ISOTOPIC FRACTIONATION OF OXYGEN BY RESPIRING MARINE ORGANISMS

We measured the respiratory isotope effect epsilon(resp) for seven representative unicellular marine organisms. The bacterium Pseudomonas halodurans, the diatom Phaeodactylum tricornutum, the phytoflagellates Cryptomonas baltica and Dunaliella tertiolecta, the heterotrophic flagellates Paraphysomoms imperforata and Bodo sp., and the, ciliate Uronema sp. exhibit epsilon(resp) values in the range 14-26 parts per thousand. We also measured epsilon(resp) for three metazoans. The epsilon(resp) for the copepod Acartia tonsa ranged from 17 to 25 parts per thousand while two larger organisms, the mollusk Mercenaria mercenaria and the salmon Salmo salmar, respire with a smaller epsilon(resp) of 5-10 parts per thousand. The average respiratory isotope effect of the dominant marine respirers (the bacteria, microalgae and zooplankton) is about 20 +/- 3 parts per thousand. An epsilon(resp) of this magnitude supports the hypothesis that the photosynthesis-respiration cycle is responsible for the 23.5 parts per thousand enrichment in the deltaO-18 ratio of atmospheric O2 relative to seawater (the Dole effect). The large value and high variability in the average epsilon(resp) limits the usefulness of a proposed method using the deltaO-18 of naturally fractionated dissolved O2 in seawater as a tracer of primary production in the oligotrophic ocean.