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demand for most algae. Most algae meet their Ci demands by the operation of so-
called carbon concentrating mechanisms (CCMs). The functional characteristics
of these mechanisms have been reviewed by Giordano et al. (
2005
)(seealso
C and N Temporal Partition
: As mentioned above, facilitated transport and
assimilation mechanisms are cost-effective, optimizing molecules and energy
invested in the process. This does not mean that the amount of energy invested
in these mechanisms is a marginal proportion of that produced during photosyn-
thesis (in addition to C assimilation in the Calvin-Benson cycle). On the contrary,
they account for quite a significant amount of photosynthetic energy producing
values for the photosynthetic quotient (PQ, number of C molecules fixed per O
2
evolved) lower than 1. In
Ulva rigida
, Gordillo et al. (
2003
) calculated that the
amount of photosynthetically derived energy used in processes other than C
fixation (operation of CCMs and other uptake and biosynthesis mechanisms)
can account for nearly as much as 50%. When cultured at high pCO
2
,CCMs
were repressed and the proportion dropped to 28% despite the fact that in this
species, the nitrate reductase activity (NR) was enhanced by CO
2
under
nonlimiting NO
3
supply. This suggests that in this species CCM could well
account for
22% of the gross photosynthetic energy. Since CCM activity and C
fixation need to be coupled, it is reasonable to question whether energy-
demanding nutrient uptake and assimilation (other than C, mainly N) would
benefit from any uncoupling mechanism.
Gordillo et al. (
2002
) suggested that in several brown algae, during the diel
cycle, the reduction of nutrient uptake at the end of the dark period and the
beginning of the light period may indicate a mechanism in which cellular energy
is differentially distributed between nitrate and phosphate uptake mechanisms on
one side and carbon uptake and fixation on the other, depending on the time of the
day (Fig.
4.1
). This strategy has been suggested by Falkowski (
1975
), Keller and
Paerl (
1980
), Lean et al. (
1982
), and Turpin (
1983
) for some microalgal species.
According to Gordillo et al. (
2002
), both the temporal partitioning of C and
nutrients, and the ability to maintain nutrient uptake in darkness are consistent
with a mechanism that reacts to the changing availability of these nutrients in an
intertidal habitat. Gevaert et al. (
2007
) found in
Ulva
that rates of biosynthetic
processes such as ammonium uptake are highest in the morning. Consistent with
this hypothesis, the maximum rate of ammonium assimilation in
U. pertusa
peaked in the morning and coincided with low levels of the photosynthetic
product sucrose, which peaked in the afternoon. There was a diurnal cycle in
the rate of ammonium uptake and assimilation in light and dark, but the ampli-
tude was much greater for assimilation than uptake. Moreover, these authors
suggest that net ammonium assimilation only occurs during the day in
U. pertusa
and that two major roles for diurnal cycles are minimization of interspecific
competition for resources and reduced metabolic costs. Further mechanistic
models for C:N partition can be found in Flynn et al. (
2001
)andWirtzand
Pahlow (
2010
).
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