Influence of organic particle addition on nitrification rates and ammonium oxidiser abundances in Baltic seawater

Nitrification is a key microbial process in the nitrogen cycle of the ocean that largely determines the pool sizes of nitrite, nitrate and ammonium but hardly affects the total pool of inorganic nitrogen (except through N 2 O losses to the atmosphere). This study explored the regulation of nitrification and its dependency on particle and ammonium concentrations. Incubation experiments were conducted in which particles of different origin were added to Baltic seawater, followed by 15 NH 4 + -labelling and droplet digital PCR. Both nitrification rates according to particle size (>3 µm and 0.22-3 µm) and archaeal and betaproteobacterial ammonia monooxygenase α-subunit gene ( amoA ) abundances were determined in order to assess the impact of high particle concentrations found in coastal areas of the Baltic Sea. The data show that the addition of particulate organic matter enhanced the nitrification rates in bulk (~805 nmol l -1 d -1 ) but not in the <3 µm fraction (<100 nmol l -1 d -1 ), indicative of a strong influence of the added particles on nitrification rates. The regulation of nitrification rates by the substrate ammonium was suggested by the inverse correlation between these 2 variables. Measurements of amoA gene abundances showed the dominance of ammonium-oxidising bacteria over ammonium-oxidising archaea during most of the 41 d experiment. Abundances of amoA were highest in the particle-associated fraction of the bacteria (≤6.9 × 10 6 amoA copies l -1 ). These findings demonstrate the importance of particle-associated ammonium-oxidising bacteria in the nitrification processes of coastal and estuarine waters containing high particle densities.