ISOTOPIC FRACTIONATION OF OXYGEN BY RESPIRING MARINE ORGANISMS

We measured the respiratory isotope effect epsilon(resp) for seven rep resentative unicellular marine organisms. The bacterium Pseudomonas ha lodurans, the diatom Phaeodactylum tricornutum, the phytoflagellates C ryptomonas baltica and Dunaliella tertiolecta, the heterotrophic flage llates Paraphysomoms imperforata and Bodo sp., and the, ciliate Uronem a sp. exhibit epsilon(resp) values in the range 14-26 parts per thousa nd. We also measured epsilon(resp) for three metazoans. The epsilon(re sp) for the copepod Acartia tonsa ranged from 17 to 25 parts per thous and 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 domi nant marine respirers (the bacteria, microalgae and zooplankton) is ab out 20 +/- 3 parts per thousand. An epsilon(resp) of this magnitude su pports the hypothesis that the photosynthesis-respiration cycle is res ponsible for the 23.5 parts per thousand enrichment in the deltaO-18 r atio of atmospheric O2 relative to seawater (the Dole effect). The lar ge value and high variability in the average epsilon(resp) limits the usefulness of a proposed method using the deltaO-18 of naturally fract ionated dissolved O2 in seawater as a tracer of primary production in the oligotrophic ocean.