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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