Mireille Consalvey PhD Thesis - University of St Andrews

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where photosynthetic rate is limited by light. Low light adapted species will have
a higher alpha to maximise the efficiency of light harvesting, but will also reach
saturation at a lower light level resulting in a lower P ..,. Species adapted to high
light do not need to be as efficient at light harvesting as light is unlikely to be a
limiting factor, therefore may have a lower alpha but have higher Pmax. To
account for changes in biomass the parameters are often normalised to biomass
e. g. chlorophyll a and this is denoted with a *.
P
Light intensity
1Lfl
Figure 1.5. P-E Curve:
Response of a photosynthetic
parameter to light
(see text for explanation).
Above the light level at which the photosynthetic rate saturates, excess
light may become damaging to the light harvesting complex/photosystem and
this is ten-ned photoinhibition (P; Warwick et al. 1984). Several reasons have
been suggested as to why photosynthesis becomes suppressed at supra-optimal
light levels: increase in photorespiration, enzymes become damaged/destroyed or
the electron transport chain is down regulated. By inactivating the chlorophyll
reaction centres, cells may be able to prevent damage by intense light (Warwick
et al. 1984). Photoinhibition has rarely been recorded in intact migratory
microphytobenthic biofilms and it has been postulated that cells within a b1ofilm
migrate downwards in response to damaging surface irradiances (Kromkamp et
al. 1998). Therefore, a biofilm can maintain high rates of primary productivity
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