ABIOTIC FIXATION OF CARBON DIOXIDE BY DISSOLVED ORGANIC MATTER
Tom P. Carsey
Experimental. spectroscopic and isotopic evidence indicated that certain
classes of dissolved organic matter in seawater are capable of incorporating
dissolved CO by purely
chemical reactions to form new carbon bonds. The
incorporation seemed to occur with unsaturated and aromatic compounds or any
other class that can easily form free radicals. The objective of the research
was to introduce various classes of marine organic substances, as well as
petroleum hydrocarbons, into sterilized seawater in the presence of added
radioactive bicarbonate, to determine the relative amounts of bicarbonate which
reacted irreversibly into the organic matter.
There were 2 reasons to investigate the abiotic incorporation of mineral
carbon, i.e., carbon dioxide, bicarbonate or carbonate, directly into dissolved organic matter. First, it represents additional fixed carbon not accounted for
by photosynthesis measurements and occurs only with pre-existing organic
matter. Second, marine organic matter produced by photosynthesis is generally
depleted in carbon-13 by about 20-25 parts per thousand relative to a mineral
standard. Dissolved mineral carbon is heavier and not substantially depleted in
carbon-13. Thus, if mineral carbon is incorporated into photosynthesized
organic matter, the resulting material has lost the isotopic signature of its
origin. The origin of much of the dissolved yellow substances in seawater,
humus, is the oxidative polymerization of unsaturated lipids which are depleted
in carbon-13 by about 25 parts per thousand. (Figure) Yet the
humic material is
heavier by about 7 parts per thousand which can be explained by the incorporation of
bicarbonate during the polymerization.
Filtered and sterilized seawater in 500 ml polycarbonate flasks containing
radiobicarbonate was spiked with small quantities of saturated, unsaturated or
aromatic compounds, marine pigments or petroleum fractions. The various sample
flasks were exposed to sunlight for several days before being acidified and
purged with nitrogen to remove unreacted bicarbonate as carbon dioxide. The
non-volatile radiocarbon that was left in the flask was measured in a
scintillation counter and the fraction of incorporation of mineral carbon was
Saturated organic compounds such as hydrocarbons or fatty acids effected no
new carbon incorporation. The unsaturated lipids, petroleum fractions and
pigments all caused incorporation of the radiocarbon. The degree of new carbon
fixation was in proportion the ability of the particular substrates to form
free radicals. The absolute amounts of bicarbonate incorporation during the
experiments were 0.01 to 10 micromoles/liter which is a small fraction of the 2
millimoles/liter in seawater but constitutes a significant addition to the
dissolved organic matter in seawater which ranges from nanomolar to micromolar.
Since the new carbon is incorporated as a carboxyl group this process would
tend to solubilize unsaturated and aromatic hydrocarbons in oil spilled on the
These experiments need to be repeated in pure water to ascertain whether the
free radical reactions are catalyzed by the trace metals in seawater.
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