Nitrogen Metabolism of Aquatic Organisms. Ii. The Assimilation of Nitrate, Nitrite, and Ammonia Bybiddulphia Aurita¹

SUMMARY Biddulphia aurita , a centric diatom, can grow on either nitrate, nitrite, or ammonia as its sole nitrogen, source. Cells remove ammonium nitrogen from the medium 2.3–2.4 times faster than either nitrate or nitrite nitrogen and, when grown for 24 hr in the ammonium medium, contain higher levels of non‐protein nitrogen than cells grown in the nitrate or nitrite medium for the same period of time. The nitrogenous compounds in the nonprotein nitrogen fraction from cells grown in the nitrate, nitrite, or ammonium medium contain the same level of soluble‐free amino nitrogen, combined amino nitrogen, and ammonium nitrogen. The high level of soluble nonprotein nitrogen in the medium of the cells grown in the ammonium medium is due to soluble amide nitrogen which represents 18% of the total soluble nitrogen present in these cells, whereas it represents only 2% in cells from the nitrite medium, and its level is negligible in cells from the nitrate medium. Cells grown in the nitrate medium have both nitrate‐ and nitrite‐reductase activity. Cells grown in the nitrite medium have only nitrite‐reductase activity in significant levels, while cells grown in the ammonium medium lack both enzymes.