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      Methane production by three widespread marine phytoplankton species: release rates, precursor compounds, and relevance for the environment

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          Abstract

          <p><strong>Abstract.</strong> The world’s oceans are considered to be a minor source of methane (CH<sub>4</sub>) to the atmosphere although the magnitude of total net emissions is highly uncertain. In recent years the origin of the frequently observed in situ CH<sub>4</sub> production in the ocean mixed layer has received much attention. Marine algae might contribute to the observed CH<sub>4</sub> oversaturation in oxic waters, but so far direct evidence for CH<sub>4</sub> production by marine algae has only been provided for the coccolithophore <i>Emiliania huxleyi</i>. In the present study we investigated, next to <i>Emiliania huxleyi</i>, other widespread haptophytes, i.e. <i>Phaeocystis globosa</i> and <i>Chrysochromulina sp.</i> for CH<sub>4</sub> formation. Our results of CH<sub>4</sub> production and stable carbon isotope measurements provide unambiguous evidence that all three investigated marine algae produce CH<sub>4</sub> per se under oxic conditions and at rates ranging from 1.6 ± 0.5 to 2.7 ± 0.7 µg CH<sub>4</sub> per g POC (particulate organic carbon) d<sup>−1</sup> at a temperature of 20 °C with <i>Chrysochromulina sp.</i> and <i>E. huxleyi</i> showing the lowest and highest rates, respectively. In cultures that were treated with <sup>13</sup>C-labelled hydrogen carbonate δ<sup>13</sup>CH<sub>4</sub> values increased with incubation time, clearly resulting from the conversion of <sup>13</sup>C-hydrogen carbonate to <sup>13</sup>CH<sub>4</sub>. The addition of <sup>13</sup>C labelled dimethyl sulfide, dimethyl sulfoxide and methionine sulfoxide – known algal metabolites that are ubiquitous in marine surface layers - enabled us to clearly monitor the occurrence of <sup>13</sup>C-enriched CH<sub>4</sub> in cultures of <i>Emiliania huxleyi</i> clearly indicating that methylated sulphur compounds are also precursors of CH<sub>4</sub>. We propose that CH<sub>4</sub> production could be a common process among marine haptophytes likely contributing to CH<sub>4</sub> oversaturation in oxic waters.</p>

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          Journal
          Copernicus GmbH
          July 1 2019
          Article
          10.5194/bg-2019-245
          80b0adce-8a01-4963-87dc-28c149cfd60a
          © 2019

          https://creativecommons.org/licenses/by/4.0/

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