Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 131

characteristics, and provide a broad explanation for why the species of phytoplankton
found in phytoplankton communities on the shelf and in the open ocean might differ.

5.1.1 Photosynthesis: light, pigments and carbon fixation

We learned in Chapter 2 that the tides produced by the Moon and the Sun dissipate
about 3.7 TW of energy in the ocean, a rate that leads to a measurable slowing of the
Earth's daily rotation. The photosynthesis of all plants on the Earth uses energy at a rate
of about 100 TW (Nealson and Conrad, 1999 ), which is roughly 0.05% of the total
energy supply from the sun (see Section 2.2.1 ). About half of the photosynthesis occurs in
the oceans, and so, using the estimate of the contribution of shelf sea primary production
to the global total from Chapter 1 , about 8 TW of solar energy is utilised by the photo-
autotrophs in the shelf seas. This corresponds to about half of humanity's total energy
demand, so primary production is a remarkable utiliser of renewable energy.
Photosynthetic organisms use sunlight to convert carbon dioxide (CO 2 ), or 'fix'
carbon, into organic compounds, particularly sugars (carbohydrates). Converting
CO 2 into carbohydrate is a reducing reaction, which requires a source of energy and
electrons. The energy is sunlight, while in aquatic photosynthesis the electrons are
taken fromwater (H 2 O). A by-product of this reaction is oxygen (O 2 ) which is released
back into the seawater. Dissolved oxygen concentrations above the 'saturated' concen-
tration that seawater can hold are an indication of active primary production. This
excess oxygen will, once exposed to the atmosphere, transfer to the air; in this way
marine primary production supplies us with about half of the oxygen that we breathe.
Fixing carbon takes place in two stages, shown schematically in Fig. 5.2 . In the
first, 'light-dependent' stage light is intercepted and used within one of many photo-
synthetic reaction centres in the cell. Each reaction centre has a light-gathering
'antennae' which uses pigments to absorb light energy. The energy is then used in
two photosystems (referred to as photosystems I and II) to make the molecules ATP
(adenosine triphosphate) and NADPH (reduced nicotinamide adenine dinucleotide
phosphate). Next comes the 'light-independent' stage (also known as the 'dark
reactions') of the Calvin-Benson cycle, where the ATP and NADPH are used to
reduce CO 2 , and ultimately to produce carbohydrates, amino acids and lipids.
A summary of the chemistry of the photosynthetic reactions is:

2H 2 O þ CO 2 þ 8 photons ! CH 2 O þ H 2 O þ O 2 : ð 5 : 1 Þ

CH 2 O on the right of Equation (5.1) is the building block for carbohydrates; 6 of
them yield glucose, while 12 of them form sucrose.
The details of photosynthesis can be found in Falkowski and Raven, 2007 , and
Kirk, 2010 . For our purposes there are four main points to bear in mind.

(i) Not all photons are useful to photosynthesis. Light between wavelengths of about
400 and 700 nm, referred to as photosynthetically available radiation (PAR), is
required by the phytoplankton. These wavelengths are in the green and blue parts
of the visible spectrum (e.g. Section 2.2.1 ), which penetrate the furthest through