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2nd ICOS-Finland Scientific Workshop Underwater biota in the northern Baltic Sea as sinks and sources of greenhouse gases Elina Leskinen, Susanna Hietanen, Jorma Kuparinen Department of Environmental Sciences, University of Helsinki, Finland In the Baltic Sea, only the littoral community functions as a real sink to carbon dioxide, CO2. The organisms involved are the photosynthetic bacteria, algae and macrophytes, and the chemoautotrophic microbes. A predicted warming of climate will enhance photoautotrophic processes with extended growth season, especially in springtime, making the littoral community an even more efficient CO2 sink. All auto- and heterotrophic organisms produce CO2 in their respiration, and thus function as CO2 sources. Methane, CH4, is produced by anaerobic microbes in anoxic sediments. The sinks of CH4 are found among the methanotrophic microbes in aerobic environments, and sulfate-reducing bacteria in anaerobic environments. CH4 released from deeper sediments is usually oxidized on its way through the water column, however, with large scale hypoxia the oxidation process is diminished and as a result more CH4 reach the atmosphere. O2, Light CO2 SOURCE Biological sources of nitrous oxide, N2O, are microbes involved in nitrogen cycling, the nitrifying and denitrifiying bacteria. They grow in sediments and water with low oxygen conditions, and produce N2O via the processes of nitrification, nitrifier denitrification and denitrification. The sinks of N2O are found among the denitrifying microbes, which reduce N2O to gaseous N2 in anoxic sediments or water. The scheme shown here is based on what is known of biological processes concerning GHGs. The relative amounts of CO2, CH4 and N2O exchange is not known. Also, it is not known how they change seasonally or due to other environmental variation. Photic zone with light for photosynthesis Pelagic community Littoral community with autotrophic, mixotrophic and heterotrophic organisms including bacteria, archaea, phytoplankton, zooplankton, nekton with autotrophic , mixotrophic and heterotrophic organisms including bacteria, archaea, micro- and macroalgae, macrophytes, micro- and macrofauna CO2 SINK in summer CO2 SOURCE Compensation point, 10 – 20 m CH4 SOURCE in anoxic sediments O2, Dark Pelagic community CO2 SOURCE CO2 SINK in summer Auto-and heterotrophic organisms including bacteria, archaea, zooplankton, nekton Benthic community CH4 SINK?? CO2 SOURCE Auto- and heterotrophic organisms including bacteria, archaea, micro- and macrofauna CO2 SINK Permanent halocline, 60 – 70 m Anoxic, Dark N2O SOURCE CH4 SOURCE?? N2O SOURCE N2O SINK N2O SINK Pelagic community CO2 SOURCE Benthic community Anaerobic, auto- and heterotrophic organisms including bacteria, archaea in anoxic water SOURCES of carbon dioxide, CO2 Autotrophic and heterotrophic organisms in aerobic waters in the pelagic, littoral and benthic areas: Aerobic respiration C6H12O6 + 6 O2 → 6 CO2 + 6 H2O Anaerobic respiration 6 NO3- + 5 CH3OH → 3 N2 + 5 CO2 +7 H2O + 6 OH- SINKS of CO2 Photoautotrophic organisms in the photic zone , chemoautotrophs also below the photic zone: Photosynthesis CO2 + H2O + light → CH2O + O2 Chemosynthesis CO2 + O2 + 4 H2S → CH2O + 4 S + 3 H2O CO2 SOURCE SOURCES of methane, CH4 Anaerobic microbes in anoxic sediments: Methanogenesis, a form of anaerobic respiration. CO2 + 8 H+ + 8 e- → CH4 + 2 H2O SINKS of CH4 Methanotrophs in oxic conditions: Aerobic oxidation of methane CH4 + O2 + 2 H+→ CH3OH + H2O Sulfate-reducing bacteria in the anoxic conditions: Anaerobic oxidation of methane. CH4 + SO42– → HCO3- + HS– + H2O Auto- and heterotrophic organisms including bacteria, archaea and some microfauna in anoxic sediments SOURCES of nitrous oxide, N2O Nitrifying and denitrifiying microbes in low oxygen conditions: Nitrification , nitrifier denitrification and denitrification. NH3 → NH2OH → NO/N2O (→NO2- →NO3-) NH3 → NO2- → N2O NO3-→ NO2- → NO → N2O SINKS of N2O Denitrifying microbes in anoxic sediments and water: Denitrification in sediments and water at anoxia. NO3− → NO2− → NO + N2O → N2