NITRATE RESPIRATION ASSOCIATED WITH DETRITAL AGGREGATES IN AEROBIC BOTTOM WATERS OF THE ABYSSAL NE PACIFIC

Rates of nitrate utilization in tube core respirometers (TCR) placed o ver aggregates on the seafloor at an abyssal site (Station M) in the e astern North Pacific Ocean increased at times of high particle flux. I n the presence of aggregates, both oxygen and nitrate were used in res piration. The ratio of O-2:NO3 concentrations in ambient waters was 3. 9, while O-2:NO3 utilization rates in TCR overlying and TCR aggregate pore waters were 2.6 and 0.6, respectively. We postulated that denitri fication was occurring in microzones of the particle-rich oxygenated ( 135 mu M) waters. To test this, nitrate respiration was measured aboar d a ship in oxygen-minimum( similar to 26 mu M) water supplemented wit h particulate matter collected by a surface net tow. Disserved oxygen consumption occurred immediately, followed by nitrate utilization whil e oxygen was still present. Calculations from cell densities indicated 0.6 mu M of the original 42 mu M of nitrate was assimilated into bact erial biomass during 36 h of incubation, suggesting the major portion of the utilized nitrate was used in respiration. Nitrate utilization r ates in the in situ incubation study and those of the shipboard experi ment were 3.1 and 2.7 mu M d(-1), respectively. The results of the pre sent studies suggest nitrate respiration occurs in microzones of aggre gates in oxygenated bottom waters at times of high particle flux and c auses some loss of fixed nitrogen. (C) 1998 Elsevier Science Ltd. All rights reserved.