The roles and functioning of the highly dynamic, changing and disturbed tropical intertidal mudflats of the north-eastern coast of South America located between the Amazon River and the delta of the Orinoco River, despite being considered the muddiest in the world, have been little investigated. Here we present a multidisciplinary study conducted in French Guiana during three consecutive days in November 2016 at the end of the dry season, aimed at describing the composition, primary production and photoregulation processes of a tropical mudflat microphytobenthic biofilm in relation to the sediment characteristics of this pristine environment. We selected three stations characterized by a marked compaction gradient due to mudflat topographic elevation and tidal cycles. The results showed that microphytobenthos are well adapted to extreme physical conditions with respect to light intensities. Indeed, the biomass of primary producers, as well as the rates of primary production, reached high levels, which could be higher than those measured in temperate intertidal mudflats. The thick biofilm of microphytobenthos that appeared on the sediment surface was dominated by common epipelic diatoms, endowed with photoregulatory capabilities, such as downward migration and efficient xanthophyll cycle, which was demonstrated for the first time for such an environment in this study via the de-epoxidation of diadinoxanthin to diatoxanthin. With a typical microbial mat/mudflat composition, prokaryotes also contributed together with diatoms to the essential biological function of the biofilm in mud consolidation, especially with the excretion of extracellular polymeric substances. Although these results are somewhat bitty for establishing general rules, they suggest that the surficial biofilm plays a key role in the functioning, consolidation and dynamics of tropical intertidal mudflats in French Guiana.