ABIOTIC FIXATION OF CARBON DIOXIDE BY DISSOLVED ORGANIC MATTER

Principal Investigator: Tom P. Carsey
Collaborating scientist(s):
Objective: Experimental. spectroscopic and isotopic evidence indicated that certain classes of dissolved organic matter in seawater are capable of incorporating dissolved CO2 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.
Rationale: 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.
Method: 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 calculated.
Accomplishment: 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 sea.
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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