7 – Primary Production, Nutrient Recycling and Energy Flow through Coral Reef Ecosystems
63
energy from the geothermal sources of reduced compounds such as sulfide stand apart from the overwhelming majority of organisms that are dependent
ultimately on solar energy trapped by photosynthetic
organisms. The myriad of photosynthetic organisms on
coral reefs provide the basis for the vigorous energy
and nutrient cycles that typify coral reefs.
Many organisms participate in photosynthetic activities on coral reefs. Prominent among these are bluegreen bacteria (Cyanophyta), macroalgae (seaweeds),
microalgae (phytoplankton) and photosynthetic protists such as dinoflagellates and diatoms. The reef habitats they occupy are also diverse, with some of the most
significant activity occurring on uncharismatic ‘mossy’
substrata such as rocks and sediments. Importantly,
their densities and actions are profuse within invertebrate host organisms such as corals, clams and foraminifera. Fundamentally, photosynthesis involves
the capture of carbon dioxide using the energy of sunlight trapped using the green pigment chlorophyll. In a
balanced equation, six molecules of carbon dioxide are
incorporated and six molecules of water split to produce one molecule of sugar (glucose) and six molecules
of oxygen.
6CO 2 6H 2 O Energy sunlight m C 6 H 12 O 6 6O 2
The matching process, respiration, acts in the reverse, and oxidises organic molecules (here glucose) to
produce carbon dioxide, water and energy.
C 6 H 12 O 6 6O 2 m 6CO 2 6H 2 O Energy metabolic
There are two parts to the photosynthetic process.
One part is referred to the ‘light’ reactions (where light
energy is trapped by chlorophyll and is converted in
the chemical energy of ATP and other molecules). The
other part involves the ‘dark’ reactions (where the
chemical energy that is trapped during the light reactions is used to fix carbon dioxide to generate organic
molecules). The dark reactions start with the fixation of
CO 2 by the abundant enzyme ribulose bisphosphate
carboxylase/oxygenase (or Rubisco) and involve the
set of reactions comprising the Calvin-Benson Cycle.
Whereas the light reactions are powered by the sun’s
energy, the dark reactions do not need light if the appropriate levels of ATP and other reduced molecules
are made available to power the enzymatically catalysed reactions involved.
Chlorophyll is the central pigment involved in the
transduction from light to chemical energy, but there is
a range of accessory pigments to assist the process. Accessory pigments interact with light in a variety of
ways and consequently add colours from blue-green
(Cyanophyta) to red (Rhodophyta) to the organisms
that contain them. As light increases, so does the rate of
gross photosynthesis (P G ). All organisms (whether photosynthetic or not) respire and release the energy of
carbon-carbon bonds in organic substrata. In photosynthetic organisms, the rate of respiration (R, usually
measured in the dark when no photosynthesis can occur) is subtracted from P G to calculate the net photosynthesis (P N ). This is essentially a measure of the rate
at which organic carbon molecules (and the associated
energy) accumulates during photosynthesis over and
above those consumed during respiration. When measured per square metre of coral reef, values of P N can be
used as a measure of the Net Primary Productivity. This
is an important number as it defines the extent of energy being added to an ecosystem like a coral reef.
The relationship between P N and light has a characteristic shape (Fig. 7.2A). In the dark, P N is negative and
equals the rate of respiration (R). As light increases, however, P N also increases until it equals zero, the co-called
Compensation Irradiance (I c ). At this point, the rate at
which organic carbon (energy) is being produced by
photosynthesis is just balanced by the consumption of
organic carbon by respiration. No net accumulation of
organic carbon (or energy) occurs at this point. I c is also
the point at which the flux of oxygen into the organism
just balances the rate at which oxygen is consumed by
respiration. The reverse is true of carbon dioxide, which
travels in the reverse direction to oxygen. The net rate of
photosynthesis continues to increase as the light levels
increase, with a net accumulation of organic carbon and
production of oxygen.
The relationship between photosynthesis and light
is linear at first and has a characteristic slope (‘A’) that
is a measure of the efficiency of photosynthesis. Essentially A is measure of the rate at which photosynthetic
activity increases with an increase in light (quanta). A
varies according to the type of organism, their light
63
energy from the geothermal sources of reduced compounds such as sulfide stand apart from the overwhelming majority of organisms that are dependent
ultimately on solar energy trapped by photosynthetic
organisms. The myriad of photosynthetic organisms on
coral reefs provide the basis for the vigorous energy
and nutrient cycles that typify coral reefs.
Many organisms participate in photosynthetic activities on coral reefs. Prominent among these are bluegreen bacteria (Cyanophyta), macroalgae (seaweeds),
microalgae (phytoplankton) and photosynthetic protists such as dinoflagellates and diatoms. The reef habitats they occupy are also diverse, with some of the most
significant activity occurring on uncharismatic ‘mossy’
substrata such as rocks and sediments. Importantly,
their densities and actions are profuse within invertebrate host organisms such as corals, clams and foraminifera. Fundamentally, photosynthesis involves
the capture of carbon dioxide using the energy of sunlight trapped using the green pigment chlorophyll. In a
balanced equation, six molecules of carbon dioxide are
incorporated and six molecules of water split to produce one molecule of sugar (glucose) and six molecules
of oxygen.
6CO 2 6H 2 O Energy sunlight m C 6 H 12 O 6 6O 2
The matching process, respiration, acts in the reverse, and oxidises organic molecules (here glucose) to
produce carbon dioxide, water and energy.
C 6 H 12 O 6 6O 2 m 6CO 2 6H 2 O Energy metabolic
There are two parts to the photosynthetic process.
One part is referred to the ‘light’ reactions (where light
energy is trapped by chlorophyll and is converted in
the chemical energy of ATP and other molecules). The
other part involves the ‘dark’ reactions (where the
chemical energy that is trapped during the light reactions is used to fix carbon dioxide to generate organic
molecules). The dark reactions start with the fixation of
CO 2 by the abundant enzyme ribulose bisphosphate
carboxylase/oxygenase (or Rubisco) and involve the
set of reactions comprising the Calvin-Benson Cycle.
Whereas the light reactions are powered by the sun’s
energy, the dark reactions do not need light if the appropriate levels of ATP and other reduced molecules
are made available to power the enzymatically catalysed reactions involved.
Chlorophyll is the central pigment involved in the
transduction from light to chemical energy, but there is
a range of accessory pigments to assist the process. Accessory pigments interact with light in a variety of
ways and consequently add colours from blue-green
(Cyanophyta) to red (Rhodophyta) to the organisms
that contain them. As light increases, so does the rate of
gross photosynthesis (P G ). All organisms (whether photosynthetic or not) respire and release the energy of
carbon-carbon bonds in organic substrata. In photosynthetic organisms, the rate of respiration (R, usually
measured in the dark when no photosynthesis can occur) is subtracted from P G to calculate the net photosynthesis (P N ). This is essentially a measure of the rate
at which organic carbon molecules (and the associated
energy) accumulates during photosynthesis over and
above those consumed during respiration. When measured per square metre of coral reef, values of P N can be
used as a measure of the Net Primary Productivity. This
is an important number as it defines the extent of energy being added to an ecosystem like a coral reef.
The relationship between P N and light has a characteristic shape (Fig. 7.2A). In the dark, P N is negative and
equals the rate of respiration (R). As light increases, however, P N also increases until it equals zero, the co-called
Compensation Irradiance (I c ). At this point, the rate at
which organic carbon (energy) is being produced by
photosynthesis is just balanced by the consumption of
organic carbon by respiration. No net accumulation of
organic carbon (or energy) occurs at this point. I c is also
the point at which the flux of oxygen into the organism
just balances the rate at which oxygen is consumed by
respiration. The reverse is true of carbon dioxide, which
travels in the reverse direction to oxygen. The net rate of
photosynthesis continues to increase as the light levels
increase, with a net accumulation of organic carbon and
production of oxygen.
The relationship between photosynthesis and light
is linear at first and has a characteristic slope (‘A’) that
is a measure of the efficiency of photosynthesis. Essentially A is measure of the rate at which photosynthetic
activity increases with an increase in light (quanta). A
varies according to the type of organism, their light