Ecology of phytoplankton 2006
551 pág.

Ecology of phytoplankton 2006


DisciplinaFitoplâncton12 materiais70 seguidores
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biomass
is still not fully clear. Setting this aside, the
direct phagotrophic transfer of photosynthetic
primary products from phytoplankton to zoo-
planktic consumers is not universally achieved
in the pelagic but is, in fact, commonly mediated
by the activities of free-living microbes. Thus, the
dynamic relationships among phytoplankton and
their potential phagotrophic consumers acquire
94 PHOTOSYNTHESIS AND CARBON ACQUISITION
a new interpretative signi\ufb01cance, which is to be
addressed in this and later chapters.
The present chapter prepares some of the
ground necessary to understanding the relation
of planktic photoautotrophy to the dynamics
of phytoplankton populations. After considering
the biochemical and physiological basis of photo-
synthetic production, the chapter compares the
various limitations on the assembly of photoau-
totrophic biomass in natural lakes and seas, and
it considers the implications for species selection
and assemblage composition.
3.2 Essential biochemistry of
photosynthesis
It has been stated or implied several times already
that the paramount requirement of photoau-
totrophic plankton to prolong residence in, or
gain frequent access to, the upper, illuminated
layers of the pelagic is consequential upon the
requirement for light. The need to capture solar
energy in order to drive photosynthetic carbon
\ufb01xation and anabolic growth is no different
from that experienced by any other chlorophyll-
containing photoautotroph inhabiting the sur-
face of the Earth. Indeed, the mechanisms and
ultrastructural provisions for bringing this about
constitutes one of the most universally conserved
processes amongst all photoautotrophic organ-
isms. On the other hand, to achieve, within the
bounds of an effectively opaque and \ufb02uid envi-
ronment, a net excess of energy harvested over
the energy consumed in metabolism requires cer-
tain features of photosynthetic production that
are peculiar to the plankton. Thus, our approach
should be to rehearse the fundamental require-
ments and sensitivities of photosynthetic produc-
tion and then seek to review the aspects of the
pelagic lifestyle that constrain their adaptation
and govern their yields.
Enormous strides in photosynthetic chem-
istry have been made, especially over the last
30 years or so, especially at the molecular and
submolecular levels (Barber and Anderson, 2002).
This progress is not likely to stop so that, undou-
btedly, whatever is written here will have soon
been overtaken by new information. At the same
time, it is possible to predict that future progress
will concern the biochemical and biophysical
intricacies of control and regulation more than
the broad principles of process and order-of-
magnitude yields, which are generally accepted
by physiological ecologists. Thus, the contempor-
ary biochemical basis for assessing phytoplankton
production will continue to be valid for some
time to come.
Photosynthesis comprises a series of reac-
tions that involve the absorption of light quanta
(photons); the deployment of power to the reduc-
tion of water molecules and the release of
oxygen; and the capture of the liberated elec-
trons in the synthesis of energy-conserving com-
pounds, which are used ultimately in the Calvin
cycle of carbon-dioxide carboxylation to form
hexose (Falkowski and Raven, 1997; Geider and
MacIntyre, 2002). The aggregate of these reac-
tions may be summarised:
H2O + CO2 + photons = 1/6[C6H12O6] + O2 (3.1)
As with most summaries, Eq. (3.1) omits not
merely detail but several important intermedi-
ate feedback switches, involving carbon, oxy-
gen and reductant, all of which have a bear-
ing upon the output products and their physio-
logical allocation in active phytoplankters. These
are best appreciated against the background
of the supposed \u2018normal pathway\u2019 of photosyn-
thetic electron transport. The latter was famously
proposed by Hill and Bendall (1960). Their z-
model of two, linked redox gradients (photo-
systems) has been well substantiated, biochemi-
cally and ultrastructurally. In the \ufb01rst of these
(perversely, still referred to as photosystem II,
or PSII), electrons are stripped, ultimately from
water, and transported to a reductant pool.
In the second (photosystem I, or PSI), photon
energy is used to re-elevate the electrochemical
potential suf\ufb01ciently to transfer electrons to car-
bon dioxide, through the reduction of nicoti-
namide adenine dinuceotide phosphate (NADP to
NADPH).
The (Calvin cycle) carbon reduction is based
on the carboxylation reaction. Catalysed by ribu-
lose 1,5-biphosphate carboxylase (RUBISCO), one
molecule each of carbon dioxide, water and
ESSENTIAL BIOCHEMISTRY OF PHOTOSYNTHESIS 95
ribulose 1,5-biphosphate (RuBP) react to yield two
molecules of the initial \ufb01xation product, glycer-
ate 3-phosphate (G3P). This latter reacts with ATP
and NADPH to form the sugar precursor, glycer-
aldehyde 3-phosphate (GA3P), which now incor-
porates the high energy phosphate bond. In the
remaining steps of the Calvin cycle, GA3P is fur-
ther metabolised, \ufb01rst to triose, then to hexose,
and RuBP is regenerated.
At the molecular level, photosynthetic reactiv-
ity is plainly sensitive to the supply of carbon and
water, the photon harvesting and, like all other
biochemical processes, to the ambient tempera-
ture. Measurement of photosynthesis may invoke
a yield of \ufb01xed carbon, the quantum ef\ufb01ciency
of its synthesis (yield per photon), or the amount
of oxygen liberated. None of these is any longer
dif\ufb01cult to quantify but the dif\ufb01culty is still the
correct interpretation of the bulk results. It is
still necessary to consider carefully the regula-
tory role of the ultrastructural and biochemical
components that govern the photosynthesis of
phytoplankton. Special attention is directed to
the issues of photon harvesting, the internal elec-
tron transfer, carbon uptake, RUBISCO activity
and the behaviour of the regulatory safeguards
that phytoplankters invoke in order to function
in highly variable environments.
3.2.1 Light harvesting, excitation and
electron capture
Light is the visible part of the spectrum of
electromagnetic radiation emanating from the
sun. Electromagnetic energy occurs in indivisi-
ble units, called quanta, that travel along sinu-
soidal trajectories, at a velocity (in air) of c \u223c 3 ×
108 m s\u22121. The wavelengths of the quanta de\ufb01ne
their properties \u2013 those with wavelengths (\u3bb)
between 400 and 700 nm (400 \u2013 700 × 10\u22129 m)
correspond with the visible wavelengths we call
light (and within which waveband the quanta are
called photons). The waveband of photosynthetically
active radiation (PAR) coincides almost exactly with
that of light. The white light of the visible spec-
trum is the aggregate of the \ufb02ux of photons of
differing wavelengths, ranging from the shorter
(blue) to the longer (red) parts of the spectrum.
Relative to the solar constant (see Sec-
tion 2.2.2), the PAR waveband represents some
46\u201348% of the total quantum \ufb02ux. The corre-
sponding photon \ufb02ux density averages 1.77 ×
1021 m\u22122 s\u22121. Division by the Avogadro num-
ber (1 mol = 6.023 × 1023 photons) expresses
the maximum \ufb02ux in the more customary
units, einsteins or mols, 2.94 mmol photon
m\u22122 s\u22121. The energy of a single photon, \u3b5´, varies
with the wavelength,
\u3b5´ = h\u2032c/\u3bb (3.2)
where h\u2032 is Planck\u2019s constant, having the value
6.63 × 10\u221234 J s (e.g. Kirk, 1994). Photons at the
red end of the PAR spectrum each contain about
2.84 × 10\u221219 J, about 57% of the content of blue-
light photons (4.97 × 10\u221219 J).
While a given radiation \ufb02ux of light of a
single wavelength can be readily expressed in
J s\u22121 (and vice versa), precise conversion across
a spectral band does not apply. The approximate
relationship proposed by Morel and Smith (1974)
for the interconversion of solar radiation in the
400\u2013700 nm band of 2.77 × 1018