Journal
ISME JOURNAL
Volume 13, Issue 8, Pages 1960-1974Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/s41396-019-0386-z
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Funding
- FORMAS [215-2009-813, 215-2010-779]
- BEAM Program
- Baltic Sea Centre at Stockholm University
- Swedish Research Council [VR 2017-03746]
- German Science Foundation [DFG-IV-124/3-1]
- Swedish Research Council [2017-03746] Funding Source: Swedish Research Council
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Ammonium is a central nutrient in aquatic systems. Yet, cell-specific ammonium assimilation among diverse functional plankton is poorly documented in field communities. Combining stable-isotope incubations (N-15-ammonium, N-15(2) and C-13-bicarbonate) with secondary-ion mass spectrometry, we quantified bulk ammonium dynamics, N-2-fixation and carbon (C) fixation, as well as single-cell ammonium assimilation and C-fixation within plankton communities in nitrogen (N)-depleted surface waters during summer in the Baltic Sea. Ammonium production resulted from regenerated (>= 91%) and new production (N-2-fixation, <= 9%), supporting primary production by 78-97 and 2-16%, respectively. Ammonium was produced and consumed at balanced rates, and rapidly recycled within 1 h, as shown previously, facilitating an efficient ammonium transfer within plankton communities. N-2-fixing cyanobacteria poorly assimilated ammonium, whereas heterotrophic bacteria and picocyanobacteria accounted for its highest consumption (similar to 20 and similar to 20-40%, respectively). Surprisingly, ammonium assimilation and C-fixation were similarly fast for picocyanobacteria (non-N-2-fixing Synechococcus) and large diatoms (Chaetoceros). Yet, the population biomass was high for Synechococcus but low for Chaetoceros. Hence, autotrophic picocyanobacteria and heterotrophic bacteria, with their high single-cell assimilation rates and dominating population biomass, competed for the same nutrient source and drove rapid ammonium dynamics in N-depleted marine waters.
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