Etude du biofilm microalgal des vasières intertidales : dynamique spatio-temporelle à micro-échelle et performances photosynthétiques

The microalgal biofilm, which is present on sediment surface, is the primary production unit on bare intertidal mudlfats. In order to improve our understanding of this primary production system, a study was undertaken at microscale to analyse the structural and functional properties of this biofilm. In the first part of this work, a detailed descriptive analysis lead to the characterisation of the spatiotemporal dynamics of the physical structure of the biofilm. It turns out that the biofilm exhibits its own dynamics at the sediment surface (as a discrete compartment in the top 200 µm), but still has a functional relationship with the total available biomass (PCB) distributed within the top 1 cm (linear relationship between the maximum size of the biofilm during an emersion and the total biomass). This descriptive survey also allowed to show the complexity of the "formation /dispersion" pattern of the biofilm during an emersion, and pointed out the fact that the biofilm tends to remain at the sediment surface all over the course of the emersion period. In the second part of this study, the aim was to assess the photosynthetic performance of in vitro biofilms. Pulse Amplitude Modulated (PAM) fluorometry proved to be a perfect tool to measure the photosynthetic activity of microalgae biofilms, especially by Rapid Light Curves (RLC) despite the complexity of the photobiological response which, in this instance, is a double function of light and time. RLC allowed to show that an increase in the size of the biofilm (the thickness) induces an under-estimation of the "true" physiological response of epipelic microalgae, but an increase of the resistance capacity of the biofilm as a whole to strong illuminations. These results clearly suggest a change in optical properties of the biofilm as a function of its size, and lead to an improvement of our representation of the biofilm during periods of photosynthetic activity.