Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 141

Box 5.1 A note on units in photosynthesis and primary productivity

Physicists describe energy flux in units of W m 2 , which correctly uses the scientific
standard of the International System of Units. Biochemists often use an alternative
unit for light called the 'Einstein', E, with a flux of light energy described in
Em 2 s 1 . More typically the flux is multiplied by 10 6 andquotedin Em 2 s 1 .

Named after Albert Einstein, who explained the photoelectric effect in terms of light
behaving as discrete 'quanta' or photons, an Einstein is one mole of photons
(i.e. 6.022

m

10 23 photons). In biochemistry it is one mole of photons within the
PAR range of wavelengths. A physicist might initially find that an odd choice of unit,
but there is good reason for using moles of photons as a unit. Reactions within the
photosystem use photons to convert energy into numbers of atoms or molecules of
different substances (e.g. O 2 ,CH 2 O); notice in Equation (5.1) that 8 photons are
required to carry out one complete photosynthesis sequence through to carbon
fixation. Biochemists use the mole as a fundamental number to describe quantities
of atoms or molecules, so the Einstein makes sense as a unit of packets of energy that
correspond to numbers of atoms or molecules in reactions. To convert a flux of PAR
from

Em 2 s 1 to W m 2 , multiply the flux by 0.2174.
Notice also that in the list of symbols at the beginning of the topic we have not
been specific about the time units used when measuring rates of primary production.
Generally rates of photosynthesis are measured in per hour (h 1 ) or per day (d 1 ).
Carbon fixation rates could also be quoted in per year (a 1 ), as in Fig. 1.1b . The
choice depends largely on the problem being addressed; just make sure you remember
to check that units are consistent when working with, for instance, Equation (5.4) .

m

uptake, and write Equation (5.4) just in terms of how phytoplankton carbon fixation
can increase phytoplankton biomass:

a q I PAR
P b max

P p ¼ P b max tanh r p þ g p

: ð 5 : 6 Þ

The parameter r p þ g p on the right of the equation represents the loss of biomass

through phytoplankton cellular respiration and grazing by heterotrophs. For the
growth rate, P p b , to be positive there needs to be sufficient photosynthesis being
carried out to account for these losses, so that

a q I PAR
P b max

P b max tanh r p þ g p

ð 5 : 7 Þ

r p þ g p

) I PAR

a q

! .

P b max P b max where we have noted that at low light intensity tanh a q I PAR = a q I PAR =

The light intensity given by r p þ g p

= a q is the compensation light intensity. If we held