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Carbon dioxide levels continue to rise in the atmosphere with predictions of a 2-3 oC increase in temperature by the end of the century. One possible strategy to prevent a climate catastrophe is to use iron fertilisation of the ocean to stimulate phytoplankton blooms to capture carbon dioxide. Studies have shown that natural iron fertilization during past glacial periods has repeatedly drawn up to 60 billion tons of carbon into the ocean depths. However, most small-scale iron fertilization experiments have not been very successful in capturing carbon. There are many reasons for this but a major one is that it is not easy to stimulate growth of large-cell diatoms, which seem to be needed to fix large amounts of CO2. Another reason is that increasing iron can also increase microzooplankton grazing, so that as soon as the CO2 is fixed by phytoplankton it is released during the grazing of the phytoplankton. Such grazing activities also affect the production and consumption of dimethylsulfide (DMS), a trace sulfur gas produced by phytoplankton and implicated in formation of low levels clouds that lower sea surface temperatures. This communication describes processes that affected dissolved DMS during the SAGE (SOLAS air-sea gas experiment).