes to CH4 oversaturation and is controlled by the availability of inorganic phosphorus. To explore these questions, we plan to conduct a combination of oceanographic, biogeochemical and microbiological studies along a shelf transect off the coast of Concepción in cooperation with our Chilean partners from the University of Concepción, in order to define areas characterized by different mixing intensities within the oxic/anoxic transition zone. Parallel to high-resolution observations of turbulent mixing, we will assess microbial CH4 oxidation, the abundance of methanotrophic bacteria, and their CH4 oxidation activity. To verify the role of MPn degradation with respect to CH4 production in surface waters, we will perform incubation experiments with surface water characterized by different nutrient availabilities.

MicroMeth - Methane production by microphytobenthos and its contribution to the benthic methane flux from the coastal zone of the Baltic Sea, DFG SCHM 2503/9-1

Increasing natural emissions of the greenhouse gas methane (CH4) substantially impact the Earth climate. In this context, near-shore waters play a key role because water column CH4 concentrations in these areas are much higher when compared with the open waters. However, little is known about CH4 emitters in shallow coastal zones or their contribution to the atmospheric CH4 flux. Moreover, a series of recent studies indicate that microbial CH4 production is not limited to strictly anoxic conditions and might be widespread in the oxic water column. This oxic CH4 production places the CH4 source close to the water surface, thereby intensifying fluxes to the atmosphere. Based on these recent investigations and a proposed linkage between primary production and CH4 formation, we hypothesize that microphytobenthos (MPB) communities, as major primary producer in many coastal systems, play a significant role in shallow-water CH4 dynamics. To explore MPB-associated CH4 production, we will investigate the potential of this CH4 source, by conducting incubation experiments and determining the rates of both primary production and CH4 production by abundant benthic diatom species. The use of a novel cavity ring-down spectroscopy-based semi-continuous method in closed incubation experiments will enable sensitive measurements of CH4 production in high temporal resolution over day/night cycles. The experiments will be undertaken under controlled temperature and light conditions and will use axenic/xenic clonal cultures of selected MPB diatoms to identify the main effectors and those species with the highest CH4 production rates. Experiments using 13C-labeled precursor substrates will be performed to identify and characterize MPB-associated CH4 production pathways. Previous studies of oxic CH4 production were mostly mechanistic laboratory studies that used culture samples; whether the measured CH4 production rates were representative of those in natural systems was not determined. To overcome these limitations in our study, the incubation experiments with clonal cultures will be accompanied by natural MPB communities sampled from the shallow waters at Askö island (Swedish Baltic Sea) and from the inner coastal waters of the Darss-Zingst Bodden Chain (German Baltic Sea), thus covering different sediment types, hydrodynamic conditions, and MPB communities. To align benthic CH4 fluxes from MPB communities with overall benthic and sea-to-air fluxes of CH4, we will determine CH4 fluxes between the sediment, water column, and atmosphere in both study areas.

NArrFix - Nitrogen Argon Measurements for the Quantification of Surface Water Nitrogen Fixation in the Baltic Sea (NArrFix ), DFG SCHM 2503/8-1

Nitrogen (N2) fixation by cyanobacteria is a common phenomenon in the Baltic Sea. It occurs in the absence of dissolved inorganic nitrogen (e.g. nitrate) during mid-summer. Different methodological approaches are used to quantify the fixation rates leading to considerable differences in the nitrogen fixation estimates. The huge range of the different estimates is a consequence of both the considerable interannual variability of the N₂ fixation and huge uncertainties associated with the different approaches (¹⁵N incubation; total N budget; pCO₂ records; phosphate excess) for the quantification of the N₂ fixation and with extrapolating the results from local studies to entire basins. Our approach is based on large-scale records of the surface water N₂ depletion during a cyanobacteria bloom, complemented by Ar measurements to account for the air-sea N₂ gas exchange. The N₂ and Ar concentrations will be determined semi-continuously by means of mass spectrometric analysis of N₂ and Ar in air equilibrated with a continuous flow of surface water (MIMS, Membrane Inlet Mass Spectroscopy). The measurement device will be attached to an established fully automated measurement system for the analysis of surface water trace gases (CO₂, CH₄, O₂, N₂O, CO) on a voluntary observation ship (VOS, “Finnmaid”). Through this the N₂ and Ar concentration between the Mecklenburg Bight and the Gulf of Finland will be obtained with a temporal resolution of 2 – 3 days. Two intense measurement periods will be performed (2021 and 2022) in order to quantify the N₂ fixation during the cyanobacteria high season from June to August. Through this we are aiming to identify the factors which trigger and possibly limit the cyanobacteria growth such as temperature, P availability and meteorological/hydrographic conditions. Concurrent records of the pCO₂ by the existing measurement system will be used for independent estimations of the cyanobacteria biomass production and thus of the associated N₂ fixation. Likewise, measurements of total N and total P will be available through cooperation with the Finnish Environment Institute (SYKE) and facilitate nitrogen budget calculations for consistency tests with our direct N₂ fixation measurements. To estimate the total N₂ fixation, the rates obtained for the upper surface layer must be integrated over depth. This will be achieved by numerical modelling (GETM) of the mixed layer depth which will be defined by different criteria. Extrapolation of the N₂ fixation obtained along the Finnmaid route to entire basins is a more challenging task. In cooperation with the remote sensing group of the Federal Maritime and Hydrographic Agency (BSH), we will make an attempt to use remote sensing data for an extrapolation procedure.

Bubble Shuttle II – Bentho-pelagic transport of methanotrophic microorganisms via gas bubbles, DFG SCHM 2530/7-1

Gas bubble releasing seep sites are relevant methane sources in aquatic systems. In the vicinity of these sites, methanotrophic microorganisms in the sediment and water column play a key role in controlling the methane flux into the atmosphere. Recent studies in the water column surrounding hydrocarbon seeps indicated an elevated abundance and activity of methanotrophic microorganisms in the near field of gas bubble plumes. During our pilot studies conducted at the Coal Oil Point Seep region in California (DFG project “Transport of methane oxidizing microorganisms from the sediment into the water column (Bubble Shuttle)” (SCHM 2530/3-1), we could show for the first time that methanotrophic bacteria were transported by gas bubbles from the sediment into the water column. Within this follow-up project we aim to support these first indications by conducting comprehensive studies on a variety of gas bubble releasing seep sites. The fundamental goal of our project is to evaluate the importance of such a transport mechanism on the pelagic methane turnover at these seep sites. Multidisciplinary studies at different seep locations located in the Coal Oil Point seep field and the North Sea will help us to discuss the different environmental factors, which control the transport efficiency of the bentho-pelagic gas bubble mediated exchange process. By integrating lab-based incubation experiments we will study the activity of seep-associated benthic methanotrophic bacteria in the pelagic environment. Additional phylogenetic analyses will be used to test our hypothesis that the gas bubble transport mechanism impacts the diversity of pelagic methanotrophic bacteria at seep sites. Field studies at a Blowout location in the North Sea and the integration of oceanographic measurements and models will be used to establish a budget for pelagic methanotrophic bacteria in the near field of seep sites. Overall, this approach will help us to discuss the impact of the bubble transport mechanism on the abundance of methanotrophic bacteria and the pelagic methane sink.

selected results

ZooM - Zooplankton associated Methane production, DFG SCHM 2503/5-1

ZooM- project details

Methane is a known greenhouse gas that severely enhances climate change on earth, yet not all methane sources into atmosphere have been identified. A process that might be of importance is the production of methane by microorganisms within the anoxic guts of certain zooplankton species and their fecal pellets. This production takes place in the upper oxygenated water column and thus could have a direct impact on the methane flux between ocean and atmosphere. We hypothesize that highly productive regions like marginal seas, which have never been studied in detail in this context before, are areas of enhanced zooplankton-mediated methane production, which most probably causes the subthermocline methane anomaly that have been sporadically identified in the oxygenated water column of the Baltic Sea. In the ZooM project, we will combine methane chemistry, microbiology, and zooplanktology in a multidisciplinary approach to investigate zooplankton-associated methanogenesis in detail using the Baltic Sea as a model system. We plan to investigate the following key questions: (1) Is the subthermocline methane anomaly a widespread phenomenon in the Baltic Sea, which shows a temporal and spatial variability? (2) Does zooplankton-associated methane production have the potential to support the methane anomaly in the shallow water and how are copepod species and environmental factors like food composition influencing methane production? (3) Which microbes are involved in zooplankton-associated methane production and can we detect differences in methanogenic assemblages and their activities between copepod guts and their fecal pellets?

selected results

Baltic Methane - Aerobic und anaerobic turnover of methane in the water column of the central Baltic Sea (Gotland-Deep und Landsort-Deep), DFG SCHM 2530/2-1

Methane is an important atmospheric trace gas with a relevant impact on earth’s climate. Although aquatic systems represent the most significant source of atmospheric methane, the importance of the marine system seems to be marginal. One effective mechanism that is limiting the flux of methane from the sedimentary reservoir into the atmosphere is the microbial oxidation of methane in the sediment. Compared to the number of studies on the microbial processes of methane oxidation in sediments, water column studies are scarce. Long-time stagnation periods within the deep basins of the central Baltic Sea (Gotland- and Landsort-Deep) have caused anoxic conditions in the deep water with strongly elevated methane concentrations. The transition zone between the oxic and anoxic water bodies (redoxcline) allows a systematic sampling of the water depth that is relevant for the turnover of methane. Thus, the detailed study of the microbial methane oxidation in the Gotland- and Landsort-Deep enables us to get new insights into the cycle of methane in the Baltic Sea that may can be used for a better understanding of the methane turnover in other anoxic/oxic basins in the world. In our multidisciplinary approach we (1) quantify the processes of the turnover of methane in the water column of the Gotland- and Landsort-Deep, (2) identify the organisms which are relevant for the turnover of methane in the water column and study their footprint in the sedimentary geological record, (3) integrate our results into hydrodynamic-biochemical numerical model.

selected results

Bubble Shuttle - Transport of methane oxidizing microbes from the sediment into the water column through gas bubbles, DFG SCHM 2530/3-1:

Bubble Shuttle - project details

The process of microbial methane oxidation in the water column is only insufficiently investigated. Water column studies in the vicinity of gas bubble releasing seep sites show, that the majority of dissolved methane is immediately oxidized by microbes after its injection into the water body, and that only a small fraction of methane is reaching the surface water and the atmosphere. In this project, our multidisciplinary approach is investigating the link between sedimentary and pelagic methanotrophy at gas bubble releasing seep sites (study area: Coal Oil Point, Santa Barbara Basin, California). We hypothesize that at these sites methane oxidizing microbes are transport by gas bubbles from the sediment into the water column. In detail we use gas chemistry and molecular biology to (1) identify the zone of methane oxidation and the organisms responsible for the turnover of methane in the sediment, (2) prove the process of methane oxidation within the surrounding water column, and (3) verify the transport of sedimentary methane oxidizing microbes by gas bubbles through the collection of gas bubbles in different water depths.

selected results

DFG 1144 - "From Mantle to the Ocean: Energy,  Material and Life Cycles on Spreading Axes" - Release and Transport of Methane and Helium at the Mid Atlantic Ridge" DFG SCHM 2530

BONUS Baltic Gas - Methane emission in the Baltic Sea: Gas storage and effects of climate change and eutrophication.