The buffering capacity towards free sulphide in sediments of a coastal lagoon (Bassin d'Arcachon, France) - the relative importance of chemical and biological processes

The Bassin d'Arcachon (south-west France) was chosen as a model ecosystem t o study the chemical and microbiological buffering towards free sulphide. D ata were collected on the vertical distribution of oxygen, sulphur and iron compounds and the vertical distribution of colourless sulphur bacteria and sulphide-producing bacteria. In addition, data on the chemical and biologi cal buffering capacity towards free sulphide were collected in sediment slu rries from defined depth layers using a biological sulphide and oxygen moni tor (BOSM) equipped with electrodes for oxygen, sulphide, redox and pH. The data showed that a substantial population of aerobic sulphide-oxidizing ba cteria was present, yet buffering towards free sulphide could mainly be att ributed to chemical processes: in particular, reactions with iron were of i mportance. Interestingly, the potential microbiological rate of sulphide ox idation was orders of magnitude higher than the rate of sulphate reduction reported for this ecosystem. The ecological implications of these observations for the Bassin d'Arcachon are that the powerful biological buffering capacity towards the free sulph ide present will become effective after the chemical buffering capacity has been depleted. Under such conditions the colourless sulphur bacteria will no longer face the competition with iron, and thus may be expected to proli ferate. The crucial factor then becomes the availability of oxygen. (C) 199 9 Academic Press.