Symbiotic association (apparently of filamentous bacteria) forming a dense sleeve (thickness of a finger) disposed around the roots of a big alder. Underwater photograph taken in a pond of a public garden in Lille (north of France). The precise location is a low-velocity zone of a smal stream, the bottom of the pound is rich in red shale particles (from black shale "cooked" in heaps burning in the region). In this place, the water that flows il often are strongly turbid after heavy rains because in part by the runoff paths red shale.