Effects of salt and light stress on the release of volatile halogenated organic compounds by Solieria chordalis: a laboratory incubation study

The red alga Solieria chordalis was used as a model to investigate the effects of salt and light stress on the release of volatile halogenated organic compounds (VHOCs). We used a purge and trap technique coupled with a gas chromatograph equipped with an electron capture detector. S. chordalis was exposed to hyposaline ( 22 psu), hypersaline ( 50 psu) and normal salinity ( 34 psu) media in combination with light stress ( 48 h in the dark followed by 24 h under continuous light). The releases of 9 VHOCs (i.e., CH3I, CH3CH2I, CH2ClI, CH2Br2, CHBrCl2, CHBr2Cl, CH2BrI, CHBr3, CH2 I-2) were measured after 0, 24, 48 and 72 h on algal sub-samples. In algae unstressed by salt, VHOC production rate decreased under dark conditions but increased with light exposure. However, long periods of light also resulted in higher production of di-halogenated compounds ( as compared to tri-halogenated or mono-halogenated compounds). The results from salt-stressed samples indicate that hypo-osmotic conditions strongly enhanced whole VHOC production rate, whereas hyper-osmotic conditions slightly reduced their production. An exception was mono-halogen production, which was higher under hyper-osmotic condition ( especially during dark periods). Furthermore, long exposure to light and hyper-osmotic conditions led to a decrease in the production of di-halogenated compounds ( this did not occur in thalli unstressed by salt). As halogen productions derive from different metabolic pathways, including an enzymatic mechanism, it is possible that the activity of these enzymes may be affected by exposure to various salinity conditions.