Indirect control of the intracellular nitrate pool of intertidal sediment by the polychaete Hediste diversicolor

In an intertidal flat of the German Wadden Sea, a large sedimentary pool of intracellular nitrate was discovered that by far exceeded the pool of nitrate that was freely dissolved in the porewater. Intracellular nitrate was even present deep in anoxic sediment layers where it might be used for anaerobic respiration processes. The origin and some of the ecological controls of this intracellular nitrate pool were investigated in a laboratory experiment. Sediment microcosms were set up with and without the abundant polychaete Hediste diversicolor that is known to stimulate nitrate production by microbial nitrification in the sediment. Additional treatments were amended with ammonium to mimic ammonium excretion by the worms or with allylthiourea (ATU) to inhibit nitrification by sediment bacteria. H. diversicolor and ammonium increased, while ATU decreased the intracellular nitrate pool in the sediment. Microsensor profiles of porewater nitrate showed that bacterial nitrification was enhanced by worms and ammonium addition. Thus, nitrification formed an important nitrate supply for the intracellular nitrate pool in the sediment. The vertical distribution of intracellular nitrate matched that of the photopigments chlorophyll a and fucoxanthin, strongly suggesting that diatoms were the main nitrate-storing organisms. Intracellular nitrate formation is thus stimulated by the interaction of phylogenetically distant groups of organisms: worms enhance nitrification by feeding on particulate organic matter, excreting ammonium and oxygenating the sediment; bacteria oxidise ammonium to nitrate in oxic sediment layers; and diatoms store nitrate intracellularly.