Nitrogenase activity in marine sediments from a temperate saltmarsh lagoon: modulation by complex polysaccharides, ammonium and oxygen

Abstract

Nitrogenase activity (acetylene reduction) was examined in marine sediments associated with seagrass beds and thalassinidean prawn burrows, in a temperate saltmarsh lagoon. Nitrogenase activity in unamended sediments from different microhabitats ranged from undetectable values to ≈0.75 nmol C 2H 4 (g dry sediment) −1·hour −1. The bulk of organic material available to heterotrophic bacteria in saltmarsh lagoons is probably derived from primary production. Amendment of sediments with structural plant polysaccharides showed that xylan and alginate stimulated nitrogenase activity on average by 5 to 18-fold relative to unamended sediments, whereas cellulose and carrageenan were less effective. Amendment of sediments with storage plant polysaccharides produced the greatest stimulation of nitrogenase activity. Addition of laminarin and glycogen (amylopectin) significantly ( p <0.05) stimulated nitrogenase activity by 19 to 92-fold. In contrast to these polysaccharides of plant origin, chitin (a polymer of prawn exoskeletons) did not significantly enhance nitrogenase activity in these sediments. Aerobic conditions stimulated nitrogenase activity by 2 to 20-fold in surface sediments, but not in sub-surface sediments, indicating that aerobic or microaerophilic respiration was a significant source of energy for nitrogenase activity in surface sediments. Oxygen-stimulation of nitrogenase activity was less marked in aerobic sediments around the rim of prawn burrow openings where subsurface sediments had been displaced to the surface by bioturbation. Ammonium did not appear to play a major role in the regulation of nitrogenase activity in these sediments as in situ concentrations were ≈20-fold lower than the concentration (50 μM) which resulted in a slight (0–15%) reduction in nitrogenase activity.