Carbohydrates produced in batch cultures of the surf zone diatom Asterionellopsis glacialis sensu lato: Influence in vertical migration of the microalga and in bacterial abundance

• Treatments without sediment and with fresh sediment had more dissolved and particulate carbohydrates, respectively.
• Lower cell numbers of A. glacialis s. l. in the treatment without sediment indicate a preference for shaded environments.
• Sugars rhamnose, xylose and arabinose, known for increasing mucilage stickiness, accumulated in all treatments with time.
• As these sugars accumulated, the formation of floating aggregates comprising cells, bubbles and sediment was observed.
• Changes in sugar amounts (day 9), accompanied by decreased bacterial counts, indicate a bacteriostatic effect of mucilage.

Surf zone diatoms thrive in the harsh conditions of exposed sandy beaches. The patch-forming Asterionellopsis glacialis sensu lato (A. glacialis s. l.) is the main producer in a rich food chain in Cassino Beach, southern Brazil. This work aimed to verify the influence of sediment particles on the quantity and composition of carbohydrates produced in cultures with A. glacialis s. l. and to investigate the relationship between carbohydrate composition and bacterial abundance. The non-axenic diatom, isolated from Cassino Beach, was batch cultivated for nine days with calcined sediment (CS), fresh sediment (FR), and without sediment (Ctrl). Higher cell counts (p < 0.05), as well as aggregates comprising bubbles, algal cells, and sand occurred in CS and FR. These results indicate that sediment influences growth and aggregate formation, possibly through shading. The highest amounts of dissolved and particulate carbohydrates were found in Ctrl (6.64 ± 1.6 mg L− 1) and FR (30.3 ± 3.3 mg L− 1), respectively. High Performance Liquid Chromatography (HPLC) characterization of particulate carbohydrates showed increases with time in rhamnose, xylose and arabinose in all treatments. The increase of these carbohydrates has the potential to cause A. glacialis s. l. to settle or float by influencing mucus thickness and adhesiveness. A significant decrease (p < 0.005) in bacterial counts occurred when particulate carbohydrates with a very distinct composition (mainly rhamnose and galacturonic acid) appeared on day 9, indicating that compositional changes in the particulate carbohydrates affect bacterial abundance.