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 fraction of NH+4 uptake in many oceans, but NO-3 uptake by bacteria is not thought to be as important. As part of the SUPER program, NH+4 and NO-3 uptake by heterotrophic bacteria in the sub-Arctic Pacific was examined and compared with use of dissolved free amino acids (DFAA). Heterotrophic bacteria accounted for 31% (range of <5–40%) and 32% (5–60%) of total uptake of NH+4 and NO-3, respectively, over 4 months of study. Although the average uptake of NO-3 by bacteria was equal to that of NH+4, bacteria actually assimilated relatively more NO-3 than NH+4 in those samples when the two processes were measured simultaneously. Uptake of NH+4 or NO-3 alone was about equal to bacterial biomass production, whereas uptake of dissolved free amino acids (DFAA) was only 40% of bacterial production, suggesting that the uptake of other dissolved organic nitrogen sources is relatively small. Uptake of NH+4 by bacteria 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 any microbial parameter and NO-3 uptake. Biomass production of heterotrophic bacteria appears to be supported largely by DIN uptake and, to a lesser extent, uptake of DFAA in the sub-Arctic Pacific. Uptake of NO-3 is more important in supporting bacterial growth than previously hypothesized, although it remains the least preferred N source.