An investigation into the release of methyl halides by phytoplankton cultures

Abstract

Methyl halides are known to contribute significantly to ozone destruction in the stratosphere and to play an important part in tropospheric chemistry. The oceans act as both a source and a sink for these methyl halides, and marine organisms have been shown to be a possible major oceanic source. This study describes the results of an investigation into the release of methyl halides by various marine phytoplankton species grown in batch cultures. Phytoplankton cultures were grown in speciHcally designed, gas-tight glass vessels, from which the headspace was analysed every 1-3 days using a gas chromatograph linked to either an electron capture detector (GC-ECD) or a mass spectrometer (GC- MS). Phytoplankton growth was monitored in the cultures by determining cell counts, chlorophyll and nutrient concentrations on frequently sampled small volumes of the culture medium. Initial experiments were conducted using the following species: Isochrvsis galbana. Phaeocvstis globosa, Chaetoceros sp., Emiliania huxleyi and Thalassiosira gravida. Methyl chloride, methyl bromide and methyl iodide production was detected for most species. Some apparent removal of methyl halides from the culture vessels was also observed. Dimethyl sulphide (DMS) production was also detected. Culture experiments were designed to examine i) the effect of different nutrient regimes and ii) the addition of grazers (Oxvrrhis marina) on methyl halide release from microalgal cells. Methyl chloride and methyl bromide production normalised to chlorophyll or cell counts generally increased with increasing initial nitrate concentration, and the introduction of grazers was shown to produce a short term increase in methyl halide release. Natural phytoplankton populations sampled from a Norwegian fjord in May 2000 were maintained in culture vessels for a few days at close to in situ conditions and methyl iodide production detected, which was apparently affected by the supplemented nutrient regime. The extent to which this study has given further insight into the factors that affect methyl halide production by marine phytoplankton and the mechanisms involved in their release are discussed.

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