Biology Reference
In-Depth Information
preferences and dynamics for each N form. The use of nitrate has a clear advantage
over ammonium since it can be stored in a vacuole with a concentration factor
(internal:external) of
1,000 (e.g., Gordillo et al.
2002
), while high concentrations
of ammonium are toxic, acting as an uncoupler (Krogmann et al.
1959
). On the other
hand, ammonium has an energetic advantage over nitrate (but this is a minor issue
when light is saturating). As a cation, less energy is required for transport, and it can
be immediately used for amino acid synthesis via glutamine synthetase/
glutamine-oxoglutarate aminotransferase (GS/GOGAT), while nitrate must be
reduced to nitrite by the highly regulated enzyme nitrate reductase, and then nitrite
must be further reduced to ammonium by the nitrite reductase, investing eight
electrons per NO
3
molecule being reduced to NH
4
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overall (e.g., Raven
1984
).
For most algae, ammonium is the preferred inorganic N form. According to Rees
(
2003
), ammonium uptake is more efficient than nitrate uptake in macroalgae. In
addition, the presence of ammonium can suppress the uptake and assimilation of
less reduced forms of N, mainly nitrate, in phytoplankton (Dortch
1990
) and
macroalgae (DeBoer
1981
; Harrison et al.
1986
; Naldi and Wheeler
2002
). Ele-
vated ammonium concentrations can also result in a decline in the activity of the
primary nitrate-assimilating enzyme, nitrate reductase (NR) in microalgae (Berges
et al.
1995
; Vergara et al.
1998
), and in a brown macroalga,
Giffordia mitchellae
(Weidner and Kiefer
1981
). Ammonium was taken up at a higher rate than nitrate
when presented alone (Lotze and Schramm
2000
; Nishihara et al.
2005
) or simul-
taneously (Naldi and Wheeler
2002
; Cohen and Fong
2004
).
>
4.4.1 Seaweed Diversity Linked to Nutrient Source
Ammonium is primarily associated with local scale regeneration, whereas nitrate
availability is predominantly determined by oceanographic processes in the adja-
cent nearshore and/or runoff from land. The physiological mechanisms by which
each form is used differ substantially, and seaweeds with different morphologies,
physiological capabilities, and life-history strategies might differ in their abilities to
utilize nitrate vs. ammonium (e.g., Naldi and Wheeler
1999
). However, Bracken
and Stachowicz (
2006
) found that, in diverse seaweeds assemblages, the uptake of
either nitrate or ammonium alone was equal to the average of the component
monocultures, but when nitrate and ammonium were available simultaneously,
total N uptake by the assemblage was 22% higher than the average from
monocultures because different species were complementary in the use of the
nitrogen forms. These results provide a mechanistic link between N use and the
diversity of primary producers in marine ecosystems. Previously, Bracken and
Nielsen (
2004
) found that in nitrate-exhausted intertidal pools, slow-growing spe-
cies tolerant to low nitrogen availability were joined by fast-growing species with
higher nitrogen requirements due to increased ammonium loadings coming from
macroinvertebrates. A fourfold increase in the ammonium loading rate was
associated with a doubling in the number of macroalgal species.
