Effects of 2E,4E-decadienal on motility and aggregation of diatoms and on biofilm formation.

Phototrophic biofilm formation and dynamics result from the interaction between several parameters, including chemical interactions. Some of the secondary metabolites released by microalgae can influence the composition of benthic communities. We determined the effects of decadienal (DD), a polyunsaturated aldehyde produced by diatoms, on a benthic diatom, Fistulifera saprophila. At 5 μg ml(-1), DD reduced cell motility by 88% and cell adhesion to the substrate by 91%. The effects occurred in less than 30 min. Using a fluorescent probe, we showed that DD could induce nitric oxide (NO) accumulation in F. saprophila cells. Cells exposed to a NO donor presented reduced adhesion and motility, which suggests the involvement of this cellular messenger in the mode of action of DD. Short-term experiments in microcosms showed that the presence of DD on a substrate strongly inhibited biofilm formation. Moreover, when the biofilm was bispecific, DD modified the proportion of the two species present. This indicates that the presence of DD-producing diatoms in a biofilm may favor the presence of certain microalgae at the expense of others. In addition to the effects on adhesion and motility, DD induced the formation of aggregates of F. saprophila cells. Aggregation was independent of NO production. Complementary experiments were performed with two other benthic diatoms, Nitzschia palea and Mayamea atomus. They showed that the effects of DD on adhesion and aggregation were species-dependent.