Does bivalve mucus favour the growth of their main food source, microalgae?

Bivalves, by releasing inorganic nutrients, can stimulate the growth of the primary producers on which they feed. Dissolved excretion and mineralization of biodeposits (pseudofaeces and faeces), two indirect processes related to their feeding activity, have already been identified. This study investigated whether microalgal growth is also dependent on the mucus secreted by bivalves and rejected with their biodeposits. An experimental study based on algal growth tests was conducted to determine whether the mucus coating the pseudofaeces and faeces of the oyster Crassostrea gigas (Thunberg) could stimulate the growth of nine marine microalgae: a Prasinophyceae [Tetraselmis suecica (Butcher)] and eight Diatomophyceae [Amphora sp. (Ehrenberg), Chaetoceros sp. (Ehrenberg), Entomoneis alata (Ehrenberg), Haslea ostrearia (Simonsen), Nitzschia acicularis (Wm Smith), N. closterium (Wm Smith), N. gandersheimiensis (Krasske) and Skeletonema costatum (Cleve)]. Oysters were fed only inorganic particles at a concentration well above the threshold of pseudofaeces production. A fraction of pseudofaeces and faeces was then inoculated into glass tubes containing a single algal species. Algal growth tests showed that mucus stimulated the growth of four of the nine species studied: Chaetoceros sp., H. ostrearia, N. gandersheimiensis and T. suecica. The strongest responses were observed with H. ostrearia and Chaetoceros sp., for which mean algal biomass was significantly higher with mucus originating from pseudofaeces than faeces. Monitoring of the release of dissolved components from mucus showed significant enrichment in inorganic phosporus. The use of an antibiotic treatment suggested that this dissolved component is released by simple solubilization rather than by bacterial breakdown. (C) Elsevier, Paris / Ifremer / CNRS / IRD.