Characteristics of a Marine Nitrifying Bacterium, Nitrosocystis Oceanus Sp. N.1

A new species of an ammonia‐oxidizing bacterium, Nitrosocystis oceanus , is described. This is the first nitrifying bacterium ever isolated from open ocean waters and the first species of the genus Nitrosocystis observed in over 30‐years. The cells are gram negative, spherical to ellipsoidal, 1.8–2.2 µ in diameter, and occur as single cells, diploids, and occasionally as tetrads. Cells at times formed cysts and zoogloea. The most consistent and outstanding generic characteristic is the ultrastructure of the cells. Cells of this genus have a cytomembrane organelle composed of a series of flattened vesicles that transverse and bisect the cell; while cells of Nitrosomonas sp. lack this complex membranous organelle, they do have one to two concentric membranes at the periphery of the cell adjacent to the plasma membrane. Nitrosocystis oceanus is an obligate autotroph using only ammonia as an energy source and carbon dioxide as a carbon source. This bacterium grows optimally at 30C and in ammonia concentrations of 5 × 10 8 – 2.4 × 10 5 µ g‐at. NH 3 ‐N/liter. It is an obligate halophile, requiring natural seawater for growth, and it lyses when suspended in distilled water. Growing optimally in continuous cultures, individual cells produced 2 × 10 ‐6 µ M of NO 2 – per day. Nitrosocystis oceunus was cultured repeatedly from offshore waters of the western North and South Atlantic Ocean since 1959. It is estimated that the standing crop of these organisms is less than one bacterium per ml. This number of bacteria in the upper 100 m of tropical waters would produce approximately 0.07 µ g‐at. NO 2 – ‐N liter −1 year −1 .