The capture of the sun's energy is a fundamental step in the energy and nutrient cycles of biological systems on Earth. Only chemosynthetic organisms that derive 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 blue-green bacteria (Cyanophyta), macroalgae (seaweeds), microalgae (phytoplankton) and photosynthetic pro-tists 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 fo-raminifera. 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.
6CO2 + 6H2O + Energysunjight ^ C6H12O6 + 6O2
The matching process, respiration, acts in the reverse, and oxidises organic molecules (here glucose) to produce carbon dioxide, water and energy.
C6HH12O6 + 6O2 ^ 6CO2 + 6H2O + EnergYmetabolic
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 CO2 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 (PG). All organisms (whether pho-tosynthetic or not) respire and release the energy of carbon-carbon bonds in organic substrata. In photo-synthetic organisms, the rate of respiration (R, usually measured in the dark when no photosynthesis can occur) is subtracted from PG to calculate the net photosynthesis (PN). 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 PN 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 PN and light has a characteristic shape (Fig. 7.2A). In the dark, PN is negative and equals the rate of respiration (R). As light increases, however, PN also increases until it equals zero, the co-called Compensation Irradiance (Ic). 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. Ic 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
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