Biological Oceanography

(^) At even higher intensities, the rate begins to fall off, an effect called
photoinhibition. This effect has different causes in different phytoplankton. One
important cause is “photorespiration”. The intermediate products of the light-
independent reactions include five-carbon, phosphate-esterified sugars, which are
photolabile. Under intense light they break down into phosphoglycolic acid (C 2 ) and
phosphoglyceric acid (C 3 ). The former is either excreted from the cell or metabolized
to CO 2 , but it cannot be returned to the photosynthetic pathway. The increase of this
process (and probably other similar ones) reduces the overall rate of net
photosynthesis.
(^) Light-intensity response curves (“P vs. E curves”) are all of this basic form, but
differ in initial slope (α), maximum rate (“Pmax”), intensity at onset of
photoinhibition, and rate of decrease due to photoinhibition. When photosynthetic
rates are normalized to (i.e., divided by) the levels of chlorophyll or cell carbon, the
initial slope and maximum rates are denoted as αB and PmaxB, where B denotes the
normalization parameter used. The intersection of the initial slope of the P vs. E curve
with Pmax is KE, the light-saturation parameter that indicates the irradiance where the
shift in the controlling factor changes. Light responses may differ between clones of
the same species cultured from different (or even the same!) habitats; between
samples from different depths at the same station; and they differ substantially
depending upon the history of light exposure of the phytoplankton. Various functional
forms have been suggested to describe the P vs. E relationship. Several equations
suggested by Platt and others are widely used. Platt and Jassby (1976) suggested a
hyperbolic tangent function, P = Pmax tanh(αE/Pmax), which works well when
photoinhibition is not obvious at higher intensities. This hyperbolic tangent function
also has some theoretical attractions. Platt et al. (1980) fitted an array of P vs. E data
for natural phytoplankton assemblages that did exhibit photoinhibition with the