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Abstract:
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.
