UPTAKE OF AMMONIUM AND NITRATE BY HETEROTROPHIC BACTERIA AND PHYTOPLANKTON IN THE SUB-ARCTIC PACIFIC

Heterotrophic bacteria are known to account frequently for a large fra ction of NH4+ uptake in many oceans, but NO3- uptake by bacteria is no t thought to be as important. As part of the SUPER program, NH4+ and N O3- uptake by heterotrophic bacteria in the sub-Arctic Pacific was exa mined and compared with use of dissolved free amino acids (DFAA). Hete rotrophic bacteria accounted for 31% (range of < 5-40%) and 32% (5-60% ) of total uptake of NH4+ and NO3-, respectively, over 4 months of stu dy. Although the average uptake of NO3- by bacteria was equal to that of NH4+, bacteria actually assimilated relatively more NO3- than NH4in those samples when the two processes were measured simultaneously. Uptake of NH4+ or NO3- alone was about equal to bacterial biomass prod uction, whereas uptake of dissolved free amino acids (DFAA) was only 4 0% of bacterial production, suggesting that the uptake of other dissol ved organic nitrogen sources is relatively small. Uptake of NH4+ by ba cteria correlated significantly with bacterial production, and both NH 4+ uptake and bacterial production correlated significantly with DFAA turnover. In contrast, there was no significant correlation between an y microbial parameter and NO3- uptake. Biomass production of heterotro phic bacteria appears to be supported largely by DIN uptake and, to a lesser extent, uptake of DFAA in the sub-Arctic Pacific. Uptake of NO3 - is more important in supporting bacterial growth than previously hyp othesized, although it remains the least preferred N source. (C) 1998 Elsevier Science Ltd. All rights reserved.