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assimilation costs of nonfacilitated vs. facilitated mechanisms for N in the form of
NH
4
þ
. In the case of the passive entry of NH
4
þ
, it requires the use of 560 times the N
and NADPH to synthesize the relevant proteins, and 1.61 times the ATP to make the
proteins and in running the transport and assimilation processes than in the case of
facilitated uptake and assimilation of NH
4
þ
. These outcomes clearly show an
optimized input of resources (N and energy) in achieving the outcome of a given
rate of NH
4
þ
assimilation in the case of a very low external concentration of NH
4
þ
.
However, for some seaweeds, the relationship between uptake rate and concentration
is a combination of a hyperbolic component due to active uptake and a linear diffusive
component (Taylor and Rees
1998
). These authors showed that seaweeds from New
Zealand have lower ammonium uptake rates than their northern hemisphere
counterparts, and suggested that uptake was due largely to passive diffusion of NH
3
.
4.2.3 N vs. P Limitation
Under conditions of coastal eutrophication (see also Chap.
21
by Teichberg), fast-
growing, ephemeral macroalgae bloom at the expense of slow-growing, perennial
macroalgae. As mentioned above, this change in community composition has been
explained by a differential ability to exploit and utilize inorganic nitrogen among
macroalgae with different growth strategies. It is generally assumed that inorganic
N availability is the main control for macroalgal growth in temperate coastal areas
(Nixon and Pilson
1983
; Oviatt et al.
1995
; Howarth et al.
2000
); however, some
coastal areas have been identified as phosphorus- rather than nitrogen-limited.
In tropical latitudes, P limitation might be established as carbonate sediments
derived from coral reefs may sequester phosphate (Lapointe et al.
1992
;
McGlathery et al.
1994
; Lapointe and Bedford
2010
; see also Chap.
16
by Mejia
et al.), although other studies have shown exceptions to this situation (Larned
1998
;
Fong et al.
2001
; Elser et al.
2007
). Teichberg et al. (
2010
) made a global survey of
the growth response of
Ulva
spp. to experimental N or P enrichment across
temperate and tropical sites and found that N and P limitation of growth was
linked directly to nutrient availability and not to geographic or latitudinal
differences as had been previously suggested (Howarth
2008
). Teichberg et al.
(
2010
) indicated that it is likely that ambient N:P ratios may be useful to predict
nutrient limitation in bloom-forming species, despite the variability in the growth
responses, uptake affinities, and tissue N and P storage capacities among
macroalgal taxa (Fujita
1985
; Pedersen and Borum
1996
; Fong et al.
2003
). Barile
(
2004
) found that macroalgae with high uptake affinities for DIN (
Ulva lactuca
,
Chaetomorpha linum
,
U. intestinalis
,
Caulerpa
spp., and
Gracilaria tikvahiae
)
were N-limited in subtropical coastal waters of east-central Florida, where water
N:P was on average 8:1. In southeastern Brazil, macroalgal growth was P-limited
where ambient N:P were greater than 16:1 (Louren¸o et al.
2005
). In contrast, in
Waquoit Bay estuaries, where the water N:P ratio was approximately 3:1 during the
growing season,
G. tikvahiae
(Teichberg et al.
2008
) and
U. lactuca
(Teichberg
