Cell coatings of surf diatoms

Many sandy beach surf-zones support characteristic accumulations of diatom cells appearing as dark brown patches at the water surface. These diatoms become buoyant during the day, adhering to air bubbles generated by wave action. This, together with water circulation patterns generated by rip currents results in the accumulation of diatoms. By late afternoon the cells lose buoyancy and the accumulations disappear. Early investigations suggested that flotation and hence accumulation was facilitated by a mucilaginous cell coating. This coating was considered to be a unifying feature of accumulating surf diatoms. However. the degree to which a mucilage coat occurs is extremely variable. For example, Attheya armatus (West) Crawford has a thick mucilaginous sheath surrounding the celL whereas Aulacodiscus spp. only have thin mucilaginous strands emerging from the rimoportulae and no surface mucilage. The nature of the mucilage coating also differs between the different species. Anaulus australis Drebes er al. Schulz has a thin layer of mucilage that continually dissolves into the water, whereas A. armatus has a permanent coating. AsterionellopsisKiacialis (Castracane) Round and A. armatushave mucilage coats that are resistant to acid removal techniques, while the mucilage layer of A. australis is readily removed. Observations suggest that the flotation mechanism of surf diatoms cannot be simply explained by rnucilages associated with surf diatom surfaces.