Isotopic fractionation of oxygen by respiring marine organisms

Abstract

We measured the respiratory isotope effect ϵ 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 Paraphysomonas imperforata and Bodo sp., and the ciliate Uronema sp. exhibit ϵ resp values in the range 14‐26‰. We also measured ϵ resp for three metazoans. The ϵ resp for the copepod Acartia tonsa ranged from 17 to 25‰, while two larger organisms, the mollusk Mercenaria mercenaria and the salmon Salmo salmar , respire with a smaller ϵ resp of 5‐10‰. The average respiratory isotope effect of the dominant marine respirers (the bacteria, microalgae and zooplankton) is about 20 ± 3‰. An ϵ resp of this magnitude supports the hypothesis that the photosynthesis‐respiration cycle is responsible for the 23.5‰ enrichment in the δ 18 O ratio of atmospheric O 2 relative to seawater (the Dole effect). The large value and high variability in the average ϵ resp limits the usefulness of a proposed method using the δ 18 O of naturally fractionated dissolved O 2 in seawater as a tracer of primary production in the oligotrophic ocean.