PT Unknown
AU Maltby, J
TI Production of greenhouse gases in organic-rich sediments
PY 2015
PU Christian-Albrechts-Universität zu Kiel
WP https://macau.uni-kiel.de/receive/diss_mods_00017688
LA en
DE Methane; Methanogenesis; Sulfate reduction; Competition; Nitrous oxide; Denitrification; Methan; Methanogenese; Sulfatreduktion; Konkurrenz; Distickstoffmonoxid; Denitrifikation
AB Methane (CH4) and nitrous oxide (N2O) are greenhouse gases, which atmospheric concentrations increased since preindustrial times by ~150 and ~20%, respectively, mainly due to the increase in anthropogenic emissions. The atmospheric increase of greenhouse gases (incl. carbon dioxide (CO2), CH4 and N2O) led to various effects on the Earth’s surface and atmosphere, summarized as global climate change. In the marine environment, temperature rise, sea level rise, ocean acidification, and decreased oxygen concentrations are the most significant effects of climate change. To predict possible changes through climate change in the future, natural and anthropogenic sources and production/consumption pathways of greenhouse gases need to be determined carefully. To date, research is sparse on the sources for oceanic CH4 and N2O, thus leading to uncertainties in global ocean emission estimates. Marine, organic-rich sediments (e.g. in coastal areas) are known to be major benthic sources for CH4 and N2O, which eventually could end up in the atmosphere. Both greenhouse gases are produced by microbial processes during the degradation of organic matter in marine sediments, namely methanogenesis and denitrification (next to nitrification), respectively. However, knowledge about magnitude and environmental controls of these microbial processes is still limited.
In the present study, benthic CH4 and N2O production was investigated in three different marine areas with the focus on the surface sediment (0-30 cmbsf=centimeter below surface): the upwelling region off Peru, the Eckernförde Bay in the southwestern Baltic Sea and the Gulf of Mexico. In sediments from Peru and Eckernförde Bay, the focus was set on surface methanogenesis within the sulfate-reducing zone, which has been thought to be negligible due to the successful competition of sulfate reducers for the mutual substrates hydrogen (H2) and acetate. In oil-influenced sediments from the Gulf of Mexico, the focus was set on benthic denitrification and sulfate reduction, with benthic N2O production as a side effect. The investigations showed the following: 
1)	In sediments off Peru, methanogenesis and sulfate reduction co-occurred within the upper 0-30 cmbsf, explained by usage of non-competitive substrates (such as methanol or methylated compounds) by methanogens. In the deeper sediment horizons (>30 cmbsf), usage of the competitive substrates H2 and acetate was confirmed, probably due to a relief of the competitive situation by sulfate depletion. Surface methanogenesis activity varied spatially along the Peruvian margin (70-1024 m water depth), with the major driving factor being the availability and variety of organic matter, followed by oxygen. Thus, highest surface methanogenesis activity was observed on the shelf, where organic carbon load was highest together with hypoxic or even anoxic conditions in the bottom water. At the same time, dissolved CH4 concentrations were also highest on the shelf, indicating a previously underestimated contribution of surface methanogenesis to benthic methane emissions. If not escaping to the water column, the methane produced by surface methanogenesis could act as important methane supplier for anaerobic oxidation of methane in surface sediments. 
2)	Surface methanogenesis was detected on a seasonal basis within the sulfate-reducing zone in sediments (0-30 cmbsf) from the Time Series Station Boknis Eck in Eckernförde Bay (SW Baltic Sea). Methanogenesis activity changed seasonally with highest rates in September/November after the summer/autumn phytoplankton blooms, and lowest rates in March after the period of low production during winter. The main controlling factor for surface methanogenesis was suggested to be the organic matter quantity and quality, followed by temperature and oxygen. The major part of surface methanogenesis was probably facilitated from usage of non-competitive substrates (e.g. methanol and methylated amines) to avoid competition with sulfate reducers, indicated by increased activity after methanol addition and concomitant detection of the family Methanosarcinaceae, whose members are known for usage of non-competitive substrates. Accordingly, usage of competitive substrates such as H2 increased in deeper sediment horizons (>30 cmbsf) when sulfate was depleted. The results revealed that surface methanogenesis could potentially fuel surface anaerobic oxidation of methane with up to 13%. In addition, not only surface methanogenesis activity but also benthic methane emissions showed seasonal variation, indicated by dissolved methane concentrations in the bottom water. 
3)	Denitrification was significantly elevated in oil-influenced (oiled=sedimented oil layer on top) sediments compared to control sediments with no oil influence, identifying this process as a major degrading process. Sulfate reduction was not elevated, hinting towards the restricted availability of sedimented oil to only specific microbial groups. Sulfide-induced N2O production, resulting from elevated sulfate reduction, was not observed. However, all investigated sediment samples revealed the potential for being a N2O source, probably resulting from elevated denitrification rates. 
In summary, the successful detection of surface methanogenesis in two, organic-rich, coastal systems shows its previously underestimated role in benthic methane budgeting, e.g. as a methane supplier for anaerobic oxidation of methane or as a contributor to benthic methane emissions to the water column. In addition to spatial variation, also seasonal variation was identified to play an important role in benthic methane production and emission, which should be included in ocean emission estimates. Production of methane in surface sediments was found to be mainly dependent on organic matter input, temperature and oxygen, and thus could be affected by predicted climate change (e.g. temperature rise). 
The findings on benthic N2O production indicate the crucial importance for studying environmental controls on denitrification in organic-rich sediments as a source of N2O to the water column, including coastal areas and cold seeps. Both systems experience high sulfide concentrations, which would also increase the potential for sulfide-induced N2O production in theses environments.
PI Kiel
ER