Biology Reference
In-Depth Information
mats that form in areas of reduced mainstream velocities take up nutrients from the
sediment. In fact, macroalgae and benthic microalgae are key factors controlling the
fluxes of inorganic and organic N fluxes between the sediment and the water
column (Tyler et al.
2003
). The high concentration gradient between the sediment
and the algal mat may lead to a high nutrient flux (Larned and Atkinson
1997
and
references therein). Kamer et al. (
2004
) found that the relative importance of the
water column vs. the sediment as a source of nutrients for
Ulva intestinalis
varied
with the magnitude of the different source. Additionally, ammonium and urea are
excreted by invertebrates living within the boundary layer on the macroalgal
surface (Probyn and Chapman
1983
; Taylor and Rees
1998
). Gerard and Mann
(
1979
) attributed the high growth rates of a wave-sheltered
Laminaria hyperborea
population to the provision of ammonium from epiphytic bryozoans, an idea
supported by the high levels of nitrogen found directly beneath bryozoan colonies
on
Macrocystis integrifolia
(Hurd et al.
1994
). Similarly, macroalgae may obtain
carbon dioxide released from respiration by bryozoans (Mu˜oz et al.
1991
;
Mercado et al.
1998
).
Another physical determinant on macroalgal nutrition is the algal surface area to
biovolume ratio (SA:V), especially at low concentrations (Rees
2007
). Hein et al.
(
1995
) suggested that N and P uptake rate and maximum capacity (
V
max
) in algae is,
at least in part, linked to their SA:V, so that algae with low SA:V show lower uptake
efficiency and
V
max
values than algae with high SA:V at both low and high nutrient
concentrations. Rees (
2003
), however, found no relationship between the affinity
constant (
K
m
) and SA:V for nitrate, ammonium, and phosphate uptake in
macroalgae. Light reaching macroalgae is (mainly) a two-dimensional phenome-
non while nutrient uptake and use is three-dimensional. Baird and Middleton (
2004
)
studied the relationship between light and N requirements and suggested that for
benthic plants the rates of light absorption to nutrient uptake varies depending on
both plant morphology and hydrodynamic conditions.
4.2.2 Neutral vs. Ionic Nutrients
Most nutrients are in ionic form, impermeable to the lipidic plasma-membrane. Main
exceptions are inorganic carbon in the form of CO
2
and ammonia, NH
3,
which can
penetrate the cell with fluxes of 7-18 mm s
1
and 6-7 mm s
1
,respectively
(S
ultemeyer and Rinast
1996
; Ritchie
1987
; Ritchie and Gibson
1987
) due to their
neutral nature. However, the proportion of inorganic carbon in the form of CO
2
and
that of ammonia in the form of NH
3
are generally much lower than their ionized forms
(HCO
3
/CO
3
¼
and NH
4
þ
respectively) in the range of pH values where most
macroalgae grow. For ionic forms, facilitated transport is necessary; in many cases,
this involves energy-demanding mechanisms. However, this does not mean that
facilitated transport mechanisms involve a lesser cost-effective cell; on the contrary,
these mechanisms allow for the optimization of resources being invested. As an
example, Raven et al. (
2008
) made a theoretical calculation on the transport and
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